shane_c wrote in post #18260620
I've seen cameras mounted to telescopes using and adapter and I've also seen some piggybacked on the telescope but using its' tracking mount. Is one way better than the other?
Also, I was reading online that not all tracking telescopes can be used for photography because the type of mount would limit you to 20-30 second exposures. Is that true? Could it still be done, just that you would need more shorter exposures vs fewer long ones? What should someone look for in a telescope they want to use for photography?
Sorry about all the newbie questions.
BEEFY SOLID GEAR... that's what you want if you want to do astrophotography with a telescope. The mount is actually more important than the scope (not that the scope isn't important, but the mount is more important.) Do not short-change the mount or you'll start to run into all sorts of issues that give you buyer's remorse (I speak from experience on this topic.) If I had it to do over again... no matter how poor I was, under no circumstances would I consider any mount that costs less than $1500... and that's the bare minimum. If I could spend $2000... or 3000... or 4000... I would.
Then there's the scope.
There are three major categories of telescope design types... "refractors" (use all lenses to focus the light), "reflectors" (use all mirrors to focus the light), and "catadioptric" (use a combination of lens and mirrors -- sometimes just called "compound" scopes for that reason.)
In refractors, there are two sub-categories... "achromatic" refractors and "apochromatic" refractors.
When light passes through any glass "lens" the curvature of the lens surface acts like a prism and creates an undesirable side-effect called dispersion in which different wavelengths of light bend different amounts and technically focus at different distances. The simple method to correct for this is to use a 2nd element in a configuration called an "achromatic doublet". The 2nd lens element has a concave curve instead of a convex curve. It doesn't completely fix the dispersion problem and you will find that objects near the edge of the field of view (stars in particular) show some color fringing as the lightwaves are all focusing in slightly different spots depending on the color/wavelength. Achromatic refractors are typically very low cost.
The "apochromatic" scopes do a significantly better job correcting. There are lots of variations of the design. Some use extra-low dispersion glass (aka "ED" glass). Some use three elements. Some use both. You can tell the difference in scopes by the price tag.... whereas an achromatic refractor might cost a couple of hundred dollars... an apochromatic refractor is typically at least $1000 (at the low end) and the price goes up from there (they can easily cost several thousands dollars.)
For visual astronomy, the achromatic scopes are good enough because you'll almost certainly just center the object that you plan to observe. But for imaging with a refractor, you _really_ want an apochromatic refractor. You (and everyone else) will immediately notice the difference.
Reflectors don't have the dispersion problem because the light never passes "through" glass. The mirrors are glass but the reflective coating is on the front surface -- not the back surface as it is in most common mirrors. That's "good" because you don't have to deal with dispersion issues. But it's "bad" because that surface is extremely easy to scratch (so handle with care... never use ordinary cleaning products on the mirror.)
One major issue with reflectors is that most reflectors are of the Newtonian variety and the eyepiece goes on the side. The challenge when thinking about these for astrophotography is that if you're using a DSLR, there's about 50mm (roughly 2") of extra distance from the front of the camera body (the lens mounting flange) back to the image sensor. This is to allow room for the reflex mirror. The scope, however, is designed to bring an image to focus at a set distance... a 500mm focal length scope means that 500mm after the light bounces off the primary mirror, the image will come to focus. The internal space of everything is designed to deliver a focused image at the eyepiece. When you attach a camera, the focus plane is really about 2" farther back. To compensate, you have to rack in the focus by 2" and most focusing mechanisms don't have enough travel... they hit the limit as the image was only starting to come to focus and then you run out of travel.
There are reflectors designed specifically for astrophotography and all they really need to do is move the primary mirror forward to compensate (and possibly fractionally enlarge the secondary mirror). The scope can still be used for visual use if you add a 2" eyepiece extension on the front.
Both refractors and reflectors tend to be somewhat short focal length scopes... 500mm... 600mm... maybe even 1200mm... but they're usually less than 1500mm (otherwise it would need to be very large).
Compound scopes (sometimes astronomers call these "CATs" - short for Catadioptric scope -- such as Schmidt Cassegrain Refractors (aka "SCT")) generally have very high focal lengths... whereas an 8" Newtonian reflector might have a 1200mm focal length, an 8" SCT will probably have a 2000mm focal length. I have a 14" SCT and it has a 3556mm focal length. These CATs are good at imaging small things. They're great for imaging planets and small deep-sky objects such as galaxies or some of the smaller nebulae. For large nebulae the object usually wont fit in the field of view.
Just remember... the higher the focal length, the more intolerant the scope is regarding tracking errors or vibrations... it takes hardly any movement at all to smear the image at these high focal lengths. My suggestion... start with a shorter focal length scope first.
On the issue of mounts...
Even among telescope mounts there's the question of how well it handles the weight. Some low-end go-to mounts aren't that solid... wind would be enough to shake the scope & camera. There's also an issue of "balance" when you piggy back. The mount can struggle to move an unbalanced load because on low-end mounts the motors, gears, and clutches aren't very strong.
In "go to" category of mounts (mounts that can point to objects and track them automatically) there are mounts in an "alt/az" vs. "ra/dec" orientation and this is a really important difference for astrophotography.
"Alt/Az" or "Altitude & Azimuth" are the simplest to understand because they look like simple turrets... you can rotate in 360º and it tips up or down. Sounds simple enough. But there is a big catch.
