This updated buyer's guide from Agena Astro helps you select the best ZWO astronomy camera for your interests and budget. This guide walks you through the key specifications of the full line of ZWO cameras including sensor size, pixel size, read noise, download rate, cooling, and color vs. monochrome. It helps you understand these specifications and why they are important in astrophotography. This guide also gives some basic tips for matching a camera to specific applications such as lunar/solar, planetary, and deep-sky imaging. After reading this guide, you will be able to make an informed choice when selecting and purchasing a ZWO astronomy camera or, for that matter, any other brand of astronomy camera as well.

• 1. Introduction
• 2. An Overview of ZWO Astronomy Cameras
  • 2.1. The Naming System for ZWO Cameras
  • 2.2. Solar-System Cameras
  • 2.3. Deep-Sky Cameras
  • 2.4. Autoguiders
• 3. ZWO Camera Specifications - A Deep Dive
  • 3.1. Size and Mechanical Footprint
  • 3.2. Port Configurations and Cables
  • 3.3. Sensor Size and Field of View
  • 3.4. Resolution
  • 3.5. Pixel Size and Critical Sampling
  • 3.6. Pixel Size and Noise, Full-Well Depth, and Binning
  • 3.7. Color vs Monochrome
  • 3.8. Noise and Cooling
  • 3.9. Shutter Speeds, Data Resolution, and Download Rates
  • 3.10. Amp Glow
• 4. Choosing a ZWO Camera for Astrophotography
  • 4.1. Overview
  • 4.2. Best Astrophotography Cameras for Beginners
  • 4.3. Cameras for Solar System Imaging
  • 4.4. Deep-Sky Cameras
  • 4.5. Cameras for EAA
• 5. Summary Table of Recommendations and Key Specifications
• 6. ZWO Accessories
• 7. Summary
• 8. Appendix

1. Introduction

Over the past several years, Chinese manufacturer ZW Optical (ZWO) has continued to introduce a series of CMOS astronomy cameras for planetary and deep-sky astronomy imaging that have developed a large following among amateur astronomers. These cameras are easy to use, affordable, compact, and they work with standard camera control and imaging software. They also have high quantum efficiencies that rival CCD-based cameras, once the preferred technology for astronomy imagers. ZWO also has a series of accessories for their cameras such as filter wheels, WiFi-enabled controllers, and adapters that make their astronomy cameras more versatile and easier to use for solar system imaging, deep-sky imaging, and electronically-assisted astronomy (EAA).

Figure 1 - A full-frame (36mm X 24mm) CMOS Sensor. (Image credit: Harry M. used under CC BY-SA 4.0).

However, as more ZWO cameras are introduced, it becomes challenging, especially for beginning astronomy imaging enthusiasts, to select a camera from the growing product line. At first glance, the sheer variety of available ZWO cameras, coupled with the array of model numbers and other specifications, can be difficult to navigate.

This Agena Buyer's Guide will help you break down all of this information into manageable pieces. It explains the key specifications of ZWO astronomy cameras including sensor size, pixel size, read noise, download rate, cooling, and CMOS color astronomy cameras vs. monochrome astronomy cameras. It also helps you narrow down your choices and select the best astrophotography camera for your interests, equipment, and budget. While this guide is geared towards the first-time astronomy camera buyer, intermediate and advanced imagers should also find this content - especially the summary specification tables - useful in making their choices.

2. An Overview of ZWO Astronomy Cameras

Figure 2 - Three types of ZWO astronomy cameras, from left to right, autoguiding cameras, an uncooled planetary camera, and a cooled deep-sky camera. Image credit: ZWO.

2.1. The Naming System for ZWO Cameras

ZWO currently offers cameras in three main categories: planetary and lunar/solar imaging, deep-sky imaging, and autoguiding cameras. This section provides a quick overview of each type of camera to help you get oriented. Later sections dive into the details of each type of camera.

For all cameras in the ZWO product line, monochrome cameras have an 'MM' designation as in, for example, ASI1600MM. Color cameras have an 'MC' designation, such as the ASI294MC camera.

The ZWO deep-sky cameras use thermoelectric coolers (TECs) to reduce the temperature of the sensor to reduce noise. These cameras have a 'Pro' designation to distinguish them from uncooled cameras for solar system imaging and the now discontinued first generation 'Cooled' ZWO cameras. ZWO 'Pro' cameras also include a memory buffer to improve and stabilize data transfer from the camera to a computer. For example, the ASI2600MC-Pro is a color camera with TEC cooling and memory buffer.

The ZWO Mini cameras, which have a 'Mini' designation, are monochrome cameras with smaller form factors and USB2.0 interfaces that are ideal for use as autoguiders. An example is the ASI174MM-Mini. These cameras fit directly into a 1.25" focuser of a guide scope.

Each ZWO camera has a number associated with it with the prefix ASI. For example, the ASI290MM is a popular planetary camera, while the ASI6200MM-Pro is a high-end deep-sky camera. These numbers refer to either the Sony sensor used in the camera or to the resolution of the camera in megapixels. The ASI290MM uses the Sony IMX290 CMOS sensor; the ASI6200MM-Pro has a resolution of 62 megapixels and uses the Sony IMX455 CMOS sensor.

2.2. Solar-System Cameras

Planetary, solar, and lunar telescope cameras (or solar-system cameras) are typically smaller, lighter, and usually less expensive than deep-sky cameras. Because planets and the Moon and Sun do not have a large angular size, these cameras usually have smaller CMOS sensors which make them less expensive. Smaller sensors, since they have fewer pixels, also have faster rates of data transfer which is important for planetary imaging. These cameras all have USB3.0 interfaces.

Solar-system cameras generally do have a mechanism to cool the sensor during operation, which is usually not required for the short exposures used for imaging bright solar-system objects. But despite the modest specification of these cameras, many experienced solar-system observers produce superb images with cameras that cost just a few hundred dollars.

ZWO solar-system cameras mostly incorporate color sensors, though a few use monochrome sensors which are typically used with sets of external filters and standard processing techniques to produce color images.

Table 1a lists the current selection of ZWO cameras for imaging solar system objects and includes the key specifications of the size of the sensor diagonal and the sensor format, a number related to the shape and size of the camera (but which is not a precise measure of the size of the sensor itself). These and other specifications will be discussed in more detail in the following sections. Many of these solar system cameras can also be used for electronically-assisted astronomy (EAA) of solar system or deep-sky objects.

2.3. Deep-Sky Cameras

Deep-sky imagers usually use telescope cameras that incorporate on-board cooling since they often use long exposures of many seconds or minutes where thermal noise becomes a problem. These cameras also use larger sensors that capture a bigger angular size of the night sky, an important consideration when capturing large celestial objects like emission nebulae or supernova remnants. As a result, cameras for imaging deep-sky objects (DSOs) are generally larger and more expensive than solar-system cameras. These cameras also all have USB3.0 interfaces.

