A Beginner's Guide to Choosing Equipment for Deep-Sky Electronically-Assisted Astronomy (EAA)
- 1. An Overview of Deep-Sky Electronically-Assisted Astronomy
- 2. What You Need for Camera-Based EAA
- 3. CMOS Cameras for EAA
- 3.1 How Astronomy Cameras Work
- 3.2 Key Specifications for an Astronomy Camera
- 3.3 Recommended Cameras for EAA
- 4. Telescopes and Focal Reducers for EAA
- 5. Telescopes and Mounts for EAA
- 6. Matching a Telescope to a Camera
- 7. Other Tools for EAA
- 8. Final Words
- 9. References
It's something every amateur astronomer thinks at the eyepiece of their telescope: "I wish I could see just a little bit more." More detail, more light, and more deep-sky objects instead of the same old showpiece sights on the same old observing list year after year. This line of thinking, which is quite understandable, inevitably leads to "aperture fever", the desire for bigger (and more expensive and cumbersome) telescopes. In the past several years, however, many stargazers looking to see more have turned to an alternative to visual observing with ever-larger telescopes: electronically-assisted astronomy (EAA).
EAA involves the use of electronic aids for the enhancement of astronomical viewing while at the telescope in nearly real time. The electronics can include night vision (NV) technology that amplifies the photons reaching the eyepiece of a telescope to make faint objects more easily visible to the eye in real time. It can also include CCD or CMOS cameras that replace the eyepiece and send an image to a video screen or computer. Some amateur astronomers suggest the term observational astrophotography or near-real-time viewing when viewing with cameras. Snapshot astrophotography is another apt description since the observer can use a camera to take a relatively quick and simple image of a celestial object while not spending a lot of time, either at the telescope or at the computer afterwards, creating stunning works of photographic art as do many serious astrophotographers.
Call it what you will, electronically-assisted astronomy is becoming a popular pastime for several reasons.
For one, EAA allows amateur astronomers to observe objects in less than ideal conditions, especially under badly light polluted skies, or even when a bright Moon is present. Deep-sky celestial objects which are unimpressive or all but invisible in an eyepiece from urban or suburban skies easily become visible, with considerable detail, and in full color if using a color camera. EAA is also ideal for older observers whose visual acuity is not what it used to be, and for public outreach to large groups of people who may not have the skill or patience to look through an eyepiece.
The night vision devices and CCD and CMOS cameras that enable EAA are also becoming more sensitive, affordable, and easy to use, and software is now available that uses image processing techniques to quickly and relatively easily enable nearly live viewing.
And there is the aperture-boosting power of EAA. By some informal estimates, using a relatively inexpensive CCD or CMOS camera has the same effect as tripling telescope aperture for a visual observer without the drawbacks of investing in a bulky large telescope. 'Seeing' 16th-magnitude quasars that are billions of light years away, or galaxy clusters hundreds of millions of light years away can be an easy observation, even with a three or four-inch telescope, and even from urban locations. Your list of interesting deep-sky objects to observe explodes from a few hundred to many thousands.
This guide covers the basics of getting started in EAA using CMOS cameras in place of an eyepiece in a telescope. It explains the equipment required to get started in EAA, and shows examples of what can be achieved with various telescopes, cameras, and accessories. The information in this guide will help you select the best equipment for your interests and budget. It assumes that you understand the basics of how to use a telescope on a motor-driven equatorial mount or altazimuth mount, and that you have a basic knowledge of how to use a computer and install software.
EAA involves a few more tools and accessories than visual observing, but it's usually less demanding than serious astrophotography. If you're new to EAA and need to get oriented, this section gives you a quick overview of the bits and pieces needed to make EAA-type observations with astronomy cameras, and how all the pieces fit together. Later sections in this guide will provide more detail about how to understand and choose these components to match your own interests and budget.
For a basic EAA set up, you will need the following pieces of equipment:
Telescope. As with visual observing, a good quality astronomy telescope is required for basic EAA of deep-sky objects. A Schmidt-Cassegrain telescope with an aperture of 6" to 8" is the most versatile and affordable choice. A small refractor with an aperture of 80mm to 100mm and a focal ratio of f/6 to f/7 also works well for mid-to-large-sized celestial objects where more field of view is required. Newtonians with a focal ratio less than f/6 can also work for EAA, although these instruments can be heavier and require a beefier mount.
Focal Reducer. As with all forms of photography, faster focal ratios (or smaller f-numbers) result in brighter images and therefore faster exposure times. Astronomy telescopes with a focal ratio of f/7 to f/10 or larger tend to be too 'slow' for capturing images of extended deep-sky objects. That's why it's useful to invest in a focal reducer, a small optical component that's placed just in front of the camera sensor to reduce the effective focal ratio of a telescope to f/3.5 to f/6 or so, fast enough for nearly real-time viewing.
Motorized Mount. Because you must expose the camera sensor to light over a period of several seconds, a motorized tracking mount for EAA is essentially a must-have to get sharp images. In most cases, either a motorized alt-azimuth mount or equatorial (EQ) mount will do the job. A computerized Go-To feature is also very nice to have since you must rely on your camera, not an eyepiece, for centering faint objects in the field of view, and the camera may have a relatively small field of view compared to a wide-field eyepiece.
Camera.The 'electronics' in electronically-assisted astronomy is a camera which takes the place of the eyepiece at the focus of the telescope. Many (but not all) modern astronomy cameras - especially those that feature a relatively small CMOS sensors, low noise, and high sensitivity -will do the job.
Computer and Software. Most CMOS and CCD astronomy cameras deliver a digital signal, so a computer is needed to acquire and process this signal and convert it to a viewable image. Any relatively new laptop computer will work fine at the telescope. The most commonly used software for EAA, a package called SharpCap, only runs in a Windows environment, so a PC or a Mac computer running Windows is required. Sharpcap also has useful features such as polar alignment to make observing a little easier. As an alternative to SharpCap and a PC, the ZWO ASIAIR PRO WiFi camera controller can capture images using ZWO cameras and enable basic image manipulation. It can be controlled with an Android or iOS smartphone or tablet.
