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Agena Buyer's Guide to ZWO Astronomy Cameras

By: Brian Ventrudo and Manish Panjwani
January 4, 2017 (Last revised: February 15, 2017)

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This buyer's guide from Agena AstroProducts 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 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.
ZWO Buyers Guide

1. Overview

As a result of their low noise and high sensitivity, CCD sensors have been the gold standard for digital astronomy cameras. These semiconductor-based sensors are based on a mature technology that is ideal for low-light applications because of the efficiency at which they convert light into electrical signals. But CMOS sensors, based on another semiconductor technology, are catching up to CCD sensors in low-light performance. These sensors are widely used in mass-produced digital cameras, and since they lend themselves to high-volume manufacturing techniques, their price continues to drop. In some applications, including astronomy, the performance of CMOS sensors enables the design of cameras with an excellent price-to-performance ratio.

ZWO Cameras Buyers Guide
Figure 1 - A Sony IMX224 CMOS sensor (Credit: Sony Corp.)

Over the past few years, Chinese manufacturer ZW Optical (ZWO) has introduced a line of CMOS-based cameras for planetary and general purpose 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 integrated ports for autoguiding, high quantum efficiencies that are beginning to rival CCD-based cameras, and a wide range of exposure times to capture nearly any celestial object.

However, as more ZWO cameras are introduced, it has become 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 confusing 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 chunks. You will understand the factors and specifications of ZWO astronomy cameras including sensor size, pixel size, read noise, download rate, cooling, and color vs. monochrome. As you read through this guide, we will help you narrow down your choices and select the best ZWO camera for your interests, equipment, and budget. While this guide is geared towards the first-time astronomy camera buyer, intermediate imagers should also find this content - especially the summary specification table and recommendations near the end of this article - useful in making their choices.

2. Types of ZWO Cameras

At present, ZWO offers 26 camera models. The subsequent sections will dissect all of these models based on various technical parameters, specifications, and suggested applications. However, as you read this guide, it will be helpful to keep in mind that at the broadest level you will need to choose between the following camera types:

  • Monochrome (B&W) or Color Cameras
  • Cooled or Non-cooled Cameras

The rest, as they say, are details. Table 1 below is a quick reference table segmenting all current ZWO cameras into one of the above categories.

Table 1: List of Cameras Currently Offered by ZWO
 Non-CooledCooled
Monochrome ASI120MM  
ASI120MM-S  
ASI174MM ASI174MM-COOL
ASI178MM ASI178MM-COOL
ASI290MM ASI290MM-COOL
ASI1600MM ASI1600MM-COOL
Color ASI034MC  
  ASI071MC-COOL
ASI120MC  
ASI120MC-S  
ASI174MC ASI174MC-COOL
ASI178MC ASI178MC-COOL
ASI185MC ASI185MC-COOL
ASI224MC ASI224MC-COOL
ASI290MC ASI290MC-COOL
ASI1600MC ASI1600MC-COOL

Note that many ZWO cameras are available in more than one version, and four are offered in all versions. Monochrome and color cameras have a suffix of "MM" and "MC" respectively after the camera model number. Cooled cameras have a suffix of "-COOL." The "-S" suffix is used only for the ASI120 cameras to denote the faster USB 3.0 versions compared to the USB 2.0 versions that do not have this suffix.

3. ZWO Camera Specifications to Consider

3.1 Footprint and Mechanics

ZWO astronomy cameras are housed in attractive CNC machined red-anodized aluminum bodies that stand up to heavy field use. The uncooled ZWO camera bodies have a diameter of 62mm (2.4") and a weight of just 100-140 grams (3.5-4.9 oz). Cooled cameras have a slightly larger diameter of 78mm (3.1") and a weight of 410 grams (14.5 oz) or 500 grams (17.8 oz).

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. Each camera includes a 1.25"-T threaded nosepiece adapter so the camera can be inserted directly in standard 1.25" telescope focusers. The front of the camera also has 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.