The smaller-sensor DSO cameras have a similar sensor size to the larger-sensor solar-system cameras, while the largest DSO cameras have large APS-C and full-frame sensors, the same as commercial mirrorless or DSLR cameras, that capture low-noise images of large expanses of sky. ZWO deep-sky cameras incorporate both monochrome and color sensors, and some models are available in both formats. Table 1b lists the current selection of deep-sky ZWO cameras and their sensor sizes. ZWO gives all these cameras a 'Pro' designation because they include on-board cooling and other advanced features.

2.4. Autoguiders

ZWO also offers a small selection of cameras for autoguiding. These autoguiders are the size of small 1.25” eyepieces and they feature slower USB2.0 data interfaces that help make them relatively affordable. Table 1c lists the current offering of dedicated ZWO autoguiders. Some astrophotographers also use ZWO planetary cameras listed in Table 1a for autoguiding.

With this overview in mind, now let's dive into the details of key specifications of all three of these types of ZWO astronomy cameras.

3. ZWO Camera Specifications - A Deep Dive

3.1. Size and Mechanical Footprint

Figure 3 – A ZWO solar system camera (ASI290MM, left) and a deep-sky camera (ASI533MC-Pro, right) attached to a Schmidt-Cassegrain telescope.

All ZWO astronomy cameras are housed in CNC machined red-anodized aluminum bodies that stand up to heavy field use. Not all cameras are the same size, however, but each fall into one of five form factors:

• The uncooled 'solar system' cameras with bodies of 62mm diameter and a length of about 35-41mm, approximately, and weighing between 100g and 140g
• The cooled 'Pro' cameras with mid-sized sensors have larger bodies of 78mm diameter and a length of 73.5mm. These bodies weigh 410g to 470g, approximately
• The largest 'Pro' cameras with APS-C and full-frame sensors have a diameter of 90mm, a length of 97mm, and weight of 700g (the ASI071MC-Pro camera has a slightly smaller (86mm diameter) and lighter body)
• The ASI183GT and ASI1600GT monochrome cameras with integrated filter wheel have bodies that are 110mm square and weigh 800g
• The small 'Mini' guide cameras have a diameter of 36mm, a length of 61mm, and a weight of 60g.

The Pro cameras are larger than the other cameras because they must accommodate large heat sinks and fans to manage heat flow. Figure 3 above shows a small uncooled 'solar system' camera and a cooled 'Pro' camera at the focuser of a telescope.

With the exception of the Mini cameras and the largest deep-sky cameras with full-frame sensors, ZWO camera bodies also feature a standard T / T2 M42x0.75 internal (female) thread interface on the telescope/lens side. This thread is very commonly used in astronomy and photography, allowing you to attach a wide variety of adapters and other accessories to the camera body. The large full-frame cameras use an M54x0.75 thread (see Figure 4).

(For a primer on astronomy threads, see our guide Astronomy Threads Explained).

Figure 4 – The front face of two ZWO cameras with M42 threads and one with M54 threads

Also, except for the larger cameras with APS-C and full-frame sensors, most ZWO cameras include a 1.25"-T threaded nosepiece adapter so the camera can be inserted directly in standard 1.25" telescope focusers. The fronts of some cameras also have a short 2" barrel that can be directly inserted into 2" telescope focusers. This barrel is 8mm-11mm in length depending on the model. However, a separate 2" T threaded prime focus adapter (not included with the camera) is recommended for a more robust and secure connection to 2" telescope focusers for those who wish to use the camera in this way (see Figure 5). The 'Pro' cameras, which have larger sensors that would be vignetted by a 1.25" nosepiece, have 2" extenders so they can mount into larger 2" focusers.

Figure 5 –A ZWO camera with a T-thread (M42x0.75) and a 1.25” nosepiece (top), 2” nosepiece with optional extender (bottom).

The opposite side of most uncooled ZWO solar system cameras features a 1/4"-20 female thread in the bottom to make it easy to attach the camera device to a photo camera tripod or mount for use as an all-sky camera without a telescope. Cooled ZWO cameras can be mounted in an optional adapter ring (available with inside diameters of 78mm, 86mm or 90mm) for mounting to a camera tripod.

Figure 6 – A ZWO ASI385MC solar-system camera with an all-sky lens (left) and the back of the same camera showing the ¼”-20 mounting hole in the center.

In each type of ZWO camera, the sensor is recessed from the front edge of the camera body or from the threaded nosepiece. Smaller-sensor uncooled cameras have a back focus of 12.5mm and the larger-sensor uncooled cameras have a back focus of 17.5mm. The cooled 'Pro' cameras have a back focus of 17.5mm.

Many cameras also incorporate a protective anti-reflection (AR) coated window in front of the sensor. Some cameras use a window instead (these are not AR-coated) that also blocks UV and IR light from reaching the sensor to help sharpen the resulting images. These cameras do not require an additional UV-IR cut filter. Tables 2a-c summarize the size, weight, and back focus of all currently available ZWO cameras and lists which type of protective window is used in each camera.

Tables 2a-c summarize the size, weight, and back focus of ZWO solar system, deep-sky, and autoguiding cameras.

3.2. Port Configurations and Cables

Every ZWO camera comes with a number of ports and cables for power and communication, but the port configuration for the cooled 'Pro' and uncooled cameras are slightly different (see Figure 7).

Uncooled camera ports include a USB port for data communications and powering the camera. The port can be used at either USB2.0 and USB3.0 data transfer rates (except for the discontinued ASI120MM/MC cameras which only have USB2.0 capability). A 2m USB3.0 cable is included with the camera. The uncooled solar-system cameras also include an autoguiding port with ST4 connector to allow the camera to be used as an autoguider. ZWO includes an autoguiding cable with these cameras.

Figure 7a - The ZWO ASI1600MM monochrome astronomy camera. In this uncooled unit, the blue USB port is at left and the ST4 autoguiding port is at right. Image courtesy of ZWO.

ZWO 'Pro' deep-sky cameras have the following ports:

• A USB3.0 port for communications and data transfer. A 2m USB3.0 cable is included
• A USB2.0 hub (two ports) to connect an electronic filter wheel (EFW) and a guiding camera to the ZWO camera where, through the communications port, these devices can be controlled with a computer. Two short USB cables are included to be used with these ports
• A 2.1mm center-positive connector for an external DC power supply for the cooler. The power supply for the camera is not included with the camera, but ZWO recommends all 'Pro' cameras use an external 12V 3A-5A DC power supply to power the camera and cooler. They do not recommend powering 'Pro' cameras with a computer through the USB3.0 interface

(Note - Some discontinued ZWO cameras with the 'Cool' designation include an ST4 autoguider port instead of the USB2.0 hub).

The ZWO Mini cameras have an ST4 port for autoguiding and USB2.0 ports with a Type C interface connection for power and control.

Figure 7b - The ZWO ASI533MC-Pro monochrome astronomy camera. In this cooled unit, the blue USB port is at the upper right. To the left of the blue USB3.0 port is a USB2.0 hub used to connect an autoguiding camera and electronic filter wheel. Image courtesy of ZWO.

Figure 7c - The ZWO ASI174-Mini camera with an ST4 port and USB port. Image courtesy ZWO.