Focusing Aid. As with visual observing, good focus is critical with EAA. It's a little trickier to focus your telescope while looking at an image on a computer screen. An inexpensive Bahtinov mask, which fits over the front of your telescope, makes focusing much easier.
Filters. While not critical, filters in front of the camera can help improve the contrast and visibility of the image. Some cameras benefit from an IR blocking filter to reduce the amount of infrared light getting to the camera sensor, resulting in a sharper image. For urban and suburban observers, a filter that reduces the effect of light pollution can help improve image contrast.
Deep-Sky Guide. With a good astronomy camera, tens of thousands of objects are accessible with a 3" to 4" or larger telescope. If you are a primarily visual observer, it might help to invest in a good deep-sky atlas or planetarium or planning software to help you find and access fainter objects in the deep sky that you may not be familiar with.
Aside from these basics, many experienced EAA observers and astrophotographers acquire more advanced tools such as electronic focusers, image processing software, and portable 'computers on a stick' to relay observations from the camera to another computer indoors through a wireless network. These advanced tools are worth considering once you learn the basics of EAA, but they will not be covered in this guide.
Astronomy cameras, like all modern digital cameras, are built with small sensors built on semiconductor chips that consist of a two-dimensional array of individual pixels. As light falls onto each pixel, the photons interact with the semiconductor to produce an analog electrical signal which is collected and converted into a digital signal, which in turn can be processed by a computer and displayed on a screen.
There are two main technologies for sensors in digital cameras, CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor). Each has its own advantages and disadvantages. In a CCD, the charge from each pixel is transported across the chip and read at one corner of the array. An analog-to-digital converter turns the electrical signal in each pixel to a digital value. Since the electronics are away from the pixel array itself, nearly all the light that falls onto the chip hits a pixel and is converted to an electrical signal. That's why CCDs are inherently very sensitive and tend to have a high quantum efficiency (QE). That's the ratio of the number of electrons produced per photon that falls on a pixel. A CCD with QE of up to 95% is not uncommon. However, one of the disadvantages of CCDs is that it takes a comparatively long time to read all the pixels on a CCD chip and deliver it to the computer. So to get a fast CCD, there must be fewer pixels and therefore lower image resolution.
In a CMOS chip, by contrast, each pixel has its own processing electronics right on the array, so readout time is very fast. However, the readout electronics takes up valuable real estate on the light-sensitive part of the chip which causes the quantum efficiency to suffer. Early CMOS chips also had uneven readout response and suffered from on-chip thermal noise, so most astronomy cameras used CCD technology until recently. But advances in CMOS design and production have solved some of these problems, and quantum efficiencies have continued to improve, even with relatively large high-resolution chips.
CMOS can also be manufactured in large quantities with lower-cost semiconductor processing techniques, so they tend to be less expensive than CCDs. That's one reason why most digital cameras, from smartphones to high-end DSLRs, use CMOS sensors, and why these sensors are also now used more widely in many astronomy cameras. A few years ago, a good CCD astronomy camera sold for thousands of dollars. Now, a CMOS astronomy camera suitable for EAA sells for as little as a few hundred dollars. While CCDs remain in use for some applications, most amateur astronomers are now turning to CMOS-based imaging for serious astrophotography and EAA.
The most affordable CMOS astronomy cameras for EAA and lunar and planetary imaging have sensors on the small side. The Sony IMX224 sensor used in the ZWO ASI224MC camera, for example, has dimensions of 4.9mm X 3.7mm and an aspect ratio of 4:3. The IMX385 sensor in the ASI385MC camera is 7.3mm X 4.1mm with an aspect ratio of 16:9. Higher-end astronomy cameras use the larger APS-C sensors (22mm X 15mm) or full-frame sensors (36mm X 24mm) used in DSLR and mirrorless cameras. These larger sensors are ideal for high-resolution and wide-field astrophotography because of their size and large pixel count, but they tend to be slower to read and far more expensive.Figure 5 shows the sensor of the ASI294MC camera which has a sensor size of 19.1mm x 13.0mm and an aspect ratio of 3:2.
As described in more detail in a later section, the sensor size is directly related to the amount of sky that can be imaged with a telescope of a given focal length. Bigger sensors take in more sky and work better for imaging larger celestial objects such as the Orion or Lagoon Nebula. But such large fields of view are not necessarily a good thing when imaging small objects like galaxies or planetary nebulae, especially with smaller telescopes with shorter focal lengths. The largest APS-C (24mm x 16mm) and full-frame(36mm x 24mm) sensors in astronomy cameras with resolutions in excess of 20 megapixels are generally overkill for EAA and can actually hinder performance because it takes a relatively long time to read each high-resolution image. It also takes a lot of computer memory and hard-drive space to process and store these images.
The aspect ratio of the sensor is not a critical specification for EAA, but it affects how an image appears on a computer screen. Most laptop screens have an aspect ratio of 16:9 or 16:10. So a sensor with an aspect ratio of 4:3 will not fill the left and right sides of the computer screen when displaying an image. Conversely, a sensor with a 16:9 or 16:10 aspect ratio that matches the computer screen can nicely fill a computer screen with an image, but much of the edges of the field of view may be empty when imaging round objects like small nebulae or globular clusters.Figure 6 shows the effect of sensor size and aspect ratio on the image of the globular cluster M3 taken with the ASI224MC camera and ASI385MC camera with a telescope of 480mm effective focal length.
Each sensor is made of a two-dimensional array of tiny pixels. The resolution of the sensor is simply the number of pixels in each dimension. The Sony IMX294 sensor in the ZWO ASI294MC camera, for example, has an array of 4144 X 2822 pixels (for a total resolution of 11.7 megapixels). The smaller IMX224 sensor in the ZWO ASI224MC camera has a much lower resolution with an array of 1304 X 976 pixels (for a total of 1.2 megapixels).
More resolution is not necessarily better for EAA where the goal is to get an acceptably pleasing image to view celestial objects in nearly real time, usually on a computer screen, and perhaps to save the image to view later. An HD computer screen has about 2 megapixels, while 4K screens have about 8 megapixels, and in most cases the image displayed in real time by a camera will not take up the full screen. To get an acceptable image, often a resolution of just 1-2 megapixels is plenty, especially for beginners or those on a budget. All the images in this article, for example, were taken with cameras with one- or two-megapixel resolution.