The opposite side of uncooled ZWO cameras features a 1/4-20 thread in the bottom to make it easy to attach the camera device to a photo camera tripod or mount for use without a telescope. Cooled ZWO cameras can be mounted in an optional adapter ring for mounting to a camera tripod.

ZWO Buyers Guide
Figure 2 - The configuration of the ZWO ASI 174MM and ASI174MC astronomy cameras. Image courtesy of ZWO

Table 2 below summarizes key physical specifications of all ZWO cameras.

Table 2: ZWO Astronomy Cameras - Mechanical Specifications
ZWO Camera ModelColor / MonochromeDiameter (mm / in)Weight (g / oz)1/4"-20 Mounting Thread
ASI034MC Color 62 / 2.4 100 / 3.5 Yes
ASI120MM Monochrome 62 / 2.4 100 / 3.5 Yes
ASI120MC Color 62 / 2.4 100 / 3.5 Yes
ASI120MM-S Monochrome 62 / 2.4 100 / 3.5 Yes
ASI120MC-S Color 62 / 2.4 100 / 3.5 Yes
ASI174MM Monochrome 62 / 2.4 140 / 4.9 Yes
ASI174MC Color 62 / 2.4 140 / 4.9 Yes
ASI174MM-COOL Monochrome 78 / 3.1 410 / 14.5 No
ASI174MC-COOL Color 78 / 3.1 410 / 14.5 No
ASI178MM Monochrome 62 / 2.4 120 / 4.2 Yes
ASI178MC Color 62 / 2.4 120 / 4.2 Yes
ASI178MM-COOL Monochrome 78 / 3.1 410 / 14.5 No
ASI178MC-COOL Color 78 / 3.1 410 / 14.5 No
ASI185MC Color 62 / 2.4 120 / 4.2 Yes
ASI185MC-COOL Color 78 / 3.1 410 / 14.5 No
ASI224MC Color 62 / 2.4 120 / 4.2 Yes
ASI224MC-COOL Color 78 / 3.1 410 / 14.5 No
ASI290MM Monochrome 62 / 2.4 120 / 4.2 Yes
ASI290MC Color 62 / 2.4 120 / 4.2 Yes
ASI290MM-COOL Monochrome 78 / 3.1 410 / 14.5 No
ASI290MC-COOL Color 78 / 3.1 410 / 14.5 No
ASI1600MM Monochrome 62 / 2.4 140 / 4.9 Yes
ASI1600MC Color 62 / 2.4 140 / 4.9 Yes
ASI1600MM-COOL Monochrome 78 / 3.1 410 / 14.5 No
ASI1600MC-COOL Color 78 / 3.1 410 / 14.5 No
ASI071MC-COOL Color 78 / 3.1 500 / 17.8 No

3.2 Port Configurations and Cables

Every ZWO camera comes with a series of ports and cables for power and communication, but the port configuration for the cooled and uncooled cameras are slightly different.

Uncooled camera ports include:

  • A USB port for data communications and powering the camera. The port can be configured for both USB2.0 and USB3.0 communications (except for the ASI120MM/MC and the ASI034MC cameras, which only have USB2.0 capability). A 2 meter USB3.0 cable is included with the camera
  • An autoguiding port with ST4 connector to allow the camera to be used as an autoguider. ZWO includes an autoguiding cable.
ZWO Buyers Guide
Figure 3a - The ZWO ASI1600MM monochrome astronomy camera. In this uncooled unit, the blue USB port is on the lower left and the ST4 autoguiding port is on the lower right. The top of the camera has female M42x0.75 threads for a 1.25" nosepiece adapter. Image courtesy of ZWO.

Cooled ZWO cameras have the following ports:

  • A USB3.0 port for communications and powering the camera. A 2 meter 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. ZWO recommends a 12V 3A DC power supply for the cooler.

(NOTE: Some ZWO cooled cameras include an ST4 autoguider port instead of the USB2.0 hub).

ZWO Buyers Guide
Figure 3b - The ZWO ASI1600MM-COOL 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 a autoguiding camera and electronic filter wheel. Image courtesy of ZWO.

