How Do I Decide on Focal Length for Astrophotography?

It's amazing how you can be heavily involved in photography for decades, but when you start getting interested in astrophotography you begin to feel like you're completely starting over.

Anyway... First, I realize I have a LOT to learn before I plunk down money for a telescope but I'm trying to get my mind around focal length as it relates to this field of photography. BTW, I do understand the importance of aperture as it relates to image quality but right now I'm more curious about field of view. I'm wondering what would be the best mirror size for each type of celestial object. In other words, assuming I wanted to shoot the moon, is there a focal length that would more than fill a full frame DSLR such as a 5D III or 6D? I''m looking at a Celestron 11" Schmidt–Cassegrain (2800mm, f/10), I assume that would be great for planets and deep field objects but is it too much for the moon? Would an 8" (2032mm, f/10) be a better all around telescope?

Or, am I asking the right question?

It's a combination of the focal length and sensor size which creates your true field of view.

Assuming an APS-C size sensor DSLR camera is attached to the scope...

The moon is about 1/2º from edge to edge... a 1500mm focal length creates a field of view which is .6º in the narrow direction so that would be a tight fit... Maybe relaxing the fit to about 1200mm would look good. Even 1000mm looks pretty good and you can crop slightly.

The Andromeda galaxuy is fairly wide... A little over 3º from edge to edge (about 6x wider than the moon) and that works well with a scope at about 500mm focal length.

Some objects are very tiny. I shot the dumbbell nebula (M27) using 3900mm to get that to fit nicely.

The bottom line is there's no "best" focal length because it depends on the object. You can use focal length multipliers or focal length reducers to help. But there's no one scope that's best for everything.

I use Starry Night Pro Plus 7 and overlay field of view for my telescope, camera, focal-reducer/multiplier combinations to determine what to use to shoot a target.

A full-frame sensor camera like a 6D (Hutech sells a modified 6D that's had it's terrestrial filter removed so you get full-spectrum) or 5D III will get field vignetting (fairly severe) using a typical 2" nosepiece with a T-thread (clear aperture through a t-thread is about 37mm), but if you get a one-piece nosepiece (2" nosepiece has the camera's bayonet mount built in and doesn't screw together with a t-thread) buys you about 10mm more aperture (usually closer to 47mm) and helps reduce the vignetting problem.

Thank you! That is exactly the information I am trying to understand. Based on what I learned from what you've explained here, I did a little searching and found a website to calculate the answer to probably any question of this type for any celestial object. All you would need to know is the size of your camera's sensor*, what the angle of view is for the object you want to photograph (as you stated above for the moon, Andromeda, etc.). Using this information, the angular field of view calculator on this site will return the required focal length.

http://www.tawbaware.c​om/maxlyons/calc.htm

*Focal length multiplier (Canon)
Full frame: 1
APS-C: 1.6
APS-H: 1.3

I assume that somewhere there's a table listing the angle of view for virtually every object in the sky.

I do know of the tawbaware calculations site... but even easier is to use astronomy software that supports "field of view" overlays or "ocular" overlays.

On a regular computer I use Starry Night Pro Plus 7 (for a lot of reasons), but Stellarium is free and supports this functionality. I use Sky Safari Pro 5 on my iPad and it also has this functionality.

For any of these programs you enter the equipment (typically you enter a list of scopes, a list of of eyepieces, a list of cameras (basically it needs to know the sensor dimensions), and a list of any focal length multipliers or reducers.) You can then turn on an overlay for, say, any scope/camera/focal length multiplier/reducer combination and it will draw the area of the sky that would be visible in the sensor.

Camera sensors can be rotated (it'll probably initially show a perfectly level frame, but you can rotate that frame to determine the best composition of an object.)

The field of view overlays makes it really easy because you can actually "see" what should show up in your frame.

I will definitely check into these software packages. I've always assumed the software is something you worry about AFTER you've chosen a telescope. Mainly because I thought software was primarily for controlling your telescope. Clearly, I've got a long way to go. Thank you for the tips. I'm open to as many as I can get.