The size of the millions of pixels on a sensor is related to resolution and sensor size. The pixels ('picture elements') are square and all the same size, and they are immediately adjacent to each other with no gap in between. That means the size of each pixel and the distance between adjacent pixels (called the pitch) are the same. For CMOS astronomy cameras, the pixel size/pitch is usually between about 2 microns and 6 microns, approximately, depending on the sensor. The pixels on the sensor of the ZWO ASI224MC sensor, for example, are 3.75 microns across while the ASI290MM/MC has 2.9-micron pixels (see Figure 7), as does the QHY5III-290M autoguiding camera, a very sensitive camera for EAA (Figure 8).
Pixel size and sensor resolution are also related to the sensor size. Take the pixel size and multiply it by the resolution of each side of the sensor, and you get the dimensions of the sensor. This also means that for a given pixel size, a sensor with the higher resolution is larger than a sensor with smaller resolution. For example, the ZWO ASI224MC and ASI385MC cameras both have sensors with 3.75 micron pixels. The former has a resolution of 1304 X 976 pixels and a sensor size of 4.9mm X 3.7mm, while the latter has a size of 7.3 X 4.1mm and a resolution of 1936 X 1096. When used with a telescope of a given focal length, the ASI385MC will present a wider field of view (see Figure 6).
Does pixel size matter for EAA? Larger pixels collect more light relative to the amount of noise generated which usually gives a cleaner-looking image with less noise. Larger pixels are also more tolerant to slight mount tracking errors, although they may sometimes render 'block-shaped' stars. Smaller pixels may generate more noise relative to the signal and may be less tolerant to tracking errors, but they may have the advantage of a 'smoother' looking image with rounder stars. A camera with pixels between 3 microns and 4.5 microns is a good bet for most EAA applications.
In a perfect digital camera, each photon that strikes a pixel would create one electron. But in practice, not all photons falling onto a pixel do so. Some photons are lost and scattered by the wiring and circuitry within the pixel. Others may be lost or reflected away at the surface of the pixel. The ratio of electrons generated in the pixel to the number of photons incident on the pixel is called the quantum efficiency (QE). Bigger QE is usually better. For CMOS cameras, the QE is generally between 50% and 90% depending on the pixel and sensor design, which means 50% to 90% of the photons contribute to an electrical signal in each pixel.
The QE of a sensor peaks at a particular wavelength which for digital cameras and astronomy cameras is usually near the center of the band of visible light around 500-600nm. The QE drops off at shorter (bluer) wavelengths around 300-350nm because such light does not penetrate as far into the semiconductor. Unlike the human eye, most CMOS sensors are also sensitive to infrared light out to a wavelength of about 1,100nm. Light of longer wavelengths moves through the semiconductor without being absorbed.
All CCD and CMOS sensors render inherently monochrome images. That's because each pixel collects and detects all light between about 350nm and 1100nm. This light generates an electrical signal which is converted to a digital signal.
To produce a color image, experienced astrophotographers often use cameras with monochrome sensors to take images through three or more color filters (red, green, and blue at a minimum), then combine each filtered image into a single color image with the help of image processing software. This is not a practical approach for EAA.
Instead, most snapshot imagers use 'one shot' color (OSC) cameras to produce color images. These cameras, like nearly all color digital cameras, use a so-called Bayer filter mounted on the face of the monochrome sensor. The Bayer filter is made of millions of individual color filters that cover each pixel. Green filters cover half the pixels across the sensor and red and blue filters each cover a quarter of the pixels across the sensor. A clever interpolation algorithm (the 'debayering' process) fills in the details of how color is distributed across the image and renders a single digital color image for each exposure. No external color filters are needed.
Single-shot cameras have one big drawback: they are less sensitive. When vendors specify a camera with a sensor with a quantum efficiency of up to 90%, for example, they mean for the monochrome sensor. The Bayer filter reduces the light hitting each pixel by 30% to as much as 60% compared to a monochrome sensor. That means longer exposures are needed to reclaim this light, but longer exposures with OSC cameras are more sensitive to background skyglow, light pollution, and tracking errors, and may produce more noise on the image. In badly light-polluted sky, OSC cameras also display an sky color that is unnatural and displeasing and it takes some work to correct this by altering the white balance of the image.
If color is important for your EAA observing interests, a color CMOS camera is a perfectly good choice, and it's likely the preferred choice for outreach. But if you want the highest possible sensitivity for finding faint fuzzies and getting maximum detail, then choose a monochrome camera with a high quantum efficiency. While you may miss color at first, monochrome images have their own beauty and subtle elements of contrast and most visual observers are accustomed to seeing faint objects in monochrome through the eyepiece.
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 2.9 micron pixels has a full-well depth of 14,600e. The ASI1600MM with 3.8 micron pixels has a full-well depth of 20,000e. And the ASI294MC camera has 4.63-micron pixels and full-well depth of 63,700e.
The ratio of full-well depth of a pixel to the noise generated in each pixel is a measure of the dynamic range of the camera. However, larger pixels to do not imply larger dynamic range because larger pixels can also generate more noise.
In electronics, where there is signal, there is noise. The ratio of the two is called, pragmatically, the signal-to-noise ratio (SNR), and like signal processing engineers, astrophotographers strive to maximize the former while minimizing the latter.
Astro-imagers deal with small signals, especially in the fainter regions of galaxies, nebulae, and star clusters that may deliver, through a small telescope, only a few hundred photons to each pixel on the sensor. That's not a big signal! Only a fraction of these photons are converted to electrical signal depending on the QE of the sensor. To get more light on each pixel, the imager can choose a telescope with more aperture or a faster focal ratio at the expense of weight, cost, and complexity, and do their work on nights of exceptional atmospheric clarity.
Noise, on the other hand, is defined as the irregular fluctuations that accompany an electrical signal but which are not part of it. It's a little trickier to understand and minimize. In astro-images, noise appears as a 'graininess', especially in areas with low signal, that obscures or overwhelms faint detail.