The "catch" is that neither of two axes of movement are in any way related to how the Earth spins UNLESS you happen to be located on either the North pole or South pole (we can probably safely assume you wont be.) If you think about the rectangular "frame" being level to the horizon at sunrise in the northern hemisphere, the Sun does rise "straight" up... it move at a diagonal moving up and to the right. And yet at sunset, it's moving at a different diagonal... this time down and to the right. The Sun's axis isn't really change... only our perspective angle of the Sun is changing and that's due to the rotation of the earth. This means the axis of the sun must be slowly rotating through the day. I picked on the Sun, but it applies to every object in the night time sky. If you view the sky through the camera you wouldn't quite notice it because it happens very slowly... but a long enough exposure WILL notice it -- the sky appears to "twist" as the camera tracks. This "twist" is called field rotation.
Major professional observatories do use alt/az mounted telescope because it's easier to support the massive weight of these enormous scopes. So to counter act the problem, the camera is actually mounted on a device that can rotate the camera itself as the telescope tracks. This is called a "field de-rotator". The speed at which it has to rotate depends on where it is pointing in the sky. Objects near the horizon seem to rotate less than objects near the zenith. A computer has to calculate the proper rate of rotation (and it's constantly changing.) Meade used to sell such a device for their LX200 series scopes (it used to cost about $600) but it's a discontinued product.
Equatorial mounts are tilted over at an angle. The lower axis (called the "Right Ascension" axis) it tilted over at an angle that matches your latitude on Earth. So if you live at +40º north of the equator, then that means the celestial pole is 40º above the horizon. So the right ascension axis is tilted so that if you could draw the imaginary axis upon which everything rotates, that axis would go through the north celestial pole in the sky (a point very close to Polaris).
In doing this, the axis of the Earth's rotation and the axis of the mounts rotation are now EXACTLY parallel to each other. As the Earth spins from west to east, the mount spins from east to west (opposite directions) and they also rotate at exactly the same rate. This perfectly cancels out the spin of the Earth AND also does it in a way that you have no field rotation. You can image as long as you want (assuming a precise alignment) and get no blur caused by the movement or twist of the sky.
While it's a bit difficult to wrap your head around what an equatorial mount is doing... it's actually easier to use once you realize what it's actually doing (if your brain is still stuck in the alt/az world then it will frustrate you). The right ascension axis is actually move the scope or camera in a PERFECT east-west direction. In other words if think of Earth's lines of latitude and longitude -- except not on the ground... in the sky (we call this the equatorial grid) the axis is moving along those lines of imaginary "latitude" (east-west running lines).
The other axis is called the "declination" axis. It's simply adjusting how far north or south the mount is pointing. A declination of +90 means you're pointing at the sky directly above the north pole. A declination of -90 means you're pointing at a section of sky directly over the south pole. A declination of 0º means you're pointing at the sky somewhere directly above the Earth's equator.
In other words... moving in RA is simply an east/west adjustment and moving in Dec is simply a north/south adjustment.
While it's not the way our brains normally like to think of a two-axis mount (we like things to be level with respect to the ground we're standing on and the equatorial mount is basically "level" to the axis of the Earth itself... never mind the particular piece of Earth you happen to be standing on) this arrangement works out EXTREMELY well for astrophotography.
Camera "tracking heads" like those made by iOptron, Sky Watcher, Losmandy, and Astrotrac are all designed to be tilted on that angle (just like an equatorial mount) but they only have a single motorized axis -- the RA axis. Once your tracker is set up correctly with a proper polar alignment (so it's rotation properly cancels out the rotation of the earth) you can attach a ballhead to the tracker and this lets you point the camera to any piece of sky you want -- you don't have to point the camera at the north pole.
Here's a single 8-minute long test shot (I stopped down to f/8 with a 135mm lens) to get this. In reality I used f/2 and much shorter exposures to capture the data for this region of sky, but the 8 minute shot let me validate that my stars were not elongating ... which meant I had done a good job aligning the axis of my tracking head to the north celestial pole -- even though THIS region of sky is nearer to declination 0º (above the equator - so I'm not pointed anywhere near the north pole.)
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You can attach the camera to a telescope, but again, that telescope should be mounted on an "equatorial" mount. I used to think the Celestron Advanced VX mount would be good for this (often just abbreviated Celestron AVX") until I talked to guys who said it doesn't have decent bearings on the RA axis so it's a bit jumpy (not smooth enough tracking). Somewhere around the $1500 price point these mounts get high enough in quality that they're smooth and they can handle a respectable load as long as the gear isn't too beefy (Like a Celestron CGEM). The high end of these things are mounts made by companies like Astro-Physics... costing well over $10k. I use a Losmandy G11 mount -- about $3200... with enough upgrades that mine is probably nearer to $4k. But I was so impressed by many design features of Losmandy that this is what convinced me to buy he Losmandy StarLapse tracker (it's extremely well engineered, machine tooled, and built. There are no cheap components in it.)
It is MUCH easier to get a good alignment and tracking at lower focal lengths then it is with a higher focal length scope. At low focal lengths (e.g. scopes or big lenses around 500mm or so) you can get a pretty good alignment and almost not even need an auto-guider. With a very large scope ... e.g. 2000 or 3000mm or above, you pretty much must have an auto-guider because even with a perfect alignment, minor mechanical variations inherent in all mounts regardless of how much you paid for them) will show up in the images.