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I am an amateur astronomer looking to upgrade from a simple point and shoot camera looking through a Newtonian telescope to a more sophisticated set-up. I lecture at a local college and I am lucky enough that they may lend me a schmidt-cassegrain telescope (Celestron CPC series telescope).

I am looking to invest into my first DSLR camera and while I know Canon has their own dedicated astronomy camera (EOS Ra) such a camera is out of my budget. I have narrowed it down to four options and I would like to have more expert advice on which I should choose:

I am lost to choose between those options. May you please shed some light? Thank you very much.

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    $\begingroup$ One feature which is very handy to have is "live view" With it, you can connect the camera via a USB cable to see a live image on your computer. This makes getting perfect focus much easier. Can you also elborate on what kind of imaging you'd like to do, such as science or pretty pics? Here is a link which may help you with some of your decisions: astropix.com/html/i_astrop/software.html $\endgroup$ – AstroShannon Aug 28 at 20:13
  • $\begingroup$ A mix of both science and pretty pictures. I plan to start small Andromeda perhaps then work my way up to deeper space objects $\endgroup$ – Mark Pace Aug 28 at 21:01
  • $\begingroup$ AstroShannon said, "...live view." That's DSLR talk. If you look at so-called "mirrorless" cameras, those effectively are in "live view" mode all the time, but they don't actually use those words to describe it in the literature. $\endgroup$ – besmirched Aug 29 at 17:25
  • $\begingroup$ Do you know which specific telescope size you would be using? M31 is a wide-field target. Most SCT scopes with an APS-C size sensor would only capture a very tiny piece of it at a time. $\endgroup$ – Tim Campbell Aug 29 at 23:47
  • $\begingroup$ Personally, I would shy away from a DSLR unless you want to use it for other purposes. A good astronomy camera can be resold on Cloudy Nights with very little loss if you decide it's not for you. The very most important questions to ask are: What do you hope to photograph? Do you have access to dark skies? With planets, light pollution doesn't hurt too much. Hauling a scope to a dark location requires more purchases, such as a power supply and a sturdy case for the scope. The more you can narrow down what your AP goals are, the better luck you'll have. But beware - it's addictive! $\endgroup$ – AstroShannon Aug 30 at 0:40
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Based on how you described your situation, I surmised that you do not have prior experience or equipment for astrophotography through a long focal-length telescope. There are a few things you did not specifically ask, but should know before you invest in an expensive camera. Also, it sounds like you would have preferred a dedicated astrophotography camera -- rather than a general purpose camera -- if budget would allow for it.

Altitude/Azimuth Mounts and Field Rotation

The Celestron CPC series scopes are all SCT's... but on an Altitude/Azimuth style mount. The mount's axis of rotation is not parallel to Earth's axis of rotation and this means that when you perform long-exposure astrophotography, you'll experience an issue called "Field Rotation". The amount of field rotation you experience depends on where you point the telescope in the sky BUT ... to avoid field rotation, the maximum exposure duration is about ... 20 seconds (tops).

The scope has an f/10 focal ratio (virtually all SCT's are f/10 scopes... but just occasionally f/8 ... that's fairly uncommon). This means 20 seconds will not be adequate (you'll likely need 5 minutes or longer).

Very large professional telescopes (e.g. 1 meter and larger) are almost always on alt/azimuth mounts, but those telescopes are equipped with field-rotators which rotate the camera while the scope tracks to counter-act the field rotation effect. The rotation rate is highly dependent on the location of the sky being imaged -- so this is computer controlled. Field rotators are expensive.

The less expensive way to solve this issue is to add an equatorial wedge. This tilts the entire alt/az setup over on an angle such that the azimuth axis becomes a right-ascension axis and can be aligned parallel to Earth's polar axis. In this orientation, the motion of the mount exactly cancels the motion of the Earth and the object is held in position with no field rotation.

Guiding

It is possible to take long-exposure images at short focal lengths (e.g. I've done as much as 8 minutes un-guided at around 500mm focal length). But even an 8" SCT would have a focal length of around 2000mm. It isn't possible to take long exposures without an auto-guider ... even on a quality mount. This requires a separate guide-camera. The good news is that guide cameras usually aren't very expensive. A starter camera is around 150 USD (e.g a ZWO ASI120MC-S is around 150). The Celestron CPC mount is designed to be affordable for visual use. While you can use it for astrophotography, it is not a high-performance mount ... and may present challenges.

Commonly the guide-camera is attached to a guide-scope ... a separate telescope usually piggy-backed to the main imaging scope (or on a side-by-side mounting plate). The focal length would ideally be around 1/3rd of the focal length of the main imaging scope (e.g. for an 8" SCT with around 2000mm focal length, the guide-scope would ideally have a focal length of around 700mm). People do push this limit ... some people will push it up to around 1/10th (e.g. a 200mm guide scope).