Phil Light wrote in post #17920489
I will definitely check into these software packages. I've always assumed the software is something you worry about AFTER you've chosen a telescope. Mainly because I thought software was primarily for controlling your telescope. Clearly, I've got a long way to go. Thank you for the tips. I'm open to as many as I can get.

considering the scope you mentioned in your opening post, you should be getting ( it will most likely be) a goto mount system.
That will take care of the tracking and object location

having software like Stellarium, Sky Safari etc on your PC / ipad etc just help you decide what objects are above your
horizon at any given time of the year to help you choose which ones to observe / image

Dave

Thanks Dave. Over the past few weeks I've been immersing myself in information, trying to get my head around this stuff. I still don't know that I completely understand how to determine what focal length that would be the best, but I do understand that like you said, I need a computerized equatorial mount with goto capability (that might be redundant). It also seems like having a finder scope with an autoguider can really help keep the scope rock-solid for long exposures. I'm also weighing the pros and cons of Schmidt-Cassegrains vs. Newtonians. Based on what I've learned so far, with regard purely to astrophotography, an SCT seems to have more advantages than a Newtonian.

I think my confusion with the focal length is that the only thing I've got to compare it to in my experience is prime vs. zoom lenses. Telescopes seem to have a fixed focal length like a prime, but not if you take into effect the different eyepieces you can use. This lets you in effect, "zoom" the telescope. But you don't use eyepieces with a camera, right? So you're limited to whatever focal length the telescope is - IF I understand correctly.

Achromatic refractors will give top-notch image quality and typically shorter focal lengths (around 500mm).

Newtonian reflectors often have focus issues unless you get a Newtonian that was designed to be used as an astrograph (a scope made with astro-imaging in mind). When you image with a newtonian, you'll usually end up with stars that have the 4-point diffraction spikes and these are caused by the spider vanes that support the secondary mirror at the front of the scope. Refractors and Schmidt Cassegrains don't have spider vanes so they don't have these diffraction spikes (some people love the spikes... some people hate them.)

Schmidt Cassigrains are sometimes called "Compound" telescopes (they're in a category called "catadioptric" which simply means the telescope uses both lenses AND mirrors (that front piece of glass that looks flat... actually does have a shape to it. It's called a "corrector plate" and it's technically a lens -- not just a piece of flat glass.) But these scopes typically have longer focal lengths. They're also usually higher focal ratios. An 8" f/10 SCT (very common) would have a 2000mm focal length. I have a 14" f/10 and it has a focal length just of 3556mm. This means we see a very very tiny piece of the sky. Often we use something called a focal reducer to show us a wider area of the sky and decrease the focal ratio. But the other thing to keep in mind is that the higher focal ratio implies that you'll need longer exposure times (so all your sub-exposures take longer to collect.)

When using a high-focal length scope, instead of the auto-guider being in a separate guide-scope, it's usually in something called an "off axis" guider adapter.

Here's a link so you can see what it looks like: http://www.telescope.c​om …hy/c/4/sc/60/p/​102812.uts

Notice the tiny little pick-off mirror in the edge of the frame? Most light passes through straight back to the camera. The pick-off mirror is on the edge of the frame (out of the way of the image sensor for most cameras -- including even an APS-C DSLR) and it bounces light 90º out to the side where you have our guide-camera. Some spacers are needed to make sure that the distance from pick-off mirror to the image sensor on the guide-camera is identical to the distance from the same mirror to the main imaging camera (because both cameras need to be in sharp focus.)

When using a separate guide-scope, the focal length of the guide scope should be some respectable fraction of the main imaging scope. Typically it should be at least 1/3rd of the focal length (1/2 of the focal length would be even better.) That means if you had, say, a 600mm focal length imaging scope, then you'd want your guide scope to be at least 200mm.

It is "easier" to image at lower focal lengths. All the stuff we say about the quality of the mount being even more important than the quality of the scope... is especially true when you get to very long focal lengths. Said differently: If a person were to be planning to spend less than $1000 on a mount... then I wouldn't go with a long focal length scope.

TCampbell, I've been reading and re-reading your information ever since you posted it, and looking for additional information regarding your advice. (Side note, even though I've been a member of this forum for nearly 10 years, I am constantly amazed by the level of friendly, technical knowledge freely shared here. I don't know what I'd do without it. THANK YOU!)

Diffraction spikes - if they're good enough for Hubble, they're good enough for me.

Thank you for the deeper dive on the 3 major categories of scopes. This helps.

I had never heard of the off-axis guider adapter. That sounds like an outstanding idea. I'm almost surprised that something like that wouldn't put a standard auto-guider in a separate scope out of business. I wonder if there are any downsides to it.

