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M45 - not as much nebulosity as I had hoped - advice welcomed
Attached is my capture of M45 with a 51mm W.O.'s space cat Petzval refractor mounted on an iOptron skyguider pro tracker. So, in APP, I processed 30 light frames (@ 120 sec per frame at ISO 1600) along with 20 dark frames + 50 bias frames + 25 flat frames + 25 dark flat frames. The dark and bias were shot immediately after the lights. So I am confident about those. But I took the flats and dark flats next morning using the "white T-shirt" method and pointing the scope up at a clear blue sky. The histogram of the flat looked good i.e. a peak very nearly centered in the histogram which I took as a good sign. I also applied light pollution correction, background calibration and star color calibration with the Tools in APP. But, as you can see, I am missing some nebulosity around some of the stars in the cluster which is what makes M45 such a nice thing to look at. What would this be due to? Not enough exposure time maybe? Lousy flats? (In future I will be improving on the flat issue with an Aurora flat panel.) Should I have taken more darks and more bias? Should I have shot the flats also in the same session? Although I wouldn't have thought that it would have made that much of a difference. Thank you in advance for any constructive comments.
The dark and bias were shot immediately after the lights. So I am confident about those.
This is not necessary, you can make those at any time you want. You do have to make them at a similar temperature, so it's nice to have a camera that's able to cool. Otherwise, try to replicate similar conditions as the night you took the photo's. It's not something you have to be super precise about, I used to make my DSLR darks at 5-6 oC intervals and put them in a library. I made new ones at each season as average temperatures are quite different then. It will depend on your sensor, I had a Canon 6D which behaved very predictable and clean, so that can differ. APP also is able to scale a masterdark, if you don't have the proper darks for a certain session, but I prefer non-scaled, actual data usually.
What camera do you use?
The histogram of the flat looked good i.e. a peak very nearly centered in the histogram which I took as a good sign.
Regarding flats, it's very important to talk about what kind of histogram you're looking at. On the camera itself (e.g. on a DSLR) that histogram is non-linear and half-way may be too low. On a linear histogram, which you can check in software (e.g. in Sequence Generator Pro which I use for flat generation) you can see if it truly is at around 50%. SGP uses 16-bit values no matter what camera you use, so it's always a target of around 30.000 ADU. Very important as well is the focus, you should be in the exact same focus as when you were taking the lights, which is why flats are usually done either with the t-shirt method in the early morning without touching the setup. I personally like a flat-panel more, because I can always have the exact same illumination of my sensor instead of having to rely on the sky which can have clouds in it etc. But if it works well for you, that's totally fine. 😉 You can check the performance by looking if dust is removed properly and the only gradient that is left is the sky background (nice gradient from bottom to top).
What would this be due to? Not enough exposure time maybe? Lousy flats? (In future I will be improving on the flat issue with an Aurora flat panel.)
Not enough integration time indeed. 30 x 120 s is just 1 hour total and the nebulosity is relatively faint, apart from the stuff just in front of the stars. It might also help to increase exposure time per sub a bit, and in total you would look at closer to 80 or more frames. Also have a look at your background, it contains a clear noise pattern, this will be less when you dither in between your lights (say every 2 or 3 lights a good dither). I'd also advice to go for more darks, make a masterdark of at least 40, preferably more and create a BPM (Bad Pixel Map) from your longest dark and flats (switch that on when creating the master calibration files).
Definitely not enough integration time. You did not mention light pollution. This can have a huge impact on how much total exposure you need. I have Bortle 5 skies (suburban). Using my William Optics ZenithStar 71 f/5.9 and Atik 314E, I need 10 hours of integration time for "bright" objects like M81 Bode's Galaxy.
Every combination of telescope, camera, observatory, and DSO must be taken into consideration. A good rule of thumb is "fast scope, dark skies" gets you the best image in the least time. If you are trying to image a faint target in light polluted skies then you've hit a "wall". No amount of integration time will give you the image you truly desire. In this case search for darker skies within driving distance.
What camera do you use?
Canon Rebel T7i “astro modified” by Spencer’s Camera and with an Astronomik light pollution clip filter inside it.
this will be less when you dither in between your lights
I agree but my equipment is primitive. I do not have a way to send a signal to the scope to dither. If I am forced to I will have to manually slightly move the telescope to dither. It’s doable but a hassle.