3.3 Sensor Size and Field of View

The physical size of the sensor on an astronomy camera governs the field of view of the camera, that is, how much of the sky you can fit in an image with a given camera and telescope. The field of view does not depend on the size of the pixels or the number of megapixels in the sensor. 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.

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 DL

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).

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 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 3 below gives approximate sizes of some common celestial objects.

Table 3: Apparent Size of Some Common Celestial Objects
Object NameApproximate Apparent Size
Mars 20 arc-seconds (at opposition)
Jupiter 40 arc-seconds (at opposition)
Ring Nebula (M57) 1.4x1.0 arc-minutes
Dumbbell Nebula (M27) 8.0x6.0 arc-minutes
Hercules Cluster (M13) 15 arc-minutes in diameter
Wild Duck Cluster (M11) 14.0x14.0 arc-minutes
Moon/Sun 30 arc-minutes in diameter
Orion Nebula (M42) 80x60 arc-minutes
Andromeda Galaxy (M31) 190x60 arc-minutes

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 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 to 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.

Example 1:
The diagonal size of the sensor in the ZWO ASI224MC camera is 6.09mm. If you have a 127mm f/7 refractor and use it with a 2x Barlow lens, the effective focal length of the system is 127x7x2=1778mm. So this ASI224MC camera and the telescope and Barlow will result in an image with a diagonal size of 3436x6.09/1778=11.8 arc-minutes.

This field is about 1/3 the diameter of the full Moon or the disk of the Sun and about 17x the apparent diameter of Jupiter at opposition. So the entire Moon or Sun will not fit into the frame of this system, while Jupiter would be reasonably well framed, and could even benefit from an optical system with more focal length to give a larger image. The image size can be adjusted with this telescope to some extent by using different Barlow lenses or focal reducers.

Example 2:
The ASI178MC camera has a sensor with a diagonal size of 8.92mm. When used with a small 80mm f/6 refractor, which has a focal length of 480mm, the camera has a field of view of 3436x8.92/480=63.9 arc-minutes, or a little more than 1 degree.

A one degree field of view easily fits the full-disk image of the Moon and Sun, and it nicely frames moderately-large deep-sky objects like the globular cluster Messier 13. With the same telescope, the ASI1600 camera, which has a sensor with a large 21.9mm diagonal, the field of view along the diagonal is 157 arc-minutes or about 2.6 degrees. That is large enough to frame even larger celestial objects like the Orion Nebula.

3.4 Pixel Size and Resolution

In consumer cameras for everyday use, 'resolution' usually refers to the total number of pixels in a sensor. So a 10 megapixel camera has 10 million pixels on its sensor, for example. But this tells us nothing about the size of the sensor or the size of the individual pixels. 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 imply a lower resolution across an image of a planet or other object projected on your sensor by your telescope.

ZWO Buyers Guide
Figure 4 - A schematic configuration of a camera sensor showing the dimensions of the sensor and the pixels. Image credit: VS Technology

The effect of pixel size for a particular telescope is governed by the image scale, which is 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 sL

where s is the pixel size in microns and L is the focal length in millimeters.

For imaging planets and small deep-sky objects, where the overall image size is small to begin with, you want to have small pixels to get high resolution across the planet's disk. Experienced planetary imagers recommend a telescope-camera combination that gives an image scale of close to 0.25 arc-seconds/pixel in good sky conditions where the air is steady. In the steadiest air when seeing is excellent, an image scale of 0.1 arc-seconds/pixel can work well.

For full-disk lunar and solar imaging, and for basic deep-sky imaging, since the image is larger and since larger sensors are used compared to planetary imaging, the pixels can be larger without sacrificing resolution. When imaging larger objects, larger pixel sizes on larger sensors give better signal-to-noise ratio and larger fields of view. An image scale of 1 to 2 arc-seconds per pixel works well for lunar, solar, and basic deep-sky imaging in good sky conditions.