3.3. Sensor Size and Field of View

How much sky can you image with your ZWO camera? The answer depends on two factors: the effective focal length of your telescope and the size of your camera's sensor. If you know the size of the sensor, you can calculate the field of view of a camera using this formula:

Field of View (in arc-minutes) = 3436 x D/L

Here, D is the dimension in millimeters of the sensor, either the length or width of the sensor, for example, or the dimension of the sensor's diagonal. The quantity L is the effective focal length of the telescope system in millimeters. (Note: There are 60 arc-minutes in a degree and 60 arc-seconds in an arc-minute). The field of view does not depend on the size of the pixels or the number of megapixels in the sensor or the aperture of the telescope. It only depends on the size of the sensor and the focal length of the telescope, along with any Barlow lenses or focal reducers used in the optical path.

Tables 3a-c give the dimensions, resolution, and pixel size of currently available ZWO cameras.

When considering a ZWO astronomy camera, it is important to match the size of the diagonal and the focal length of your telescope to the size of the type of object you wish to image. Planets, for example, are quite small. Jupiter grows to a size of 40-50 arc-seconds at opposition. Galaxies, smaller star clusters and nebulae range in size from 3-4 arc-minutes to 20-30 arc-minutes or more. And the sprawling North America Nebula is nearly 180 arc-minutes at its longest dimension. Table 4 below gives approximate sizes of some common celestial objects.

Given the wide range of the size of astronomical objects, no single camera and telescope can frame all objects in an optimum way. But if you wish to image planets, you generally need a telescope system with long focal length and a camera with a small sensor size so that the sensor is not underfilled. If you are interested in the Moon and Sun, you want a telescope with intermediate focal length and a slightly larger sensor so that you can get full-disk images. And if you are after large deep-sky objects, you want a sensor of intermediate to large size and a telescope with a relatively fast focal ratio to achieve bright images of extended objects. Let's look at a few examples.

Example 1 – Lunar/Solar/Deep-Sky Imaging
With an 80mm refractor of 560mm focal length and an ASI533MC-Pro camera that has a square sensor 11.3mm on each size, the above equation shows this telescope-camera combination has a field of view of 69'x69' (or 1.16° x 1.16°). This field of view is ideal for framing the full disk of the Sun or Moon, and it nicely frames moderately-large deep-sky objects like the globular cluster Messier 13.

Example 2 – Planetary imaging
Since planets appear quite small, to get a good planetary image, you want a camera with a small sensor and a telescope with a long focal length. An 8” Schmidt-Cassegrain telescope operating at its native focal length of f/10 has a focal length of 2000mm. When used with an ASI662MC camera which has a small sensor of dimensions 5.6mm x 3.2mm, this telescope will deliver a field of view of 9.6' x 5.5' (or 576” x 330”). Recall that even Jupiter, which presents the largest apparent diameter at opposition, spans an apparent diameter no larger than 50”. Using a 2x Barlow lens in this telescope with the same camera delivers a field of view of 288” x 165”.

Do these examples mean cameras with larger sensors have no place in planetary imaging? Not necessarily. Some software used to capture images from astronomy cameras has a feature that allows you to capture a 'region of interest' or ROI that is just a subset of the pixels on a larger-sensor camera. By selecting an ROI that frames a planet, it is possible to get a respectable image. Of course, larger sensor cameras are more expensive so for imagers who are primarily interested in planetary imaging, a smaller-sensor camera is more cost effective.

Figure 8 – A wide-field image of the Veil Nebula captured with an ASI533MC-Pro camera and an Askar ACL200 (200mm) f/4 astronomical camera lens and an Optolong L-eNhance filter. This set-up delivers a field of view of 194'x194'. Image credit: Brian Ventrudo.

Example 3 – Wide-Field Deep-Sky Imaging
Now let's say you want to capture an image of the entire Veil Nebula complex (Figure 8), a supernova remnant in the constellation Cygnus. The entire object spans about 3°, so to frame it nicely you need a field of view of about 5°. That implies a big sensor and a short-focal-length telescope. Let's pick a full-frame camera, the ASI2400MC-Pro, with a sensor of 36mm X 24mm. To get a field of view of 5° (300') across the smaller edge of the sensor, we use the above equation to calculate that we need a telescope with a focal length of < 275mm. The Askar FMA230 with a 230mm focal length, for example, or the Sharpstar 61EDPHIII with 0.8x focal reducer (268mm focal length) would do the job.

3.4. Resolution

In consumer cameras for everyday use, 'resolution' usually refers to the total number of pixels in a sensor. A 10-megapixel (MP) camera has 10 million pixels on its sensor, for example. ZWO astronomy cameras come in a wide range of resolutions, from 1.3 MP for the ASI120MC-S to a huge 62MP for the deep-sky ASI6200MC-Pro.

Figure 9 - A schematic configuration of a camera sensor showing the dimensions of the sensor and the pixels. Image credit: VS Technology.

Does more resolution make for a better astronomy camera? Like many things in astrophotography, it depends on the application and the end goals.

Higher-resolution cameras do offer the possibility of images with more detail, especially when such images are presented on a large screen or when enlarged and printed. But if you view your images mostly on a computer screen, keep in mind that a 4k computer screen has a resolution of 3840x2160 pixels or 8.29 MP. Uncropped frames taken with cameras of higher resolution do not look any better or more detailed on such a screen. Of course, the frames from high-resolution cameras can also be cropped or zoomed without experiencing a loss of detail. And using the 'region of interest' function on camera control software, a high-resolution camera can 'zoom' in on detail in a solar-system or deep-sky object while still retaining enough detail.

On the downside, cameras with higher resolutions need to transfer a larger amount of data after capturing each frame, which is why higher resolution cameras have relatively slow data transfer rates. The 2.1 MP AIS290MM camera, for example, has a data transfer rate of 170 frames per second (fps), while the 62MP ASI6200MC-Pro transfers data at just 2.0 fps. Fast data transfer rates are critical for planetary, lunar, and solar imaging, which is why cameras for this application tend to have smaller, lower resolution sensors. Deep-sky objects are essentially fixed in appearance so fast rates are not important. Another drawback - each frame on a high-resolution camera takes up a lot of memory on a computer. Such cameras also tend to cost more because their sensors are larger.

3.5. Pixel Size and Critical Sampling

The resolution of a camera tells us little about the size of the sensor or the size of the individual pixels. And when it comes to astronomical imaging, the size of the individual pixels is important. Larger pixels can capture more light with less noise, but they offer a lower resolution across an image of a planet or other object projected on your sensor by your telescope. Smaller pixels can, under certain conditions, capture more fine detail in celestial objects. This is especially important for imaging the planets, Sun, and Moon.

This sub-section is a little more technical than most in this buyer's guide. But once you gain an understanding of the influence of pixel size in astrophotography, you will be able to better understand the differences between some ZWO camera models that, at first glance, may appear otherwise similar or aimed at a similar application.

A little 'quantification' is in order here. The pixel size of a camera's sensor for a telescope of a given focal length influences the image scale, the angle subtended by each camera pixel with a given telescope system. It is usually expressed in arc-seconds per pixel, and it's calculated with this simple formula:

Image scale (arc-seconds/pixel) = 206 x s/L

where s is the pixel size in microns and L is the focal length in millimeters. For example, the ASI294-series cameras have pixels that are 4.63 microns on each edge. With an 80mm telescope of focal length 560mm, this gives an image scale of 1.7”/pixel.