The guide scope need not be high-quality ... it's staggering how bad the optics can be and still result in exceptionally good guiding quality (this has to do with the algorithms used ... they don't assume the Star is a pin-point source of light).

Another option is to use an off-axis guide adapter. This are gadgets mounted between the telescope and main imaging camera. It has a 45° prism which acts as a "pick off" mirror. It is position just outside the field of view of the main imaging camera and sends light out at a 90° angle and into a guide-camera mounted right next to the main imaging camera. An off-axis guide adapter has the advantage of sharing the same telescope as the main imaging camera -- which means no differential flexure issues between two scopes resulting in elongated stars. The down side of this is that the pick-off mirror has to be very small so as to not present an obstruction in the view of the main imaging camera. And that means you have to try to find a suitable guide-star in that very tiny field of view (sometimes you have to rotation the off-axis adapter to try to find a star.)

Schmidt Cassegrain Optics

The telescope wont have a "flat" field. Stars near the edges of the field of view will exhibit off-axis aberrations ... which will include some loss of focus and also the addition of coma and possibly a bit of astigmatism. Celestron does make a line of SCT optical tubes they refer to as their "EdgeHD" line (more expensive). These telescopes are not coma-free but they do have a flatter field with respect to focus and overall do a decent job of correcting the field of view well enough for use with an APS-C camera. When using an APS-C camera with traditional non-correct Schmidt Cassegrain optics, expect to see some reduction in off-axis image quality (edges and corners of the frame).

Cameras

The Canon EOS Ra is a full-frame sensor which has been modified for astrophotography use (the primary difference is the band-pass on the filter -- specifically at the Hydrogen alpha wavelength). Full-frame cameras are a can-of-worms. They require well-corrected optics because of the larger physical sensor size. A DSLR camera is typically attached to a telescope via a nose-piece that slides into the eyepiece receiver, but has the bayonet-style mount required to mate with the camera body. This often means either a 1.25" or 2" diameter nose-piece is used. A 1.25" outer-diameter loses a few millimeters on the inside due to wall-thickness ... but generally offers something like 28mm clear internal diameter. That is large enough for an APS-C sensor. But a full-frame camera needs nearly 44mm of clear aperture -- so a 1.25" nose-piece would result in extreme vignetting. A 2" nosepiece is generally large enough as long as the clear unobstructed diameter is larger than the diagonal measure of the sensor (just about 44mm). Given that the CPC doesn't offer a well-corrected field of view with a 44mm diameter, you would likely be disappointed with the results of such a large camera.

The issue with non-modified DSLR's for deep-space astrophotography is that the built-in IR filter doesn't allow full-pass of light in the visible spectrum. These cameras trim light transmission in an effort to match the sensitivity of the human eye. This "trimming" is rather substantial. A camera designed for traditional photography is trimming around 75-80% of the photons at the Hydrogen alpha (~656nm wavelength) band. This means deep-space nebulae that glow in Ha will require substantially longer exposure times.

Dedicated astrophotography cameras don't have UV or IR filters. They are full-spectrum cameras with only the limitations imposed by the sensitivity of the Bayer matrix -- and usually are sensitive well into the near IR spectrum. A hard-cut IR filter is recommended as an accessory.

A second issue is that long exposure astrophotography results in a build up of heat which another source of noise.

I still have and use my Canon 60Da, but I've added a number of dedicated CMOS astrophotography imaging cameras.

To get an APS-C size sensor in such a camera, the popular sensor is the Sony IMX071 (this is the same sensor used in, say, a Nikon D5100 ... except without Sony or Nikon firmware (so the RAW images are true RAW images... not cooked RAW's like you'd get in a Nikon or Sony camera -- where stars go missing because they try to clean up noise and mistake faint stars for noise.)

From ZWO, the camera is called a ZWO ASI071MC-Pro. This is a color (all ZWO cameras with 'MC' in the name are color. Cameras with 'MM' in the name are monochrome) CMOS imaging camera designed specifically for long exposure deep-sky astrophotography. List price in USD is about 1480 USD. This is a few hundred above the price of a Canon 90D (about 1200 USD). (Incidentally, monochrome cameras actually cost more because these sensors are manufactured with the Bayer color-filter array on the sensor, and they are factory-modified to remove the filter (a manual process) -- which increases the price.)

The camera is has a built-in cooling system that is able to chill the sensor to as much as 40°C below ambient temperature to reduce noise.

The camera uses a USB 3.x connection but also includes a built-in USB hub that provides two additional USB ports. This allows connection of an auto-guider camera and/or filter-wheel without having to run separate cabling up to the scope.