The ZWO ASI120, ASI224, and ASI290-series cameras have both small sensors and small pixel size, so they work well for imaging planets and small deep-sky objects. The ZWO ASI174, ASI178, and ASI185-series cameras have larger sensors and larger pixels, and work well for imaging the Moon and Sun and slightly larger deep-sky objects, depending on the focal length of your telescope, and they work reasonably well with planets.

Example 3:
The ASI224MC camera has a sensor with 6.09mm diagonal and 3.75 micron pixels. Let's calculate the image scale with an 8" (200mm) aperture f/10 Schmidt-Cassegrain telescope with a 2x Barlow lens.

The telescope and Barlow have an effective focal length of 200x10x2 = 4000mm. The image scale of this system is 200x3.75/4000=0.19 arc-seconds per pixel. When imaging Jupiter at opposition, for example, when its apparent size might be 40 arc-seconds, the image of the planet would span 210 pixels on the camera's sensor, which has a dimension of 1304x976 pixels.

Table 4 below summarizes key sensor specifications for all ZWO cameras.

Table 4: ZWO Astronomy Cameras - Sensor Specifications
ZWO Camera ModelColor / MonochromeSensorSensor
Diagonal
Size (mm)
Sensor
Dimensions (mm)
Sensor
Resolution (MB)
Sensor
Dimensions
(pixels)
Pixel Size
(microns)
ASI034MC Color ONS ASX340CS 5.0 4.1x2.9 0.34 728x512 5.60
ASI120MM Monochrome ONS MT9M034 6.09 4.8x3.6 1.2 1280x960 3.75
ASI120MC Color ONS AR0130CS 6.09 4.8x3.6 1.2 1280x960 3.75
ASI120MM-S Monochrome ONS MT9M034 6.09 4.8x3.6 1.2 1280x960 3.75
ASI120MC-S Color ONS AR0130CS 6.09 4.8x3.6 1.2 1280x960 3.75
ASI174MM Monochrome Sony IMX174 13.4 11.3x7.1 2.3 1936x1216 5.86
ASI174MC Color Sony IMX174 13.4 11.3x7.1 2.3 1936x1216 5.86
ASI174MM-COOL Monochrome Sony IMX174 13.4 11.3x7.1 2.3 1936x1216 5.86
ASI174MC-COOL Color Sony IMX174 13.4 11.3x7.1 2.3 1936x1216 5.86
ASI178MM Monochrome Sony IMX178 8.92 7.4 x 5.0 6.4 3096x2080 2.40
ASI178MC Color Sony IMX178 8.92 7.4 x 5.0 6.4 3096x2080 2.40
ASI178MM-COOL Monochrome Sony IMX178 8.92 7.4 x 5.0 6.4 3096x2080 2.40
ASI178MC-COOL Color Sony IMX178 8.92 7.4 x 5.0 6.4 3096x2080 2.40
ASI185MC Color Sony IMX185 8.58 7.3x4.6 2.3 1944x1224 3.75
ASI185MC-COOL Color Sony IMX185 8.58 7.3x4.6 2.3 1944x1224 3.75
ASI224MC Color Sony IMX224 6.09 4.8x3.6 1.2 1304x976 3.75
ASI224MC-COOL Color Sony IMX224 6.09 4.8x3.6 1.2 1304x976 3.75
ASI290MM Monochrome Sony IMX290 6.46 5.6x3.2 2.1 1936x1096 2.90
ASI290MC Color Sony IMX290 6.46 5.6x3.2 2.1 1936x1096 2.90
ASI290MM-COOL Monochrome Sony IMX290 6.46 5.6x3.2 2.1 1936x1096 2.90
ASI290MC-COOL Color Sony IMX290 6.46 5.6x3.2 2.1 1936x1096 2.90
ASI1600MM Monochrome 4/3 CMOS 21.9 17.7x13.3 16.0 4656x3520 3.80
ASI1600MC Color 4/3 CMOS 21.9 17.7x13.3 16.0 4656x3520 3.80
ASI1600MM-COOL Monochrome 4/3 CMOS 21.9 17.7x13.3 16.0 4656x3520 3.80
ASI1600MC-COOL Color 4/3 CMOS 21.9 17.7x13.3 16.0 4656x3520 3.80
ASI071MC-COOL Color Sony IMX071 28.4 23.6 x 15.7 16.2 4928x3264 4.80