But what is the 'best' image scale, and why does it matter? To answer this question, we need to understand the concept of sampling, a quantity which describes how a camera's pixels are able to reproduce fine detail in an image produced by a telescope.

For example, let's say we're trying to capture an image of the Moon where the smallest feature, say a small crater, appears about 3” wide with our telescope. We need to make sure that the image scale of our camera sensor is sufficient to sample across this small part of the image. If we have an image scale of 4”, we won't see any detail in the crater because each pixel samples a wider angular diameter than the crater. This is an example of undersampling. But if we try for a very small image scale of, say 0.2”, by increasing the effecting focal length of our telescope (or choosing a camera with smaller pixels) to get 15 pixels sampling the crater's image, we may just be using extra pixels to detect detail that our telescope can't resolve (or it may just sample greater detail in a blurry image if the atmospheric seeing is bad). This is called oversampling.

The 'sweet spot' is called critical sampling and it's calculated by specifying the number of pixels (N) across the smallest feature our telescope can resolve. For a given telescope with effective focal ratio FR, and for light at wavelength λ (in microns), to get make sure our image is critically sampled, we need a camera with pixel size s (in microns) that satisfies this equation:

s_c = 1.22λF_R/N

In world-class seeing conditions, we might use N=5, that is, we have 5 pixels sampling the smallest resolvable detail. For average to good seeing, N=3 and we'll use this quantity in the rest of the argument and examples. That means 3 pixels will sample the smallest resolvable image. If our pixel size is less than sc then we are oversampling. If our pixel size is greater than s_c then we are undersampling.

Here's an even simpler form of the above equation, taking the wavelength of light as 500nm (0.5μm) in the middle of the optical spectrum, and N=3:

F_R ≅ 5s_c

This says that to achieve critical sampling, make sure your telescope has an effective focal ratio of five times the pixel size (in microns). You often see this rule of thumb mentioned in books on astrophotography and in the online astronomy forums.

To achieve critical sampling, we can pick a camera with the right pixel size to match our optics. Or if we already have a camera with a fixed pixel size, we can use the above equation to figure out what our telescope's effective focal ratio should be to achieve critical sampling. We have some control over the focal ratio by adding a Barlow lens or focal reducer.

Example 1:
Let's use our ubiquitous 80mm telescope with a focal length of 560mm and also use it with a 2x Barlow lens. This gives a working focal ratio of f/14. At the center of the visible spectrum in green light where λ=0.5μm, the above equation says we are critically sampling our image when we choose a camera with a pixel size of 2.8μm. This also gives us an image scale of 0.52”/pixel. This pixel size matches quite well to the 2.9μm pixel size of many ZWO cameras such as the ASI290MM, the ASI662MC, or the ASI585MC. If we choose a camera with smaller pixels we are oversampling and, with larger pixels, we are undersampling.

How important is critical sampling? It is important for imaging maximum detail with planets, the Moon and Sun, and small deep-sky objects like planetary nebulae and some galaxies. For getting dramatic wide field images of large nebulae, we're usually not worried about capturing the smallest degree of detail, so undersampling is nothing to worry about. In most cases, however, we want to avoid oversampling because it does not improve the image and, if the pixels are small enough and the signal on the camera is small, we may make noise worse. In fact, as we see in the next section, choosing cameras with larger pixels can help with noise and other specifications, especially for deep-sky objects, even if it does result in undersampling.

Let's do a couple more examples.

Example 2:
Let's look again at our effort to capture the entire Veil Nebula in Example 3 from Section 3.3. We have a full-frame camera, the ASI2400MC-Pro, a camera with 5.94μm pixels. Let's also use the Askar FMA230 that has 230mm focal length and focal ratio of f/4.6. This scope takes in the entire nebula with this camera. If we image the nebula with an OIII filter, which passes light at about 500nm (0.5μm), the above equation says that, for critical sampling, we need a camera with pixels that are 0.94μm wide. But our camera has 5.94um pixels, so we are vastly undersampling our image. But we don't care because our goal is to capture an aesthetically-pleasing image the entire nebula. If we want to capture finer detail in the Veil, we need a camera with smaller pixels or a telescope with a slower (larger) focal ratio.

Figure 10 – An image of the Sun captured with a ZWO ASI290MM solar system camera with an H-alpha solar telescope. For critical sampling, the 2.9μm pixel size of this camera matches well to a focal ratio of f/10.9, close to the f/10 focal ratio of the telescope used to capture this image. Credit: Brian Ventrudo.

Example 3:
Now let's consider capturing solar images using the popular Daystar Quark H-alpha solar filter with a 120mm f/8 refractor (a focal length of 960mm). Our goal is to get the best possible detail on the solar disk, so we want to achieve critical sampling. The Quark has internal optics that increase the effective focal ratio of our telescope by 4.2x for an effective focal ratio of f/33.6. At the H-alpha wavelength of 656nm (0.656μm), and again choosing N=3 (three pixels for each barely resolvable detail), the above equation says our image is critically sampled with a pixel size of 9.0μm. This is the exact pixel size of the ZWO ASI432MM camera, an excellent choice for this application. However, with a sensor size of 14.5mm X 9.9mm, we will get a field of view (using the equation in section 3.3) of 12' x 8.4' which does not capture the entire 30' wide solar disk. That is the trade-off we make since ZWO does not make (nor does anyone) a camera for amateur astronomy with 9μm pixels that has a sensor large enough to give a full-disk solar image.

Tables 3a-c above summarize key sensor specifications for all ZWO cameras.

3.6. Pixel Size and Noise, Full-Well Depth, and Binning

Sensors with larger pixels tend to have better noise performance than those with smaller pixels, especially at low light levels. That's why, for example, the camera on a smartphone shows much noisier (or grainy) images taken at night compared to a DSLR or mirrorless camera. The phone might have pixels just 1.5μm across while a DSLR might have 4μm or even 6μm pixels. Bigger pixels also have noise, but they collect more light and therefore have a higher signal-to-noise ratio. ZWO cameras intended for bright solar-system objects tend to have smaller pixels. Deep-sky cameras have larger pixels, on average.

Larger pixels also tend to have a larger full-well depth. During an exposure, light generates electrons in each sensor pixel. But there's a limit to how many photoelectrons each pixel can hold before no more are generated. This is the full-well depth. When this number is exceeded, signal can spill over into adjacent pixels causing the images of bright stars to 'bloom' and appear large and bloated. Larger pixels can, in general, hold more electrons. For example, the ZWO ASI290MM camera with 2.9μm pixels has a full-well depth of 14,600e-. The ASI294MC camera has 4.63-micron pixels and full-well depth of 63,700e-.

Tables 5a-c below list the maximum full-well depth of the current selection of ZWO cameras.

Most ZWO astronomy cameras also enable 2x2 binning of pixels. This is the process, controlled in software, of combining four pixels together to effectively make one larger pixel. The signal increases by a factor of four, but the read noise also increases slightly, so the all-important signal-to-noise ratio improves by less than a factor of four. Binning is accomplished at the expense of image resolution.

