Imaging DSOs with an f/8.3 'scope

[AstroMum]

I'm about to start dipping my toes into the AP pond and after a bit of research have decided on a used Canon 1000D or 1100D to go with my 'scope.

I know my f8.3 refractor isn't ideal for imaging DSOs, but how bad is it exactly? What objects would be suitable for me to try and conversely, which ones should I not even bother with? Am I better off just doing afocal photography?

I used the DSLR calculator here to work out the magnification one of the aforementioned cameras would give me combined with my 'scope which looks to be roughly equivalent to a 27mm eyepiece. However, I don't know how to work out what the actual field of view would be and I understand this would be the limiting factor.

Thanks for any advice. I can't afford to spend any more on kit, but would love to make the most of what I've got.

[Cornelius Varley]

Try this Field of View calculator. Select your telescope on the left and your camera on the right and an object from the lists below.

Peter

[ollypenrice]

F8.3 is slow but not totally impossible. The bigger problem will be chromatic aberration which will cause stars to bloat, notably in the blue. You'll also need long exposures and that means accurate tracking. Your longish focal length will be more demanding regarding tracking.

Just have a go. A Bahtinov mask will help you to focus and you can make one or buy one quite cheaply. Try prime focus and see how you get on.

Olly

[RogerTheDodger]

Grab a copy of CCD from here http://www.assne.org/download.htm

Click update Database and add in your scope details, and the camera details, you can see the 1100d details below, the 1000d is as near as dammit the same.

As you can see it will give an indication of how big the object will be on your CCD and therefore the image.

[Shibby]

I have been reading up about the f/ratio myth a bit recently... I think all you need to know is that although the scope is capable, you will require long exposures if you don't want to be battling with noise all the time. So accurate tracking will be key, particularly with the narrower FOV you get with a long focal length.

The fringe killer can help with the blue halos you'll get, though I'm not sure how much you'd really want to invest in this particular scope for AP.

Planetary nebulae spring to mind as they are normally small and bright (meaning your exposures won't necessarily need to be quite as long). Also, the heart of the Orion nebula might be a good target.

[AstroMum]

Thanks for the tips and tools everyone, looks like I might do alright if I choose my targets carefully and work with long exposures. I'm glad I invested in a motorised mount!

If I find I really enjoy imaging, I will almost certainly invest in a more suitable 'scope. I don't want to do that yet as I don't have the funds and I want to determine what objects are most interesting to photograph. For all I know, I might enjoy doing the good old moon the most.

[Tinker1947]

Get your mount Polar aligned, pick some a bit bright Pleiades are high and bright, put you camera on the focus tube with nothing else but the T-Ring/Adapter, then give if different lengths of exposure, expect to spent a lot of time getting it right..

[Catweazel]

Hi again! I have just been running a thread about doing the same with the 200P, problem there seems to be that the scope is Ok but the mount may well struggle. Didn't really think about the Evostar due to f8.3 issue, but like you say, it may be ok for moon, although I get yellow fringing type CA. As Olly said, have a go at it, I'm going to; what's the worst that could happen - blurred pictures

[AstroMum]

Absolutely, I've got the equipment (minus the camera, but I'll be buying that whatever I do) so it doesn't hurt to give it a try.

Tinker, it's certainly going to be a tough learning curve. My husband used to teach photography, but he's rather rusty now and doesn't have a clue about AP! Hopefully I'll get the hang of it eventually....

[Ags]

Perhaps you or your husband has some nice 100 or 200mm lenses from his photography teaching days? These would be ideal for imaging!

[ollypenrice]

^^ Dead right. Camera lenses and a decent mount are superb.

Beware the F ratio myth. If you pin the focal length of any two telescopes there is no myth when you compare them.

Olly

[AstroMum]

Some nice lenses would be awesome, but it was over a decade ago and my husband has long since sold all his kit.

If I had the money now, I would seriously consider skipping the scope completely for imaging and use a camera exclusively.

[brantuk]

Have a read of this Becky - it's a very good starter guide to using Canon dlsr's for astro imaging.

Astronomiser - Automated Astronomy and AstroImaging Solutions

[AstroMum]

Thanks for the link, I have bookmarked it and will have a read in the morning.

[dph1nm]

What really matters is the pixel scale of your camera. You can eliminate the problems of longer focal lengths by using a camera with larger pixels. An f8 scope with 8um pixels is basically an f4 scope with 4um pixels - same exposure times, same tracking errors. Unfortunately most DSLRs seem to have similar pixels sizes these days, so if you are going down the DSLR route your are a bit limited. You can bin up your pixels, but again, due to the bayer matrix, DSLRs are not ideal for this.

NigelM

[Catweazel]

Have a read of this Becky - it's a very good starter guide to using Canon dlsr's for astro imaging.

Astronomiser - Automated Astronomy and AstroImaging Solutions

every time I look at one of these sites about AP it scares the bejesus out of me , the amount of scopes & equipment being used in some of those pictures is staggering!