3.5 Pixel Number and Binning

Sensor size and pixel size, as explained above, are important parameters when choosing an astronomy camera. The number of pixels in an astronomy camera is, of course, a direct consequence of these two specifications. But does the number of pixels matter? Large pixel counts make it easier to obtain pleasing large images in print or on a computer screen without the obvious effect of pixelation. And large pixel counts also make it easier to crop images while retaining reasonably high resolution. So when imaging extended objects like galaxies and nebulae, a larger pixel count is often better. The ZWO ASI1600 and ASI071MC cameras, for example, which are optimized for deep-sky imaging, have sensors with 16 megapixels.

There is one disadvantage to large pixel counts, however. It takes the camera longer to download data from all those pixels, so a larger number of pixels tends to make overall download times longer. This can be a disadvantage when imaging planets, especially Jupiter and Saturn, because they have rapid rotation rates. That means an ideal camera for planetary imaging needs to have fast download speeds to achieve good image sharpness and counteract the effect of planetary rotation. Many cameras optimized for planetary imaging, such as the ASI174, ASI224, and ASI120 cameras do not have sensors with more than 1 or 2 megapixels.

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 3.6 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.

3.6 Color vs Monochrome

Most ZWO cameras come in a color version as well as a monochrome version. Which should you choose?

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.

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.

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. For deep-sky objects, especially nebulae, multiple images are collected through color and narrowband filters such as H-alpha or OIII ("oh-3") 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.

The disadvantage of imaging with monochrome cameras? It takes much 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.

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. But 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.

ZWO Buyers Guide
Figure 5 - 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.

3.7 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.

ZWO Buyer's Guide
Figure 6 - 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.

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 amount of noise goes up with temperature, so this noise can be reduced, or at least held to a tolerable level, by controlling the temperature of the sensor with a thermoelectric cooler (TEC).

Cooled ZWO 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 500 ms. 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.

ZWO Buyer's Guide
Figure 7 - M42 imaged with a ZWO ASI224MC-Cool with and a 60mm f/4.3 refractor. Image courtesy ZWO and Matej Mihelcic.

3.8 Other Specifications to Consider - 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 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, 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. Only the ASI174-series 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 above, most cameras optimized for planetary use only have sensor sizes of 1 or 2 megapixels, so the data can be downloaded fairly quickly. Larger sensors generate more data and have slower download rates.

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 12 bits or 10 bits 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 the ASI034 and ASI120MM/MC 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.

Table 5 below summarizes some additional specifications discussed above.

Table 5: ZWO Astronomy Cameras - Additional Specifications
ZWO Camera ModelColor / MonochromeADC Data
Resolution (bits)
Max Download
Rate@Max Resolution (fps)
USB Data InterfaceTEC Cooling
ASI034MC Color 8 95 2.0 No
ASI120MM Monochrome 12/10 35 2.0 No
ASI120MC Color 12/10 35 2.0 No
ASI120MM-S Monochrome 12/10 60 3.0 No
ASI120MC-S Color 12/10 60 3.0 No
ASI174MM Monochrome 12/10 164 3.0 No
ASI174MC Color 12/10 164 3.0 No
ASI174MM-COOL Monochrome 12/10 164 3.0 Yes
ASI174MC-COOL Color 12/10 164 3.0 Yes
ASI178MM Monochrome 14/10 60 3.0 No
ASI178MC Color 14/10 60 3.0 No
ASI178MM-COOL Monochrome 14/10 60 3.0 Yes
ASI178MC-COOL Color 14/10 60 3.0 Yes
ASI185MC Color 12/10 108 3.0 No
ASI185MC-COOL Color 12/10 108 3.0 Yes
ASI224MC Color 12/10 150 3.0 No
ASI224MC-COOL Color 12/10 150 3.0 Yes
ASI290MM Monochrome 12/10 170 3.0 No
ASI290MC Color 12/10 170 3.0 No
ASI290MM-COOL Monochrome 12/10 170 3.0 Yes
ASI290MC-COOL Color 12/10 170 3.0 Yes
ASI1600MM Monochrome 12/10 23 3.0 No
ASI1600MC Color 12/10 23 3.0 No
ASI1600MM-COOL Monochrome 12/10 23 3.0 Yes
ASI1600MC-COOL Color 12/10 23 3.0 Yes
ASI071MC-COOL Color 14 10 3.0 Yes