Binning is commonly used, for example, to achieve an image scale that is more realistic for the seeing conditions. For example, if your telescope and camera give an image scale of 0.3 arcsec/pixel, but your seeing is only 1.5 arcseconds, then this will result in decreased sensitivity without improved resolution, and you need to attend more carefully to guiding during the image. Binning can help improve this situation. Binning is also used in color imaging with a monochrome camera and appropriate color filters (see section 3.7 below). Many imagers take the monochrome images without binning, then capture the color images binned. The binned pixels are four times more sensitive, so the time needed to capture color data is reduced by a factor of four. In post processing, the color images are blended with the more detailed mono images.

3.7. Color vs Monochrome

Should you choose a color camera, or a monochrome camera?

If you're just starting out, or you want to keep your astrophotography workflow simple, a color camera is a good bet. A color astronomy camera uses the same sensor as its monochrome counterpart, but it incorporates a color filter array, often a Bayer filter, permanently fixed over the sensor. The filter passes red, green, or blue light into each pixel on the sensor, and an algorithm in the camera interprets the intensity of light on each pixel and produces a full-color image. They are sometimes called one-shot color (OSC) cameras.

Figure 11 – Three monochrome images taken of Mars with a ZWO ASI290MM camera combined into a single color image. Image courtesy of ZWO and Milika and Nicholas.

A color astronomy camera lets you capture a full-color image of planets or deep-sky object in a single shot without much additional processing and without the need for additional filters. Using standard astro-imaging software, color images can be stacked, sharpened, and enhanced as needed. However, you are restricted to the colors provided by the color array filter and processing techniques. There is no easy way to add information from colors outside the visible spectrum - IR data, for example - which may enhance the image. And there can be some loss of image sharpness and resolution when working with images from color astronomy cameras, especially when using narrow-band OIII (green) or H-alpha (red) filters that only pass one color of light and which, therefore, only register a signal on the pixels with that particular color filter in front of it.

That's why most serious astrophotographers use monochrome cameras for their best work. A monochrome camera produces a single monochrome image of a planet or DSO. But more commonly, a monochrome camera is used to make a series of images through color or narrowband filters. For planets, separate images are captured through a series of color filters then combined using standard image-processing techniques to produce full-color images with far more detail than is accessible with a single-color image (see Figure 8). For deep-sky objects, especially nebulae, multiple images are collected through color and narrowband filters then combined into a single image. Again, far more details are accessible in such objects with this approach using multiple images through filters and a monochrome camera.

Figure 12 - Monochrome image of IC 405 captured with a 60mm f/4.3 refractor and ZWO ASI174MM monochrome camera and narrow-band H-alpha filter. Image courtesy ZWO and Matej Mihelcic.

The disadvantage of imaging with monochrome cameras? In some cases, it can take more time to gather multiple images through filters, and it takes more time to combine the images in post processing. You need to buy the filters, of course, and also a manual or automated filter wheel to hold and swap the filters into the optical path. And when imaging planets, to achieve the sharpest images, you must work quickly to capture each image before a planet's rotation smears the image.

While monochrome has been the choice of serious imagers, even expert astrophotographers appreciate the benefits and ease of using color cameras, especially when shooting the deep sky. The latest 'Pro' level ZWO cameras are built around larger color sensors that allow experienced imagers to capture 'one-shot' color images that are of impressive quality. These are the same sensors used on high-end DSLR cameras. Because of the large sensors, these cameras, which include the ASI2400MC-Pro, ASI2600MC-Pro, and ASI6200MC-Pro, tend to be quite expensive. These color cameras, and smaller color cameras like the ASI294MC-Pro and ASI533MC-Pro, work quite well on nebulae and supernova remnants when matched with dual or tri-band filters such as the Optolong eNhance filters that pass blue light (from H-beta), green light (from OIII) and red light (from H-alpha).

One word of warning – it's often difficult to get good color balance when using OSC cameras in skies that suffer from moderate to serious light pollution, and gradient across the frame can also be a problem. For this reason, monochrome cameras with filter sets are recommended for such conditions.

The bottom line? If you value convenience and speed, if you're just interested in casual imaging, or if you are just starting out in astrophotography, then a color ZWO camera is the best bet for planetary, lunar/solar, and deep-sky imaging. If you want to get the sharpest possible images and you don't mind investing in additional equipment like filters and a filter wheel, as well as extra complexity in acquiring and processing images, then consider a monochrome camera. And if you have deep pockets and value the ease of 'one-shot' color imaging, a high-end color 'Pro' camera is an excellent choice.

3.8. Noise and Cooling

Noise is a key specification in astronomy cameras, and there are many types of noise that arise in digital cameras. Some, like photon noise and quantization noise, are inherent to the detection process in the semiconductor electronics and the conversion of the signal to a digital format. Other types of noise are a consequence of the design of the sensor or of the operation of the sensor in various environmental conditions.

Read noise, for instance, is generated by electronics on the sensor and in the camera as the electric charge produced by light in the pixels is converted to a signal. Read noise is inherent in the design of the CMOS sensor and the amplifier and associated electronics that create the digital output of the camera. Low read noise is essential to accurately detecting small signals from faint objects or a dark background. Read noise tends to dominate the signal-to-noise ratio of an image for short exposures of less than a second, approximately. It's expressed in the number of unwanted electrons (e-) produced. For example, the ZWO ASI1600MM camera has a read noise of 1.2e- when the camera's electronics is set to 30 dB gain. This is a very low read noise and it's ideal for getting good contrast images of deep-sky objects against a dark sky. Cameras tend to have lower read noise and higher gain levels.

Tables 5a-c list the minimum read noise of the current selection of ZWO cameras.

Then there is thermal noise. This is produced by processes in the semiconductor that produce unwanted electrons that are not caused by a signal. The thermal noise is strongly influenced by sensor temperature. It tends to double, approximately, for every increase in temperature of 6°C or 7°C. The only way to decrease thermal noise is to cool the sensor, usually with a thermoelectric cooler (TEC). All ZWO 'Pro' cameras include regulated TEC cooling. This is a big advantage because you can set the target temperature of the camera and take dark frames that include the thermal noise. If the temperature is regulated, the noise in the dark frame matches the light frame during imaging, and you can more easily calibrate your image. If the temperature is changing, as it might in an uncooled camera, while you take your calibration frames and during your image capture, you cannot calibrate your image very well.

Cooling is only important for exposures longer than about 500ms. That means it is not critical, usually, when imaging the planets, Moon, or Sun. But for longer exposures of deep-sky objects, active thermal control is a big advantage when trying to achieve the best image quality.

The noise sources mentioned above can affect any pixel in the sensor equally. But fixed pattern noise, as its name implies, is a result of some pixels giving a signal of higher intensity above the general background noise. This is caused by a variation in some of the millions of pixels in the sensor. "Hot pixels," pixels that show a signal even in the absence of a real signal, are an example of this type of noise. The advanced CMOS sensors used in ZWO astronomy cameras are designed to keep fixed-pattern noise to a minimum. In most cases, it can be removed through the use of dark frames when processing your images.