I hear what you're saying regarding the sub-$1000 mount. It's really the same argument that I've used for buying a high-quality tripod first, rather than starting cheap and working your way up to a good one; a waste of money in the long run. So I definitely don't want to "under-buy" to begin with. But, I've seen all kinds of great reviews and testimonials of the Celestron Advanced VX Equatorial Mount. (http://www.telescopes.​com …anced-vx-equatorial-mount). Thoughts?

Jeff

Phil Light wrote in post #17928083
TCampbell, I've been reading and re-reading your information ever since you posted it, and looking for additional information regarding your advice. (Side note, even though I've been a member of this forum for nearly 10 years, I am constantly amazed by the level of friendly, technical knowledge freely shared here. I don't know what I'd do without it. THANK YOU!)

Diffraction spikes - if they're good enough for Hubble, they're good enough for me.

Thank you for the deeper dive on the 3 major categories of scopes. This helps.

I had never heard of the off-axis guider adapter. That sounds like an outstanding idea. I'm almost surprised that something like that wouldn't put a standard auto-guider in a separate scope out of business. I wonder if there are any downsides to it.

I hear what you're saying regarding the sub-$1000 mount. It's really the same argument that I've used for buying a high-quality tripod first, rather than starting cheap and working your way up to a good one; a waste of money in the long run. So I definitely don't want to "under-buy" to begin with. But, I've seen all kinds of great reviews and testimonials of the Celestron Advanced VX Equatorial Mount. (http://www.telescopes.​com …anced-vx-equatorial-mount). Thoughts?

Jeff

I went through two mounts that I thought would be adequate, but underperformed, before deciding to stop messing around and get some thing that was really solid. I ended up with a Losmandy G11 mount and I've been extremely happy with the tracking.

I'm not sure if you've seen my Andromeda (M31) image (I've posted this before) but this is composed of all 8 minute "light" exposures ... but this was unguided.

IMAGE LINK: https://flic.kr/p/AZ1K​5j

TCampbell wrote in post #17928516
...............
So you don't necessarily "need" a guider and a guider isn't a substitute for having a nice precise polar alignment (my good fortunate was that I had spent time working on the accuracy of the polar alignment.)

...............

yup, that is all good for a home observatory based scope

but if you are like me and others, where light pollution and or a rental accommodation situation doesn't allow for that and we have to travel
out to dark sites to do anything worthwhile, an auto guider helps a lot when it would take ages each session to try and polar align accurately

That said, there's a wonderful new device I have only yesterday learnt about .....

http://www.qhyccd.com/​PoleMaster.html#PoleMa​ster

designed to take the pain out of polar alignment when setting up in away from home locations
A fellow astro-tog from South Australia says he can set up polar alignment within minutes with it


Dave

TCampbell, again, excellent information and extremely helpful.

Years ago, on the wall of my kids dentist office, I saw a shot of Andromeda almost identical to yours. I was blown away. THAT shot was what put the bug in my brain that someday, I HAVE to learn astrophotography. Your shot is the holy grail for me. If I can eventually get something like that (of M31), all of the expense, education and work will be worthwhile. (And thank you for posting that. I had not seen that before.)

Regarding what Dave said, I am in the same situation. I live in Indianapolis and will always have to travel a fair distance to find dark skies. From everything I can gather, that probably means an autoguider will help due to the fact that I'll have to be constantly setting it up and tearing it down. I will never be able to just dial it in and leave it. Needing to travel like that every time I want to shoot something is the only real bummer, and one of the reasons I've talked myself out of this for years.

Dave, thank you for the link. I'd like to know what your (or anyone's) thoughts are on this automatic alignment device. http://www.telescopes.​com …y-for-automatic-alignment. I'm sure these things don't take the place of needing to understand and perform polar alignment, but it sure seems like it could cut down on time spent doing it.

Phil,

Thanks for sharing the story of seeing the image of M31 on the wall at the dentist's office. For me (and I'm in my 50's now) it was a visit to a science center when I was 19 years old and they happened to have a clear sky and they opened their observatory. I saw Saturn through a telescope for the first time and I was hooked. It's amazing how astronomy images work like a highly addictive drug... after the first taste you always want more.

Early on, it helped me when another astronomer broke down the idea that astrophotography is really TWO things...

1) Image acquisition: This is the part where you learn how to refine the techniques of aligning your telescope mount, scope, auto-guider (if an auto-guider is even being used) and camera to "acquire" enough good images of your subject to create the image you want. It also includes the collection of "dark" frames, "flat" frames, and "bias" frames. It's basically all the "data collection".