4. General Recommendations

So which ZWO camera is right for you? The general features and strengths of each camera are summarized in Table 6 below. 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.

Table 6: ZWO Astronomy Cameras - General Recommendations
ZWO Camera ModelColor / MonochromeAll-Sky Lens IncludedBest ApplicationsPlanetarySolar/LunarDeep-SkyAll-Sky
ASI034MC Color No Planetary imaging ★★★ ★★
ASI120MM Monochrome Yes Planetary imaging ★★★★★ ★★★★ ★★★ ★★★
ASI120MC Color Yes Planetary imaging ★★★★★ ★★★★ ★★★ ★★★
ASI120MM-S Monochrome Yes Planetary imaging ★★★★★ ★★★★ ★★★ ★★★
ASI120MC-S Color Yes Planetary imaging ★★★★★ ★★★★ ★★★ ★★★
ASI174MM Monochrome No Solar and lunar imaging ★★★★ ★★★★★ ★★★ ★★★
ASI174MC Color No Solar and lunar imaging ★★★★ ★★★★★ ★★★ ★★★
ASI174MM-COOL Monochrome No Solar and lunar imaging ★★★★ ★★★★★ ★★★★ ★★★
ASI174MC-COOL Color No Solar and lunar imaging ★★★★ ★★★★★ ★★★★ ★★★
ASI178MM Monochrome Yes Planetary imaging;solar/lunar ★★★★★ ★★★★ ★★★ ★★★★
ASI178MC Color Yes Planetary imaging;solar/lunar ★★★★★ ★★★★ ★★★ ★★★★
ASI178MM-COOL Monochrome No Planetary imaging;solar/lunar ★★★★★ ★★★★ ★★★★ ★★★★★
ASI178MC-COOL Color No Planetary imaging;solar/lunar ★★★★★ ★★★★ ★★★★★ ★★★★★
ASI185MC Color Yes Planetary imaging;solar/lunar ★★★★★ ★★★★★ ★★★★ ★★★★
ASI185MC-COOL Color No Planetary imaging;solar/lunar ★★★★★ ★★★★★ ★★★★★ ★★★★★
ASI224MC Color Yes Planetary imaging ★★★★★ ★★★★ ★★★★ ★★★★
ASI224MC-COOL Color No Planetary imaging ★★★★★ ★★★★ ★★★★★ ★★★★★
ASI290MM Monochrome Yes Planetary imaging ★★★★★ ★★★★ ★★★★ ★★★★
ASI290MC Color Yes Planetary imaging ★★★★★ ★★★★ ★★★★ ★★★★
ASI290MM-COOL Monochrome No Planetary imaging ★★★★★ ★★★★ ★★★★★ ★★★★★
ASI290MC-COOL Color No Planetary imaging ★★★★★ ★★★★ ★★★★★ ★★★★★
ASI1600MM Monochrome No Deep sky ★★★ ★★★★ ★★★★ ★★★★★
ASI1600MC Color No Deep sky ★★★ ★★★★ ★★★★ ★★★★★
ASI1600MM-COOL Monochrome No Deep sky ★★★ ★★★★ ★★★★★ ★★★★★
ASI1600MC-COOL Color No Deep sky ★★★ ★★★★ ★★★★★ ★★★★★
ASI071MC-COOL Color No Deep sky ★★★ ★★★★ ★★★★★ ★★★★★

If you are just starting out in astrophotography, and if you are on a budget, then the ASI120 series of cameras is a great place to start. These cameras can do planetary, lunar/solar, and some deep-sky imaging at a good entry-level price. The ASI120-S series have a faster USB 3.0 interface than the standard ASI120s.