3.9. Shutter Speeds, Data Resolution, and Download Rates

Like any digital camera, ZWO astronomy cameras have a range of user-selectable shutter speeds. Most ZWO cameras have shutter speeds ranging from 32 microseconds to 1000 seconds. While the extremes of this range of shutter speeds are enabled by the design of the camera's electronics, they may not be required for most imaging applications. In general, longer exposures of many seconds to minutes are used for faint deep-sky objects while shorter speeds on the order of milliseconds to hundreds of milliseconds are used for the planets, Moon, or the Sun (through a telescope with a safe solar filter).

As ZWO cameras use CMOS sensors, most are equipped with rolling shutters that scan the image sequentially, from one side of the sensor (usually the top) to the other, line by line. This is not a problem with deep-sky cameras since the targets are fixed in appearance. Planets, however, rotate appreciably during exposures and some solar features changes relatively quickly, so rolling shutters may cause a slight distortion of the image, though with careful technique this is not usually a problem. Most ZWO solar-system cameras use rolling shutters. The ASI174-series and ASI432MM cameras use a global shutter which scans the entire area of the image simultaneously.

The download rate of an astronomy camera defines how quickly an image frame can be downloaded from the camera to a computer. Fast download rates are essential when imaging objects like planets that may rotate quickly during image capture. The download rate is governed by the sensor and readout electronics, but for a given camera and sensor, the more data there is to download, the longer it takes. As mentioned earlier, most cameras optimized for planetary use only have sensor sizes of 1 or 2 megapixels which means the data can be downloaded fairly quickly. Larger sensors generate more data and have slower download rates. ZWO's 'Pro' cameras include a large onboard 256MB DDR3 memory buffer. This enables quick and stable data transfer off the camera and has the effect of reducing 'amp glow' that degrades image quality around the edge of the sensor, especially at high gain.

The data resolution also affects transfer rates. Digital cameras translate analog images from a telescope into numbers that can be read by a computer. The larger number of bits in each digital number enable a larger range of tonality and shades in the grayscale which may make for better images. A 12-bit resolution has 2 bits more levels of gray than 10-bit, but the larger number of bits means a longer transfer time and more data to store on your hard drive. ZWO cameras have user-selectable data resolution of 14 bits, 12 bits, or 10 bits (depending on the camera) so you can configure the camera for what's best for your situation.

All ZWO cameras also have a user-selectable image resolution to enable you to trade off faster data transfer against image resolution. For example, the ASI224MC camera has a maximum data transfer rate of 64 frames per second when set for the maximum resolution of 1304x976 at a data resolution of 12 bits. But when the camera is software-configured to render an image resolution of 640x480, for example, the transfer rate doubles to 127 frames per second at the same 12-bit data resolution.

Except for a few discontinued planetary cameras, all ZWO cameras enable a USB 3.0 interface for fast download speeds. But they can be software-configured to work at USB 2.0 speeds with computers with slower communications ports.

Tables 5a-c list the maximum frame rate and bit depth of the current selection of ZWO cameras.

3.10. Amp Glow

Finally in our tour of key astronomy camera specifications, a word about amplifier glow. This term arose when describing an unwanted signal from a CCD camera sensor caused by the amplifier electronics that were usually situated at the edge of the sensor itself. With CMOS astronomy cameras, the amplifiers are associated with each pixel, so 'amp glow' is generally a type of unwanted signal caused by the sensor or camera circuitry. The amp glow grows more noticeable in longer exposures and at higher gain. It is usually not dependent on temperature. Figure 13 shows an example of amp glow from an ASI183-series camera.

Figure 13 – An example of an image with amp glow (image courtesy ZWO).

While annoying, amp glow can usually be removed by carefully capturing and processing with dark frames. However, several of the most recent (after 2019) cameras from ZWO incorporate an anti-amp glow function that combines software or hardware design that reduce the power consumption of the CMOS sensor and its support circuitry. This allows for a substantial reduction or even elimination of amp glow in many applications. You can learn more about amplifier glow in this article at the ZWO website .

Tables 5a-c indicates which ZWO cameras have amp-glow reduction circuitry.

4. Choosing a ZWO Camera for Astrophotography

4.1. Overview

Armed with the knowledge you gained in Section 3, you are now in a position to sort out which ZWO camera is best for a number of astrophotography applications. However, there is no single best astrophotography camera from ZWO for all astrophotography applications. Nor is there a single best camera for planets, lunar/solar, deep-sky, and EAA imaging. Nor are there any 'bad' ZWO cameras – if there were, they would not be on the market. But picking a camera for a particular application depends on a number of factors including the focal length and focal ratio of the telescope, the primary targets of interest, the skill level of the imager, and of course, the budget. In this section we will look at the current crop of cameras and match them to a particular application with a little help from what we learned in Section 3.

4.2. Best Astrophotography Cameras for Beginners

For newcomers to astrophotography, especially for those on a tight budget, a choice from ZWO's line of planetary cameras is a good place to start. The ASI120MC-S or ASI224MC cameras deliver acceptable lunar, solar, and planetary images, and even pretty good images of brighter deep-sky objects subject to the available field of view from your telescope. Both can also be used as autoguiders.

If you have slightly deeper pockets, you can go right into the ASI385MC with the faster download rate and state-of-the-art sensor. It's just as easy to use as the ASI120 cameras and is a good general-purpose small-sensor camera, though it does suffer from a bit of amp glow at higher gain and longer exposures. The relatively affordable ASI662MC camera would also deliver similar results with less amp glow though it has a slightly smaller sensor and smaller pixels which must be matched to the right focal ratio to achieve best results.

For beginners committed to deep-sky observing and who want to jump right away into a cooled 'Pro' ZWO camera, the ASI533MC-Pro is the best place to start and may be the only camera required for years of enjoyment. This 9-megapixel camera does require an additional power supply for the cooler, but its otherwise relatively easy to use.

4.3. Cameras for Solar System Imaging

There's no lack of choice from ZWO for cameras for lunar, solar, and planetary imaging. The problem is sorting out which matches best to a particular application, sometimes a challenging task because there's a significant amount of overlap in specifications and performance for some of these cameras.

The best way to evaluate these cameras is in terms of pixel size since that defines image scale for a given focal length and because image scale is important for solar-system imaging. Read noise, the maximum data transfer rate (or frame rate), and full-well depth are also important. And the sensor size itself governs the field of view of each camera for a given focal length.

Figure 14 – Image of the waning gibbous Moon captured in daytime with a ZWO ASI290MM solar system camera. Image credit – Brian Ventrudo.

The least expensive ZWO solar system cameras are the ASI120MC-S (also available in monochrome) and the ASI224MC. Both have a resolution just over 1MP and feature 3.75μm pixels. The ASI224MC allows much faster download rates than the ASI120MC/MM, which is very important in 'lucky imaging' of solar system objects. The ASI385MC has the same pixel size as the ASI224MC but a slightly larger sensor which may be helpful for lunar and planetary and casual deep-sky imaging.