There are several sources of noise, but shot noise is one of the most important. This type of noise is inherent to detecting a signal. It's caused by discrete particles (photons) falling onto the pixels like, for example, rain falling onto a tiled floor. Even in a steady rain, in each fixed interval of time, a slightly indeterminate number of raindrops falls on each tile because the drops are discrete and arrive randomly. As it turns out, if N photons are collected on a pixel, the corresponding shot noise is the square root of N. When only shot noise is considered, the signal-to-noise ratio therefore increases as square root of the number of photons collected.
Shot noise is not a property of the camera sensor itself. The only way to improve the ratio of signal to shot noise is to collect more signal. For example, if each pixel collects 25 photons in one second, the shot noise is 5 photons and the SNR=5. Increase the collection time to four seconds so each pixel collects 100 photons (4x as many), and the shot noise increases to 10 photons but the SNR=10 (2x as big). In 100 seconds, 2500 photons are collected (100x as many as in one second) and the SNR=50 (the square root of 2500).
Thermal noise is caused by the thermal jostling of atoms within the sensor that results in the creation of electrons. The read-out circuitry in the camera can't tell these thermally-generated electrons from electrons generated by photons from a distant galaxy. Thermal noise is sometimes dark current because it is present even in the absence of light falling on the detector. 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).
Figure 12 shows the thermal noise for the ASI294MC-Pro camera as a function of temperature in units of electrons/second/pixel. At a temperature of 22°C, for example, the thermal noise is about 0.25 electrons/second/pixel. The thermal noise builds over time, so in a 60 second exposure for this camera at 22C, the thermal noise adds up to 15 electrons in each pixel.
The strategy used by most digital astrophotographers to reduce the effect of noise involves 'stacking' or averaging many separate frames to improve the SNR (see Figure 13). For random noise such as thermal and shot noise, the SNR increases with the root of the number of subframes. In astrophotography, each sub is typically several minutes long and the telescope mount is guided (or autoguided) to maintain tight tracking.The camera is kept at a relatively low gain setting to ensure a high dynamic range to bring out fine detail in astronomical objects.
For EAA, you generally want much shorter subframes (or subs) of about 5s to 30s in length so you don't have to wait around too long for an image to appear on your screen. Shorter subs also relax or eliminate the need for guiding and make it easier to get away with a relatively casual polar alignment and a less complex and expensive mount. To compensate for shorter subs, the camera gain is turned up which reduces the dynamic range. But in EAA we're after fast images – snapshots – not magazine-quality images.
Short subs mean the signal from the camera has to be 'read' more often. This puts into play another type of noise: read noise. Like thermal noise, read noise is generated within the camera itself. It's a consequence of the uncertainty generated by the readout electronics on the sensor. Unlike thermal noise, read noise does not increase over time. A camera delivers the same read noise for a 5s exposure or a 500s exposure. But every time a subframe is captured, the read noise increases.
Fortunately, many current CMOS cameras have a very low read noise of just 1e to 2e (or less) at a high camera gain of 30-40dB. The ZWO ASI224MC camera, for example, has a read noise of just 0.8e. This compares to read noise of 7e to 10e for many CCD astronomy cameras. This low level of read noise is the reason that many CMOS cameras work so well for EAA applications and why CMOS cameras are on the way to replacing CCD cameras for near real-time imaging.
Other forms of noise bedevil astrophotographers such as fixed-pattern noise and quantization noise, but these are more advanced topics. There are also many types of noise that are in fact unwanted signals such as signal generated by light pollution and other sources of skyglow which is some cases can be removed by optical filters.
With the discussion of thermal noise above, it may seem fairly obvious that it's best to choose a CMOS camera with a built-in TEC cooling system to keep thermal noise as low as possible. Cooled cameras have extra components so they're more expensive and require a source of power for the thermoelectric cooler (TEC) and fans, and they work just fine for EAA. But is cooling really a must-have for EAA in all cases? Not necessarily, for four reasons:
- Perfectly good images for EAA are within reach of uncooled cameras (all images in this guide are taken with uncooled cameras, for example)
- Many cooled cameras have larger sensors intended for serious wide-field astrophotography rather than casual viewing of many smaller deep-sky objects, especially when used with smaller telescopes (more on this in a later section); only a few cameras with smaller sensors come in cooled versions
- Thermal noise increases as exposure time increases, and in EAA the subframes are relatively short, especially with cameras with very high sensitivity and QE
- When observing under light-polluted urban and suburban skies, the effect of light pollution often has a much greater effect on the quality of the image than thermal noise. In badly light-polluted sky with short sub-frames, you may not even notice the effect of cooling on the image.
An exception to the last point: if you do your EAA work on nebulae through narrowband filters that minimize the effect of light pollution, then cooling MAY help when capturing longer subframes of >10s, especially on warmer nights.
Noise in astrophotography is a big topic, and this section just scratches the surface. A good resource for understanding the effects of noise in EAA and astrophotography is this video of an excellent lecture on deep-sky CMOS imaging by Dr. Robin Glover, the creator of the SharpCap software package used by many EAA enthusiasts. It's packed with useful information and expands on many of the points made in this section.
|Camera||Sensor Size (mm)||Pixel Size (um)||Resolution||Aspect Ratio||QE(%)||Read Noise
(at Max Gain)
(Cooled version available)
(Cooled version available)
|ZWO ASI1600MM/MC||17.7x13.4||3.8||4656x3520||4:3||60||1.2||20,000||No-Cooled version available|
What about DSLR cameras for EAA? It is possible to use such cameras, especially those from Nikon and Canon, although it does require some extra software and steps. Also, DSLR cameras have larger sensors, either APS-C or full-frame, which means they provide a larger field of view than the cameras listed in Table 1. To control the camera, you need software such as Backyard EOS or Backyard Nikon (available from OTelescope.com). This software can deposit images in a folder set up in Sharpcap for subsequent processing. Or it can be used with the application AstroToaster and Deep-Sky Stacker Live to enable nearly real-time adjustment. All these applications run concurrently on a Windows PC. The multiple steps and software packages required imply that EAA with DSLRs requires a little computer savvy and patience than simply using a dedicated astronomy camera and Sharpcap.