If you have slightly deeper pockets, you can go right into the ASI290MC 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 camera.

At present, the ASI224MC is an excellent color planetary imaging camera. It has a similar sensor size to the ASI120MC-S, but the sensor allows much faster download rates, which is very important in capturing planetary images. The ASI290MC is also very close in performance for planets and it has faster frame rate and higher resolution. The ASI290MM is the best mono planetary imaging camera.

For lunar and solar imaging, as well as planetary imaging at longer focal lengths, you can consider higher resolution cameras like the ASI178-series, ASI174-series, and ASI185-series cameras. These cameras, especially in their cooled versions, also work well for small to medium sized deep-sky objects.

The best ZWO cameras for deep-sky imaging are the ASI1600 (color or monochrome), especially the cooled versions which reduce the background noise of the image, and the color ASI071MC-COOL. The cooled ASI1600 and the ASI071MC-COOL cameras are also excellent for deep-sky imaging when outfitted with an external camera lens using optional adapters.

While the cooled ASI1600-series and the ASI071MC-COOL are excellent cameras for deep-sky imaging, they differ slightly. Unlike the ASI1660-series cameras, the ASI071MC-COOL is only available as a color camera; it has no monochrome equivalent. It has a larger sensor than the ASI1600-series at the expense of a slightly slower download time and slightly higher noise. The ASI071 has 14-bit ADC for better image contrast over the 12-bit ASI1600-series, and it has a heated glass window to discourage dew formation.

5. Consolidated Specification and Recommendation Table

For your convenience, all of the ZWO camera specifications and recommendations listed above have been compiled into one master table (Table 7) below. Click on the table image below to see a larger pdf version. We hope to soon replace the pdf file with an interactive data table that you can sort and filter to focus on the cameras you are most interested in.

Table 7: ZWO Astronomy Cameras - Complete Specifications and Recommendations
ZWO Buyer's Guide
Click on the table image above to enlarge.

6. Accessories for ZWO Cameras

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.

All-Sky Lenses: Many ZWO cameras come with an all-sky lens to let you capture wide angle images of the night sky without a telescope. These small lenses give a 150° view of the sky so you can capture aurorae, meteors, and the wide band of the Milky Way. You can also remotely monitor sky conditions from an observing location.

[Coming soon] Canon Lens Adapters: ZWO also manufactures EOS-T2 adapters so you can attach Canon camera lenses directly to a ZWO astronomy camera. There's also a Nikon AI-EOS adapter to allow you to attach Nikon lenses to the EOS-T2 adapter, which then attaches to the camera. These adapters permit you to shoot very wide-angle images of the heavens.

Filters: ZWO has recently introduced its own line of color and bandpass filters for astronomical imaging. These are offered in standard 1.25" and 2" 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 850 nm 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).

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 a 5-position electronic filter wheel.

ZWO Buyer's Guide
Figure 9 - The ZWO manual filter wheel holds up to four color filters. Image courtesy of ZWO

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 saves you a little bit of money as compared to buying the individual items.

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 astroimaging 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.

Brian Ventrudo
About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites OneMinuteAstronomer.com and CosmicPursuits.com.

Manish Panjwani
About the Author

Manish Panjwani has been an active amateur astronomer since before Halley's Comet last flew by our neighborhood. A former wireless communications consulting engineer and management consultant to various Fortune 500 companies, Manish started Agena AstroProducts in 2003. Since then, Agena has become one of the leading online retailers of telescopes and astronomical accessories worldwide. Besides observing from his heavily light polluted backyard in Los Angeles, Manish enjoys conducting astronomy outreach programs in local schools. Manish also holds a Master's degree in Electrical Engineering from Virginia Tech and an MBA from the Kellogg School of Management at Northwestern University.