The ASI662MC is a step-up in terms of resolution compared to the ASI224MC. This 2.1MP camera has smaller 2.9μm pixels and very fast data transfer rates. This camera is a newer version of the ASI462MC and features zero amp glow, larger full well depth, higher IR sensitivity, and lower dark current. These improvements come at the cost of slightly higher read noise and lower data transfer rates.

The ASI290MM has been on the market now for some years, but it's still a superb monochrome camera for planetary imaging and for even lunar and solar imaging with scopes with somewhat faster focal ratios.

The 8.29MP ASI585MC incorporates a sensor with the same 2.9μm pixel size as the ASI662MC and ASI290MM. But the sensor is larger which offers a larger field of view for capturing full-disk solar images or for live all-sky views or casual deep-sky imaging.

For even smaller pixels, which suits planetary imaging at faster focal ratios, the ASI678MC and its early incarnations, the ASI178MM and ASI178MC offer a promising solution. The newer ASI678MC features slightly smaller 2μm pixels, zero amp glow, lower readout noise, and high sensitivity especially in the IR at the expense of a slower data transfer rate.

The ASI183MM and ASI183MC cameras are more of an all-around camera, with a larger 20MP sensor, more than is needed strictly for planetary use, and small 2.4μm pixels. The pixel size suits faster focal ratios for capturing maximum detail, and shorter focal lengths deliver large fields of view for casual deep-sky observing. For more dedicated deep-sky imaging, a 'Pro' version of the 183MC/MM is also available.

The ASI482MC and ASI174MM cameras have larger 5.8μm pixels and are good choices for lunar and solar imaging with longer focal ratios. The ASI432MM camera, with very large 9μm pixels and 1.7MP resolution. It works especially well with long focal ratios. It's a great match for solar imaging with the Daystar Quark H-alpha solar filter (with its internal 4.2x telecentric Barlow) and telescopes with an inherent focal ratio of around f/7 or f/8.

And how about the larger sensor ASI1600MM, ASI294MM, and ASI294MC cameras? These cameras are a great match for full-disk lunar and solar imaging and casual deep-sky imaging and electronically-assisted astronomy. With the region of interest (ROI) feature on your data collection software, these cameras can also work for imaging planets.

And while ZWO intended most of the above cameras for solar-system imaging, that's not to say they cameras won't bring in deep-sky objects, especially the cameras with bigger sensors – the table reflects the utility of these cameras for casual DSO imaging. But for more serious deep-sky imaging, cooled cameras are recommended. Also, many solar-system cameras come with small removable 'all-sky' lenses that used for imaging the entire sky to monitor weather or auroral or meteor activity.

Table 6 summarizes the strengths of each uncooled ZWO planetary/solar-system camera. In this table, each camera is given a somewhat subjective rating out of five stars for its relative performance for planetary, lunar/solar, deep-sky, and all-sky imaging. This five-star rating is not intended to evaluate the camera in absolute terms; the rating suggests the performance of the camera for each application relative to other ZWO cameras for a particular application.

4.4. Deep-Sky Cameras

ZWO's line of cooled 'Pro' cameras are a little more streamlined than their solar-system camera offerings. The ZWO deep-sky cameras can be grouped, roughly, according to sensor size (and therefore price). All are excellent cameras – but the best one, as described in Section 3, depends primarily on the focal length and focal ratio of your scope.

Figure 15 – An image of the Western Veil Nebula (NGC 6992) captured with the square-sensor ZWO ASI533MC-Pro camera, a TeleVue 85 refractor and 0.8x focal reducer, and an L-eNhance filter. Image credit – Brian Ventrudo.

The 9MP ASI533MC-Pro and ASI533MM-Pro cameras have the smallest sensor size of the deep-sky cameras. They're also the only ones to feature a square sensor which suits the shape of many deep-sky objects. These cameras also have zero amp-glow and relatively low read noise.

The ASI183MC-Pro and ASI183MM-Pro are similar to the 533-series in price and sensor class, but they have smaller 2.4μm pixels and a higher 20MP resolution. These cameras match well to shorter focal-length telescopes. They also suffer somewhat from amp glow which can be removed through dark frames. The ASI183MM camera is also available in a GT-version which includes a monochrome camera and integrated filter wheel.

Moving up in sensor size we get to the ASI1600MM-Pro (16MP) and ASI294MC-Pro and ASI294MM-Pro (11.7MP) cameras with 3.75μm and 4.63μm pixels and micro-4/3” class sensors, both good choices for deep-sky work. ZWO also offers the ASI1600GT, a monochrome camera with integrated filter wheel. These cameras offer a wider field of view than the 1”-class cameras mentioned above.

Then there are the cameras with APS-C (24mm x 16mm) sensors, the same found in some commercial DSLR and mirrorless cameras. The ASI071MC-Pro features an APS-C sensor with a 16MP resolution and big 4.8μm pixels. This is the most affordable of the big-sensor ZWO cameras and works well for deep-sky imaging. As an older camera, it doesn't have zero-amp-glow circuitry. The newer and slightly more expensive 26MP ASI2600MM-Pro and ASI2600MC-Pro cameras do include this circuitry and feature slightly smaller 3.8μm pixels.

At the top end of their product line, ZWO also offers cameras with full-frame (36mm x 24mm) sensors. The ASI2400MC-Pro features a 24MP sensor with large 5.94μm pixels, ideal for getting a high signal-to-noise ratio when imaging faint deep-sky objects. They also offer a 62MP camera in both color and monochrome versions, the ASI6200MM-Pro and ASI6200MC-Pro. This camera makes sense for those looking for maximum detail when enlarging and printing images or when cropping images. This is the best camera from ZWO money can buy at this time. But save some money for a new computer or hard drive as well – the ASI6200-series delivers large 120MB files for each captured frame!

(Note: All ZWO 'Pro' cameras require an external 5V power supply – they cannot be powered with the ZWO ASIAIR-Pro/Plus controllers or through a computer USB port).

All ZWO deep-sky cameras can be used for solar system imaging, especially lunar and solar imaging, but also for planetary imaging using the ROI feature on your image capture software. But if solar-system imaging is your primary interest, it's more cost effective to stick with uncooled cameras.

Table 7 summarizes the strengths of each 'Pro' deep-sky camera. In this table, each camera is given a somewhat subjective rating out of five stars for its relative performance for planetary, lunar/solar, deep-sky, EAA, and all-sky imaging. Again, this five-star rating is not intended to evaluate the camera in absolute terms; the rating suggests the performance of the camera for each application relative to other ZWO cameras for a particular application.

4.5. Cameras for EAA

Most astrophotographers are interested in capturing data with their cameras and doing significant post-processing in the hours and day after their observing session to achieve a final image. But some observers simply want the extra boost in sensitivity of an astronomy camera compared to the human eye to see celestial objects right away, in more or less real time, on their computer screens or on a TV monitor in the field. The image need not be optimized and highly processed, but it needs to be pleasing enough to enjoy and to show others, for example, at an outreach event, and it needs to be updated continuously. This sort of astrophotography is often called electronically-assisted astronomy (EAA) because the camera sensor is used to see more detail than the human eye, and the image is delivered by the camera in more or less real time.