Which telescope works best for EAA? And which focal reducer works best with your telescope? This section and the next will explore the answers to these questions. But here's one useful rule of thumb that will save you hours of head scratching, frustration, and perhaps thousands of dollars spent on the wrong equipment. For most beginners and many experienced 'snapshot astrophotographers' interested in deep-sky sights, the best choice of telescope is one with an effective focal length (including focal reducer) of 500mm to 1000mm (and preferably no more than 800mm) and an effective focal ratio of about f/3.5 to f/6.
What's the reasoning behind this?
In a nutshell, this range of effective focal length produces an image size on many camera sensors that nicely frames many photogenic deep-sky objects including galaxies, globular clusters, and larger planetary nebulae. Also, keeping the effective focal length under 800-1000mm makes it easier to find and track objects with an affordable and relatively light-weight telescope mount without the need for autoguiding. And a focal ratio faster than f/6 makes for brighter images and shorter image capture times, while any faster than about f/3.5 tends to compromise image quality with most amateur scopes.
Before we get into telescopes for EAA, let's have a quick look at focal reducers. A focal reducer is the opposite of a Barlow lens or focal extender. It's a lens or group of lenses placed in front of the camera that causes the light from a telescope objective to converge at a steeper angle to the focal plane as if it were coming from an objective with short focal length and therefore a faster (lower) focal ratio. A shorter focal length means a wider field of view. A faster focal ratio means a brighter image of extended objects (although to get brighter images of point sources such as stars requires a larger objective).
You do not absolutely need a focal reducer to do EAA. With some telescopes such as small ED refractors with a focal ratio of f/6 or Newtonians with a focal ratio of f/4 to f/6, you can get by without a focal reducer. But with slower telescopes of f/7 to f/10, a focal reducer is essentially a must-have. Snapping an image of an extended object like a nebula or galaxy with an f/10 Schmidt-Cassegrain telescope takes four times longer than if the same telescope is matched with a focal reducer that makes the effective focal ratio f/5. The difference between a 10s subframe and a 40s subframe in this example is not only a matter of getting an image four times faster. It also makes it less likely your mount will cause a tracking error during each subframe.
Focal reducers are usually specified by their design reduction factor, the amount by which they reduce the focal length and increase the focal ratio of the telescope. They also have a specified working distance or back focus usually specified in millimeters. This is the distance at which the camera side lens of the reducer must be placed in front of the camera sensor in order to operate at the design reduction factor. For example, the Tele Vue Optics TRF-2008 focal reducer works with many refractors of about 400mm to 600mm focal length. When used at its specified working distance of 55mm, it reduces the focal length and focal ratio of the telescope by 0.8x. This reducer and many others must be placed within a few millimeters of the working distance to produce a good image. Other focal reducers can be placed, with the help of spacer rings, at a different operating distance from the camera to achieve a slightly greater or lesser amount of reduction.
While most Barlow lenses can be used with nearly everytype of telescope, this is not the case for focal reducers (with a couple of exceptions). Schmidt-Cassegrain telescopes work with focal reducers designed for Schmidt-Cassegrains. Ritchey-Chretien scopes use another type of focal reducer. Refractors require focal reducers designed to work with refractors. In some cases, each refractor manufacturer makes reducers that work best only with their telescopes.
Some of the most commonly used focal reducers for EAA are listed below.
GSO 0.5x Focal Reducer. As close as it comes to a 'one-size fits all focal reducer', these inexpensive single-lens focal reducers are available in 1.25" or 2" cells. They thread onto the nosepiece of the camera using spacers that place the element at a working distance, typically about 50-55mm, to achieve a reduction factor of 0.5x. You can also place them at other working distances to get slightly larger or smaller reduction factors. Priced at less than $50, they produce an acceptable image when used with cameras with small sensors such as the ASI224MC and ASI290MC/MM cameras from ZWO or equivalent cameras from other vendors. They are not recommended for use with cameras with larger sensors where image distortion and vignetting become evident away from the center of the field of view. They work best with telescopes with intrinsic focal ratios of about f/7 or slower.
Mallincam MFR-5II. This focal reducer uses two separate C-threaded elements that can be configured along with spacer rings to achieve a range of reduction factors from about 0.3x to 0.8x by varying the working distance of the reducer to the camera sensor. It works with refractors and Schmidt-Cassegrains. Initially intended for use with Mallincam CCD video astronomy cameras, they can also be used with small-sensor CMOS cameras similar to the GSO focal reducers above. These reducers are hand-selected, mounted, and tested, so they are much pricier than the GSO reducers. The MFR-5II has superior anti-reflection coatings compared to the earlier MFR-5, but the latter is still quite usable for EAA with small-sensor cameras. These focal reducers have moderately acceptable performance even with EdgeHD telescopes with small-sensor cameras (see Figure 10).
Celestron and Meade f/6.3 Focal Reducers. With a reduction factor of 0.63x, these reducers only work with Schmidt-Cassegrain scopes. They're mounted in a cell that threads to the back of many Schmidt-Cassegrain scopes. Since they also correct for field curvature of the telescope, they are good performers even with sensors up to APS-C size (24mm x 16mm). They don't quite get the effective focal ratio to the sweet spot for EAA, but some observers combine two of these reducers to get a faster focal ratio. NOTE: These 0.63x reducers cannot be used with Meade ACF or Celestron EdgeHD telescopes, which have their own 0.7x specialized field flatteners for astrophotography that produce an effective focal ratio of f/7, which is a little slow for EAA.
Starizona Night Owl. Specifically designed for EAA, this 0.4x focal reducer works with Schmidt-Cassegrain scopes and produces excellent images for camera sensors with a diagonal up to 16mm across. Not for use with Meade ACF or Celestron EdgeHD scopes.
Meade f/3.3 Focal Reducer. No longer produced, these 0.33x reducers work great for EAA with Schmidt-Cassegrain scopes since they take the focal ratio down to f/3.3. Ideal for use with smaller sensors if you can find a gently used specimen.