Lastly, it has a 256MB memory buffer -- which is why ZWO tag this as a 'Pro'. Many dedicated astronomy cameras do not include a built-in buffer. If reading out the sensor across a slow USB 2 bus to the computer, the delay can result in a problem called 'amp glow' (almost looks like a like flare coming out of the edge of the frame). The buffer allows for instantaneous read-out to the buffer ... and the computer's speed is no longer an issue. Amp-glow is usually not an issue if the computer has a fast bus and fast storage).

Here is a link to the manufacturer's website: https://astronomy-imaging-camera.com/product/asi071mc-pro (Although nearly every astronomy dealer I am familiar with probably carries these cameras ... the more difficult issue is finding it in-stock during the pandemic).

For slightly less money, the same sensor is available from another vendor: QHYCCD. The QHYCCD QHY168C is the very same sensor. It sells for roughly 1400 USD (just slightly cheaper than ZWO) but it's on-board buffer is 128MB. Also is does not include the USB hub to piggy-back an auto-guider camera or filter wheel.

Here is a link to the manufacturer's website: https://www.qhyccd.com/index.php?m=content&c=index&a=show&catid=94&id=13

If the 1400-1480 USD price tag is out of budget, it is also possible to get a 4/3rd's sensor camera with the same technology ... for just around 1000 USD.

If you inspected either of those links, you might have noticed that these cameras have absolutely no controls of any kind on the body -- not even a power switch. They are completely controlled by computer -- which is fine for astrophotography since using a computer is preferred even with a traditional camera. They are powered via the USB bus with the exception of the cooling system (it runs on 12v power ... and usually the 12v power supply is not included.)

Traditional Cameras

It is somewhat less risky to use a traditional camera ... even with the short-comings -- because if you decide that astrophotography isn't for you... you've still got a camera that you can use for other things.

When you use a camera for astrophotography, you generally will not use any automatic features of the camera. The focus system does not matter -- you will have to manually focus the telescope. The burst-rate of the camera doesn't matter... you'll be taking long-exposure images generally only in Bulb mode. Basically ... you just want a decent sensor.

The EOS 250D and the M50 actually have the same sensor. (A 24.1 megapixel sensor). the 77D has a 24.2 megapixel sensor (it may actually be the same sensor). Only the 90D has a higher resolution sensor but that is not necessarily an advantage in astrophotography (often, lower resolution sensors will have less noise on long-exposure imaging).

Affordable alternatives

A highly affordable way to do deep-sky astrophotography is to use a decent (solid) photo tripod and add an equatorial tracking head. Today the common models are the Sky Watcher "Star Adventurer" head or the iOptron "Sky Guider Pro" head. These only have a Right-Ascension motor (no equatorial... you just manually re-point the camera. You could use, for example, a ball-head attached to the tracking head for this). The counter the rotation of the Earth. You use a camera with traditional camera lenses and these allow for long exposure imaging and do not require an auto-guider. If you own a decent photo tripod and camera, then the tracking head is typically in the 300-400 USD price range.

This option is far more forgiving (easier) than starting astrophotography with a long focal-length telescope.

Planetary Imaging

I didn't previously mention this because you did not specifically ask about it. But planetary imaging does work well through an SCT and does not require auto-guiding or an equatorial mount. This is because the planets are bright enough that they are best captured via video (e.g. about 30 seconds is typically long enough). Tiny sensors are more than large enough to handle this -- such as the ZWO ASI120MC-S at around 150 USD. But if you have deeper pockets... an ASI290MC or an ASI178MC, etc. are even better options (in the 300 to 400 USD price range).

Summary

  1. Be aware of the issues of using an alt/az mounted SCT telescope without an equatorial wedge and the need for a guiding solution.

  2. The Canon EOS Ra is a full-frame camera modified for astrophotography, but pairs well with telescopes that can provide a well-corrected flat field with at least a 44mm diameter.

  3. A dedicated CMOS astrophotography imaging camera with an APS-C size sensor would certainly be a huge advantage and ... for not significantly more than the price of the Canon EOS 90D. A dedicated CMOS astrophotography imaging camera with a Micro 4/3rds sensor would be even more affordable (less than the price of the Canon EOS 90D)

  4. A tracking head for a photo tripod is significantly easier and more affordable than all the gear acquisition needed to use a high focal length telescope.

  5. Planetary imaging does not require auto-guiding or an equatorial mount ... and the planetary imaging cameras are significantly more affordable.

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  • $\begingroup$ Thank you for your very thorough explanation, Tim. I appreciate you taking your time to properly explain this to me. For now, I'm just dipping my toes in however I shall keep your comments in mind for future expansions hopefully! $\endgroup$ – Mark Pace Aug 30 at 18:14

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