(Note: Learn more about choosing cameras and other equipment for EAA with this guide from Agena Astro ).

Figure 16 – A stacked EAA image of the Crescent Nebula (NGC 6888) captured with a ZWO ASI385MC camera. Image credit – Brian Ventrudo.

The most common type of camera for EAA has been astronomy video cameras such as the Revolution Imager or one of the many versions of a Mallincam. These devices use CCD sensors and processing electronics that output an analog video signal to a TV or monitor or, with analog-to-digital conversion electronics, to a computer. But as CMOS sensors improve in sensitivity and drop in price, it has become possible to use CMOS-based astronomy cameras for EAA of solar system and deep-sky objects.

The best CMOS cameras for EAA offer fast data transfer rates (and therefore a smaller number of pixels, say, about >10MP) since EAA usually involves stacking several images to reduce noise. For this same reason, a good EAA camera should also have low read noise (< 1.5e-), especially at high gain. EAA cameras should also have high sensitivity to build up images quickly in nearly real time.

Real-time lunar and solar EAA can be done with many of the small-sensor ZWO cameras listed in this guide. The ASI224-series, ASI290MM, ASI662MC, and ASI385MC are particularly good cameras for EAA because of their rapid data transfer rates and high sensitivity. These cameras have resolutions of 1304x976 (ASI224) and 1936x1096 (ASI290 and ASI385). However, they only have 6mm or 8mm diagonals, so they have limited fields of view for observing extended celestial objects.

To capture larger objects or wider fields of view, the ASI533MC-Pro or ASI294MC or ASI294MC-Pro cameras are better choices. These cameras have larger sensors but still have a fast data transfer rate for real-time imaging. The Pro versions of these cameras with two-stage TEC cooling are ideal for low-noise EAA of fainter deep-sky objects, especially in warm ambient conditions. The ASI533-series cameras also has virtually no amp glow which makes it easy to capture pleasing EAA images without having to apply dark frames. It also has a good resolution of 3008x3008 pixels which makes it excellent for extended nebulae, star clusters, and other larger objects. The larger micro-4/3 sensor of the ASI294-series with 4144x2822 resolution (11.7 megapixels) and 4.63-micron pixel size can capture data at 16fps at full resolution. The larger pixels also collect more light and offer the promise of better signal-to-noise ratio. The ASI183MC and ASI183MC-Pro cameras also work for EAA applications but suffer from considerable amp glow.

Each of the recommended cameras are available in color or monochrome. For EAA, most observers will choose a color version. If you can afford it, the cooled 'Pro' versions of these cameras are recommended to help control thermal noise, especially for those who observe from a dark location. But perfectly acceptable 'snapshot' images are possible with the uncooled versions of these cameras also.

Tables 6 and 7 also include ratings for each ZWO camera for electronically-assisted astronomy (EAA).

5. Summary Table of Recommendations and Key Specifications

Table 8 provides a summary of the key specifications and recommendations related to ZWO planetary, deep-sky, and autoguiding cameras mentioned in this guide.

Table 8 - ZWO Astronomy Cameras - Complete Specifications and Recommendations. Click on the table image above to enlarge.

6. ZWO Accessories

ZWO has a wide range of accessories available for their cameras including adapters, wide-angle lenses and lens adapters, as well as filters and filter wheels. While ZWO astronomy cameras include everything you need to get started in astrophotography with a telescope, several accessories are worthy of consideration depending on your application. Some popular ZWO accessories include:

Figure 17 – A ZWO ASIAIR Plus controller. Image credit: ZWO

ASIAIR Plus Controller: The ZWO ASIAIR Plus is a small controller that provides wireless control of a ZWO camera and accessories as well as a power hub for some cooled ZWO cameras and all filter wheels. Through an installed app on an Android or iOS smartphone or tablet, the ASIAIR-PRO provides WiFi control and connectivity making it unnecessary to use a computer or laptop for astrophotography and EAA. (Note – The discontinued ASIAIR-Pro and the current ASIAIR-Plus controllers cannot power cooled 'Pro' cameras directly. These cameras must be powered with external 5V power supply).

Filter Wheels: Filter wheels sit between the camera and the telescope to help you capture color images of astronomical objects with monochrome cameras. ZWO offers a range of filter wheels for all applications and budgets. At the simplest and most economical end, they offer a manual filter wheel that accepts five 1.25" filters. They have also released very popular 5, 7 and 8 position electronic filter wheels for 1.25", 31mmm, 36mm, and 2" filters. The larger 2" filter wheels are intended for use with the full-frame ASI6200-series cameras.

ZWO EAF: The ZWO electronic automated focuser (ZWO EAF) enables remote focus control through a computer or ASIAIR controller. This small accessory attaches to many telescope focusers and requires 5V from an external supply or the ASIAIR.

Canon and Nikon Lens Adapters: ZWO also manufactures special T2 adapters with camera mounts so you can attach Canon or Nikon camera lenses directly to a ZWO astronomy cameras. These adapters permit you to shoot very wide-angle images of the heavens.

Filters: ZWO has introduced its own line of color and bandpass filters for astronomical imaging. These are offered in standard 1.25" and 2" mounted filter sizes, as well as in 31mm and 36mm unmounted versions for use in their filter wheels.

ZWO's filter line-up includes a standard LRGB filter set for use with monochrome cameras, a premium LRGB filter set for monochrome cameras, IR Cut filters, and 850nm IR pass filters that can be used to enhance color and monochrome images. A very recent addition includes the narrowband SHO (Sulfur-II, Hydrogen-Alpha and Oxygen-III) filters that can be used by advanced imagers to make spectacular deep sky images such as the "Pillars of Creation" in the Eagle Nebula (M16).

Adapters: ZWO now offers an ever-increasing range of simple yet necessary mechanical adapters for a variety of applications. These could be useful for attaching accessories with differing thread formats and getting the correct spacing between the camera and accessories such as focal reducers, barlows, and so forth.

Camera Kits: Camera kits, consisting of a camera, a filter wheel, and select filters are also offered for some popular camera models. Buying the kit generally saves you a little of money as compared to buying the individual items separately.

7. Summary

As this buyer's guide has shown, ZWO has a comprehensive (and growing) line of astronomy cameras to suit imagers with an interest in the planets, Moon and Sun, and the deep sky. With state-of-the art CMOS sensors, these multipurpose cameras are designed to suit a wide range of budgets and are easy to use, so it has been easier to get into astro-imaging and start producing good results. ZWO is constantly innovating so new cameras and accessories are introduced often. As new products and accessories are introduced, this guide will be updated, so check back here regularly, or have a look at Agena's ZWO product page at this link.

8. Appendix – Discontinued ZWO Astronomy Cameras

ZWO continuously adds new cameras and occasionally discontinues some of their camera models. They have in the past also discontinued an entire line of cameras, the TEC-cooled 'Cool' line and replaced them with the more advanced 'Pro' line. Many of the phased-out ZWO cameras appear for sale on the used market from time to time. Table 9 below summarizes cameras discontinued by ZWO over the past several years.

Table 9 - Discontinued ZWO Astronomy Cameras. Click on the table image above to enlarge.