Refractor Focal Reducer/Field Flatteners. Each major refractor manufacturer also makes focal reducers to work with their apochromatic or ED scopes. Often combined with field-flattening elements, these reducers are available with a reduction factor of 0.7x-0.8x for refractors with a focal ratio of about f/6 to f/7.5. They are usually designed with a working distance of 55mm, so spacers may be needed between the reducer and camera to set the reducer at the design reduction factor. These devices are intended for use with large-sensor cameras and serious astrophotography, so they are more than enough for EAA applications with smaller sensors. Most such reducers cost a couple hundred dollars or more.
Hyperstar. A unique and expensive option (about $1,000), the Hyperstar device from Starizona replaces the corrector lens of compatible Celestron Schmidt-Cassegrain and EdgeHD scopes to achieve a focal ratio of f/2. The camera mounts in the Hyperstar optic and sits at the top of the telescope in front of the corrector plate; no light is directed out of the back of the scope. These are great for EAA because the f/2 focal ratio means very fast image capture, but they often result in very wide fields of view that are not suitable to frame smaller objects with large-sensor camera. They can be a great complement to other focal reducers.
Where to start? The GSO 0.5x focal reducer is usually an entry point for newcomers to EAA. The 1.25" model is a bargain and it's relatively easy to use. It works with most types of telescopes, and it delivers a respectably good image when used with small-sensor cameras like the ZWO ASI224MC, ASI290MC/MM, or similar cameras from other vendors. It threads onto the nose of the camera, often with an additional spacer or two to ensure the correct working distance.
Such an inexpensive component has its limitations of course: if better image quality is needed, a more sophisticated focal reducer is recommended, especially if you want use a camera with a larger sensor. As a comparison, Figure 15 shows an EAA image of M27 made with the GSO 0.5x reducer (at the specified working distance) and another image made with the far more expensive and sophisticated Tele Vue TRF-2008 focal reducer using the ASI385MC camera (7.3mm x 4.1mm sensor) and an 85mm f/7 refractor. The vignetting and distortion of the star images at the edge of the field with the GSO reducer is evident, although the image near the center of the field of still quite good. This sensor size is the largest that is recommended with the GSO 1.25" 0.5x focal reducer.
You can much learn more about the ins and outs of focal reducers with the Agena AstroProducts Guide to Focal Reducers for Astronomy.
This detailed test report from amateur astronomer and engineer Jim Thompson is also useful in understanding focal reducers with Schmidt-Cassegrain telescopes, especially the Meade/Celestron 0.63x reducer and the Mallincam MFR-5 reducer.
With a basic understanding of focal reducers in our pockets, let's look at which telescopes to consider for EAA. Each type of telescope has its pros and cons, and as with visual astronomy, there is no perfect single telescope for all types of EAA observing. It comes down to a trade-off between cost, size, aperture, flexibility, and observing interests.
Refractors. A small refractor of 70-100mm is not the ideal scope for visual observing of faint fuzzies like galaxies or planetary nebulae or globular clusters. But when used with a modern CMOS astronomy camera, these little scopes can quickly turn into powerful tools for EAA. With even the simplest CMOS camera, globular clusters are easily resolved into individual stars, and nebulae and galaxies show detail accessible to visual observers only in a 12" or larger scope. A small f/6 or f/7 apochromatic/ED refractor is a good bet for EAA, especially when coupled with a focal reducer made to match the scope to get an effective focal length of 400-600mm. They're also relatively lightweight and easy to mount and handle. Faster achromatic refractors are also a possibility, but they tend to form more bloated star images. 'Beginner' refractors with a slow focal ratio of f/10 or f/12 are not recommended for EAA because they offer a narrow field of view.
Schmidt-Cassegrains (SCTs). As is the case for visual astronomy, SCTs are a good general workhorse for EAA. For beginners, a 6" or 8" scope is an excellent balance of aperture and manageable size. A focal reducer is a must, either an inexpensive GSO reducer or the Mallincam MFR-5 for small-sensor cameras, or a dedicated SCT reducer for larger cameras. And while field-flattened scopes like the Celestron EdgeHD or the Meade ACF are fine instruments for visual use and astrophotography, with the exception of Hyperstar for Celestron EdgeHD scopes, there are few options for suitable focal reducers for EAA. Maksutov-Cassegrains are not ideal for deep-sky EAA because of their long focal ratios of f/12 to f/15.
Fast Newtonians. Appreciated by many serious imagers for their fast optics and budget-friendly price, classic Newtonians with fast focal ratios of f/4 to f/6 offer an alternative to ED refractors. While they get bulky in apertures of 8" or larger, these scopes are good choices for EAA and they may not need a focal reducer for many deep-sky objects. They do, however, require a coma corrector when used with cameras with a diagonal larger than 8-10mm (approximately). They can also be a little tricky to collimate, and the faster the scope, the more critical the collimation. For many EAA enthusiasts, even 'beginner' Newtonians with apertures of 100mm-130mm can work well when placed on a motorized mount.
Go-To Dobsonians. A modern-day incarnation of the Newtonian, Dobsonian telescopes on motorized go-to mounts may be tempting for EAA. But many of these mounts are designed with visual observing in mind and they are not accurate enough for EAA where the field of view is much smaller than with a wide-field eyepiece. Don't go out of your way to pick one of these for EAA. Although, to be fair, some observers with big fast Dobs and high-end tracking platforms do capture some pretty good images.
As with astrophotography, an accurate and solid mount is essential for EAA. Motor drive in the polar axis is essential, and go-to is a big help in finding objects, especially with smaller-sensor cameras with a limited field of view. A mount should be stable enough to minimize and damp vibrations, even in a breeze. And it should not be loaded to capacity: a mount with a 30lb rating, for example, should carry a load no more than ½ to ⅔ of this value to ensure stability during the length of a sub-frame.