In my thread about doing this with the 200P, I gave up as the old style 200P focuser won't focus with the camera apparently (if that makes sense). I might give it a try with my Evostar, but, other than that, it does seem that the cost is rather prohibitive if you are looking at getting any decent results. I need to start playing the lottery!

[AstroMum]

What really matters is the pixel scale of your camera. You can eliminate the problems of longer focal lengths by using a camera with larger pixels. An f8 scope with 8um pixels is basically an f4 scope with 4um pixels - same exposure times, same tracking errors. Unfortunately most DSLRs seem to have similar pixels sizes these days, so if you are going down the DSLR route your are a bit limited. You can bin up your pixels, but again, due to the bayer matrix, DSLRs are not ideal for this.

NigelM

That's rather interesting. Do you mean I should perhaps go for an older camera with a smaller megapixel count? If so, any in particular you would recommend?

every time I look at one of these sites about AP it scares the bejesus out of me , the amount of scopes & equipment being used in some of those pictures is staggering!

In my thread about doing this with the 200P, I gave up as the old style 200P focuser won't focus with the camera apparently (if that makes sense). I might give it a try with my Evostar, but, other than that, it does seem that the cost is rather prohibitive if you are looking at getting any decent results. I need to start playing the lottery!

Yes, it is a bit scary and and a whole different beast to plain old visual astronomy! I've actually started to consider buying a small apochromatic refractor for imaging if my appetite is sufficiently whetted. The other option is to go for a focal reducer, but that seems to introduce possible new problems and isn't exactly cheap either.

I'm going to see how I get on with the Evostar. Taking into account what I've seen online, I should be able to get decent shots of smaller DSOs such as M57. I would really love to photograph M42, but I may get just as much enjoyment out of the smaller objects.

[Shibby]

That's rather interesting. Do you mean I should perhaps go for an older camera with a smaller megapixel count? If so, any in particular you would recommend?

Actually, out of the 2 you were considering, the 1000d is 10.1mp vs the 1100d's 12.2mp!

[Catweazel]

I've actually started to consider buying a small apochromatic refractor for imaging if my appetite is sufficiently whetted. The other option is to go for a focal reducer, but that seems to introduce possible new problems and isn't exactly cheap either.

Me too, which leads me to another one of my silly questions: I notice that all of the little APO's have mounting brackets designed for camera tripods? Do you then need to get together some kind of scope ring/dovetail arrangement if you want to use it on a EQ5 type mount? I'm guessing that these are fairly costly

I'm going to see how I get on with the Evostar. Taking into account what I've seen online, I should be able to get decent shots of smaller DSOs such as M57. I would really love to photograph M42, but I may get just as much enjoyment out of the smaller objects.

Hmm..,I might have a go at this as well, I have the Canon 1000D, so will try to get it to focus with the Evostar using T-ring; no idea if it will work Like you say, smaller DSOs are a more likely target to produce a half decent result. It's finding them that's the problem

[AstroMum]

Actually, out of the 2 you were considering, the 1000d is 10.1mp vs the 1100d's 12.2mp!

Indeed, that's the main reason I'm considering the 1000D. I'm also tempted by some of the newer features on the 1100D, particularly the increased ISO limit.

Would love to get my hands on one of the older 20Da cameras, but they seem rather hard to find and hold their value well.

Me too, which leads me to another one of my silly questions: I notice that all of the little APO's have mounting brackets designed for camera tripods? Do you then need to get together some kind of scope ring/dovetail arrangement if you want to use it on a EQ5 type mount? I'm guessing that these are fairly costly

Hmm..,I might have a go at this as well, I have the Canon 1000D, so will try to get it to focus with the Evostar using T-ring; no idea if it will work Like you say, smaller DSOs are a more likely target to produce a half decent result. It's finding them that's the problem

I was looking at the OpticStar ED80S Gold which comes with a dovetail bar and rings. Seems a similar price to other 80mm APO fracs which don't typically include those accessories as standard.

Do let me know how you get on with imaging, I'd love to hear if you get some decent photos.

[dph1nm]

That's rather interesting. Do you mean I should perhaps go for an older camera with a smaller megapixel count? If so, any in particular you would recommend?

Well - I guess the trouble is that the newer cameras (and unfortunately the trend with pixels seems to be to get ever smaller - the old Canon 300D had 7.4um pixels - the latest 600D has 4.3um ) tend to have better thermal noise control, so it swings and roundabouts really.

Astro CCDs are (somewhat) better in this respect, but it is still hard to find one well matched to large focal lengths.

NigelM

[jgs001]

Becky, I invested in an RC scope last year to get some longer focal length imaging with my dSLR. Not had a lot of chance to use it, but even at f/9 I've produced results with it. You just need to be patient and have good tracking. (1370mm at f/9 is a heap load better than 1200mm at f/15, I've tried that too).

[ollypenrice]

I wouldn't get too hung up on the 20Da. They are roundly outclassed by more modern and basic DSLRs, really because the noise is so much less on the modern ones.

Olly