I have an old Celestron Celestar 8 that I would take out and do some viewing periodically. Yes I know I need a better scope - it is on my list of things to buy someday. I use it for small DSOs but mainly planet obs and some photography of the planets. I recently used all my loose cash to buy a new Canon R5 (for nature and family photography) and decided to do some DSO astrophotography with it as well. I started out with the obvious - Andromeda, N. American Nebula, Pleidies, Orion, etc. The Celestron is not going to work with its small field of view (it can just about fit the moon when I attach with a T-ring) but since it has a motor drive I polar align it - mount my camera on top with a 70-300 Canon Ultrasonic zoom lens (1:4 - 5.6) and point it at the DSO and set it to take photos for a couple hours. (I use a diffraction mask to focus) I can go 15-30 second exposures without startrails depending on the alignment.
I take a bunch of Dark, Dark Flat, and Flat frames - get rid of any bad ones - then use DSS to stack them.
According to clearoutside.com (an amazingly useful website for planning) I am under Class 7 skies (sigh) so there is a lot of light pollution. I deal with this in PS with a great plugin tool that I found related to the "Astronomy Tools Action Set". I go through the processing in PS according to the AstroBackyard Imaging Processing Guide.
And the results are.......meh
I include an example - - 280 minute exposure.
Clearly I am doing something wrong - maybe a couple things. I chatted with someone I know who gets much better results and here are the culprits we have come up with:
1) Using a zoom lens means going through a lot of glass. I should be using a small refractor that is made to take DSOs. Of course I would need a mount to go with it so we are talking $2k - $3k at least. And since I just spent a tidy sum on a new Canon that is going to be a hard sell with my wife. This may be it but I have seen other folks on the internet that have taken DSOs with zoom lenses and they seem to do much better than me. So I want to make sure it is not a different issue. Do you think this is what is holding me back?
2) I am really really bad at PS. Or at least there is something I am doing in the process that is overdoing it and I need a much gentler touch. Very possible. But I have fiddled around with the example files from AstroBackyard and gone through his process and they don't come out as good as his - but not too bad. So I don't think it is #2 - but I DO think I am screwing something else up - and in doing so I try to correct it in PS and I because the input is flawed the PS work goes wonky trying to fix it and make it look good.
3) My exposure of 30 seconds is too long. Maybe but I see pretty small star trails so I don't think this is it.
4) I am messing up DSS. Maybe....
5) This is my best guess - I am using too high an ISO. This picture uses an ISO of 6400. My thinking here is - high ISO more signal. Yes I know it also means more noise and a hotter sensor. My thinking is that the Canon goes to a crazy ISO of 52,000 or something and visual inspection of 6400 doesn't seem too noisy. And the alignment is not perfect so the DSO and stars are moving slightly from one frame to another so when I stack it I subtract the flats and darks and dark flats so that gets rid of the noise on the same pixels and the slowly shifting DSO & stars are on new pixels - so when you stack them the hot pixels will disappear (I think this is called dithering).
This sounds like it should work - but I look at what others use and they seem to all be using ISOs below 1000. But I am guessing that by using an ISO of 6400 I have too much noise and also too much light pollution and subtracting that out is helping kill my image.
So I am guessing - even though I have a Canon R5 with an amazing fast sensor I should bring my ISO way down. So my questions are - - which of the above do you think is the main problems? And if as I suspect it is #5 - what ISO should I be using? If I am really lucky and there is a Canon R5 person out there - - what ISO do you use as I figure it is camera specific what ISO is too high.
Thanks for your patience with this rambling question. I can add a photo of Andromeda which is even worse if that would help.