But do you absolutely need an equatorial mount? For astrophotography, where frames are captured over a few minutes or more, the answer is definitely 'yes.' For EAA, the answer is 'maybe' or 'it's nice to have.' If the sensitivity of the camera and the focal ratio of the telescope allow for exposures of 30s or less, a good go-to tracking altazimuth mount can work just fine, and many serious EAA practitioners work with alt-az mounts, especially with systems such as Celestron CPC and NexStar and Meade LX90, LX200, and LX65. For captures longer than about 30s, the effects of field rotation become noticeable.
If you already have a good scope on a solid motorized alt-az mount, that's plenty good enough to get started in EAA and it may be all you ever need. If you are starting from scratch, then invest in a good EQ mount and learn how to use it. It will let you make longer captures, and you'll be 'future proof' if you want to do more serious astrophotography. Any good mid-weight mount with a 25-30lb capacity will carry an 8" SCT or small refractor or 6" Newt and still be easy enough to handle and transport.
Picking a telescope is one thing. Picking a camera is another. But there's one more critical step: figuring out how your camera and telescope (and focal reducer) work together and how they match your observing interests and the deep-sky objects you wish to observe.
If you're mostly interested, for example, in views of large colorful nebulae like M42 or big open clusters like the Double Cluster, then you need to choose a combination of camera and telescope that offer a field of view of about 1.5° to 2°. That means you need a scope with a relatively short focal length and a camera with a bigger sensor. Conversely, if you want to target small galaxies or planetary nebulae that are only fractions of a degree across, you need a longer focal length and a camera with a smaller sensor.
As an example, Figure 17 shows images of the galaxy M51 taken with two telescopes and two focal lengths, a TV85 refractor with a 0.8x focal reducer (effective focal length 480nm) and a GSO f/5 150mm Newtonian with no focal reducer (focal length 750mm).
How to quantify all this? With a given camera, telescope, and focal reducer, it's possible to calculate the field of view exactly. You need to know the dimensions along each edge of the camera sensor of length and the effective focal length of the telescope including the effect of the focal reducer. The field of view (in arc-minutes, or ') is simply:
FOV (arc-min) = 3436*D/F (Eq. 1)
where D is the sensor dimension in millimeters and F is the effective focal length in millimeters. Divide by 60 to get the FOV in degrees.
Another quantity to consider is the image scale, the measure of the angle of sky captured by each pixel on the sensor. It's calculated in terms of arc-seconds/pixel as:
Image scale ("/px) = 206*s/F (Eq. 2)
where s is the pixel size in microns and F is the effective focal length of the scope in mm. As a rule of thumb, the image scale should be ½ the size of the smallest detail to be resolved in the image. That's usually limited by atmospheric conditions from about 1.5" to about 4", give or take. So ideally you aim for an image scale of 0.75"/px to 2"/px. If the image scale is larger (which is called undersampling), you lose image detail but get more light into each pixel. If the image scale is too large, single pinpoint stars fill entire pixels which leads to 'blocky' star images. If the image scale is much smaller than seeing conditions permit, then you end up using several pixels to capture a blurry image without gaining any other advantage. Better to undersample than oversample.
Experienced astrophotographers take image scale seriously. For grabbing EAA snapshots, image scale is not as critical. If possible, try to land on the side of undersampling, if possible, but don't worry too much about it.
The website Astronomy Tools has a very useful calculator for determining field of view and image scale of a telescope with eyepieces (in 'visual mode') or cameras (in 'imaging mode'). It also generates a useful graphic that shows deep-sky objects in the frame of the selected camera, telescope, and focal reducer. Spending a few minutes with this online tool is a great way to get a feel for framing deep-sky sights with various combinations and permutations of equipment.
Let's use the field of view calculator at Astronomy Tools to look at a couple of examples to see how to figure out how a camera and telescope work together.
Example 1: Matching a Camera to an 80mm f/7 ED refractor
Many vendors offer an 80mm f/7 refractor with semi-apochromatic optics. These scopes are lightweight and often affordable which makes them great choices for EAA, especially for outreach at star parties when many larger and colorful nebulae are visible in the northern summer sky. We want to match this scope to a 0.8x focal reducer to get an effective focal length of about 450mm and a faster focal ratio of f/5.6. Let's see how this scope works with two cameras, a ZWO ASI385MC color camera (with a 7.3mm x 4.1mm sensor and 3.75 micron pixels) and an ASI294MC color camera (with a 19.1mm x 13.0mm sensor and 4.63 micron pixels). Both cameras have good quantum efficiency (QE) and low read noise, making them excellent choices for EAA.
When supplied the focal length of the telescope, reduction factor of the focal reducer, and camera type, Astronomy Tools calculates a FOV of 0.93° x 0.52° and an image scale of 1.72"/pixel for the ASI385MC camera. For the ASI294MC we calculate a much larger FOV of 2.45° x 1.67° and an image scale of 2.13"/pixel. You can also use Equations (1) and (2) to calculate these numbers.
How would a small nebula like, say, the Trifid Nebula (M20) appear in these cameras and this telescope? Figure 18 shows the results of the FOV calculator in Astronomy Tools. With the smaller ASI385MC sensor, M20 frames nicely in the field of view with plenty of space around it. In the much larger ASI294MC, the nebula appears relatively tiny in the larger frame, but the field of view is large enough that, with a nudge of the telescope to the south, the nearby Lagoon Nebula (M8) would fit in the same frame, making for a compelling scene. Of course, much smaller objects like galaxies and planetary nebulae would be dwarfed by the large field of view of the ASI294MC, although there is a provision in software to get an image from a smaller part of the sensor with this camera and other cameras with larger sensors.
Example 2: Matching a Camera to an 8" Schmidt-Cassegrain
The 8" SCT is another popular all-purpose telescope, and it's a good one for EAA. With an f/6.3 focal reducer, this scope delivers a focal length of about 1280mm. Figure 19 shows how it frames the relatively bright galaxy M82 using the same two cameras used in Example 1. For the ASI385MC, Astronomy Tools calculates a fairly narrow FOV of 0.32° x 0.18° and an image scale of 0.62"/pixel, likely a little oversampled. For the ASI294MC we calculate a much larger FOV of 0.86o x 0.58° and an image scale of 0.75"/pixel (you can also use Equations (1) and (2) to come up with these numbers).