After you collect all the data, make sure you back this up (because you will likely re-use the data more than once.)

If you can collect enough data, then you should be able to create the image you want.

2) Image processing: This has two sub-parts...

2a) your image data has to be calibrated -- in other words you have to use your darks, flats, and bias frames to "calibrate" your light frames... and in order to stack the light frames they have to be "registered" (aligned -- using stars in the image to make sure they all stack up neatly) and then integrated to create your fully calibrated and integrated "master light" image.

You also have to neutralize and extract the background (get rid of the muddy background to make a dark background) and "white balance" (color calibrate) the image. These are somewhat "mechanical" steps.

2b) At some point the basic processing steps are done and now it's time to "stretch" the image data and start teasing out the details. This part of the process is perhaps less about "science" and more about "art". You want the image to look pleasing.

When I processed the M31 image I posted above, I worked the image... wasn't happy... I scrapped my work and started from scratch and re-worked the image... wasn't happy again... started from scratch a 3rd time and re-worked the image again... and finally got a result that I felt I could work with. There was some trial and error because some of the processing decisions I made early on in the workflow would have a big impact on how things turned out much later in the workflow (but weren't visible early on.)

I make pasta from scratch and my friends ask me how I learned it... I tell them you have to ruin 5 or 6 batches of pasta dough before you learn the "feel" for the texture of the dough. Learning the "artistic" part of imaging processing is a bit like this. You can follow a basic recipe and it will get you some results but you may not like those results... no worries... scrap your work and re-do it (this is why I suggested you back up the data after you acquire it). Each time you do, you'll start to pick up on little nuances and you'll learn how small decisions effect the final result.

My advice would be to start by learning the how to acquire good image data. Clear, dark, moonless nights may be infrequent... the clouds may move in on you before you finish acquiring all the image data you want (but you can always collect more data the next night, or the next new moon... or even the next year). But once you've got that data locked in... you have all the time in the world to work on processing it. (This, btw, is why it's not a good idea to rush the task of image acquisition. You do want to make sure you are collecting good quality data.)

When you've got the good quality image data in the bag... you're ready to start learning image processing. Just like the pasta dough.... don't expect the first batch to come out to your liking. But since you've saved your original data, scrapping your work and reprocessing from the beginning is no big deal. After a half-dozen "batches", things will start to click and you'll be getting results more to your liking.

One little thing... a very common mistake is to believe that you don't need a good precise polar alignment with the mount because you've got an auto-guider to correct for tracking error. Don't be sucked in to that trap. The auto-guider will struggle to save your images if your mount alignment is poor. If that means you have to spend an extra 15-30 minutes to make sure you've got a good polar alignment and make sure you've got good focus... then that's time well-spent. Because you'll be really disappointed if you spend 2 or 3 hours capturing data only to discover that your focus is mushy and your stars are all elongated because you rushed through the boring bits of mount alignment and camera focus.

There's no way you can know this but I'm reading and re-reading your posts dozens of times. Thank you for all the valuable information. I'd also like to make sure you know, I completely understand and agree with (of course) your point on polar alignment. I realize that just about everything that happens beyond that is determined by the accuracy of that step. Your M31 image is a testament to that. I'm spending most of my "research" time reading and watching tutorials on how to polar align an EQ mount. It's actually even starting to sink in. I'm also beginning to understand that it's probably not as daunting as it sounds at first, however, I'm sure I'll still be a little intimidated the first few times I do it.

I'm holding off on dropping a large chunk of $ until I'm absolutely sure I'm getting the equipment that will be best for me to start with, given my budget. I don't want to get in over my head, but I do want to get something that I can grow into, not out of, and maybe something I can build on in the future. Hopefully, I will have made a decision within the next few weeks. I have however made (ordered) my first purchases toward this goal.

A plastic star chart: http://www.amazon.com …oh_aui_detailpa​ge_o00_s00,
and a book:
Astrophotography 1st Edition - http://www.amazon.com …oh_aui_detailpa​ge_o00_s01.

I'll have those tomorrow. This should keep me busy during the next few weeks while my equipment budget is burning a hole in my pocket. I also downloaded Stellarium and I've starting getting familiar with that. It's amazing that that program is free.

Last but not least, outside of photography, my other hobby is cooking. I started making my own homemade bread and pasta several years ago. Your analogy is spot on.