With the smaller ASI385MC sensor, M82 frames nicely in the field of view with plenty of space around it. In the much larger ASI294MC, again, the galaxy appears relatively tiny in the larger frame, but the field of view is large enough that, with a nudge of the telescope and perhaps by rotating the camera, the nearby spiral galaxy M81 would fit in the same frame, making for a very good view.
It's worth experimenting with a number of cameras and telescope combinations with well-known objects of various classes including planetary nebulae, galaxies, globular clusters, and open star clusters. Many planetaries are quite small and favor longer focal lengths and smaller cameras. Many open star clusters are much larger and favor larger sensors and shorter focal lengths. As with visual observing, there's isn't a single telescope/camera combination that works well to frame all celestial objects.
Filters are handy for visual observing and imagers, EAA enthusiasts included. Filters do not make deep-sky objects brighter, but they can improve contrast or image quality by blocking unwanted light at some wavelengths and passing desirable light at other wavelengths.
There are two types of filter to consider. UV/IR-block filters, as their name implies, block UV and IR light from hitting the camera sensor. Most astronomy cameras detect light far beyond the range of the human eye into the ultraviolet to about 350nm and out to the infrared at about 1100nm. Refracting telescopes, however, cannot bring all these wavelengths to a tight focus in the same plane. The result is larger 'bloated' stars and somewhat 'fuzzy' images. The effect of IR light is particularly problematic, so an inexpensive IR block filter that cuts off light longer than about 700nm is a good investment. More sophisticated filters than block UV and IR are also an option. Some higher-end cameras include a protective window over the sensor that block UV or IR light. For newcomers to EAA, a simple IR-block filter (if the camera does not include an IR-blocking window over the sensor) is a good place to start.
Figure 20 shows an image of the Dumbbell Nebula (M27) with and without the inexpensive 1.25" GSO IR-block filter. The top image shows the nebula without a filter, and the bottom image shows the galaxy with IR block. With the filter, the stars are evidently sharper and smaller.
Bandpass filters, sometimes called 'light pollution filters', are useful for blocking unwanted light from sodium and mercury street lamps while passing light from nebulae in the red, green, and blue regions of the spectrum. They are not as effective for stars and galaxies because these objects generate light across the entire visible spectrum, but such filters can still be of some help for EAA. Since cameras are sensitive to the red H-alpha emission from nebulae at 656nm, filters for EAA should pass this important wavelength as well as OIII at 501nm and, if possible, H-beta at 486nm. When used with color cameras for EAA in moderately to severely light-polluted sky, these filters can produce unbalanced color that may look somewhat unnatural, but they can improve image contrast.White balance isn't a problem for monochrome cameras and light pollution filters work very well with these cameras to improve the contrast of nebula against the background sky.
As with visual observation, finding good focus for EAA and astrophotography is essential. It can also be a little tricky. To make it easier, the use of a Bahtinov mask is recommended. This mask is placed over the dewshield or the front of the of the telescope tube, which is aimed at a bright star. The focuser is adjusted until the central 'spike' in the diffraction pattern created by the mask lies exactly between the 'X'. The masks are available for a wide range of telescope apertures. (Tip: Don't forget to remove the mask before you start taking images)!
If EAA has one disadvantage compared to visual observing, it's that you spend a lot of time looking at a screen rather than at the sky. But a computer (or some type of computing device like a tablet) is essentially a must-have to process and display the digital signal from your CMOS camera, as is some form of software to process the signal and adjust the display so you can see and optimize the image for viewing on the screen. The most widespread software currently used for EAA is SharpCap, a package developed by British amateur astronomer Robin Glover. To unlock its most useful features, the purchase of a modestly-priced annual license is required. Figure 1 (at the top of this article) shows SharpCap in action, and nearly all the images in this article were captured and adjusted in SharpCap through is histogram and white balance controls. Compared to most astrophotography software packages, SharpCap is relatively simple to use although it does have many useful and powerful features. It works with most astronomy cameras from major manufacturers. It also runs only on the Windows platform so a PC or a Mac running Windows is a must-have.
ZWO can introduced a device that takes that place of SharpCap and a PC. Called the ASIAIR-PRO, this device connects through WiFi to a tablet with a ZWO app that controls a ZWO camera and mount. While primarily used for astrophotography, the ASIAIR-PRO (unlike its predecessor the ASIAIR) enables live viewing and basic image adjustment and recording. This makes it a very proming tool for EAA enthusiasts.
While it may seem a little intimidating to newcomers, deep-sky electronically-assisted astronomy EAA is a form of 'snapshot astrophotography' that makes accessible a huge range of celestial sights through a small telescope using CMOS astronomy cameras. As outlined in this guide, EAA requires different considerations when choosing equipment compared to visual observing, but it's much less taxing and complex than serious astrophotography. EAA can be done with many types of telescopes, basic tracking mounts, and relatively inexpensive small-sensor cameras and accessories. Like most things in amateur astronomy, it requires a bit of practice and patience. But for many, especially urban and suburban observers, the tools and techniques of EAA open up the skies to many years of pleasant viewing and rewarding study and contemplation.
Some additional articles and references that are relevant to EAA (including references mentioned earlier in this article).
- A Q&A about EAA with Jim Thompson, an Ottawa-area engineer and amateur astronomer. https://astronomyconnect.com/forums/articles/an-introduction-to-electronically-assisted-astronomy-eaa.77/
- Agena's Guide to Light-Pollution Filters for Visual Astronomy https://agenaastro.com/articles/guide-to-light-pollution-filters.html
- Agena's Guide to Astronomy Filters for Imaging https://agenaastro.com/articles/guide-to-imaging-filters.html
- Agena's Guide to Focal Reducers https://agenaastro.com/articles/focal-reducers-guide.html
- Agena's Guide to ZWO Astronomy Cameras https://agenaastro.com/articles/zwo-astronomy-cameras-buyers-guide.html
- Video of Dr. Robin Glover on Noise in EAA and Astrophotography https://youtu.be/3RH93UvP358
- Cloudy Nights EAA and Night Vision Forum https://www.cloudynights.com/forum/73-eaa-and-night-vision/