Exposure time recommendations for my setup [Deep Sky] Acquisition techniques · Rene Matjanec · ... · 36 · 831 · 3

Gaetano R. Cassarino:
I took 600" subs also with my EQM-35 mount. So your AM5 will definitely work. Just be sure to balance well your setup and also polar align the best you can.

So far my total guiding error is around 0.5". From what I have seen that is pretty good. I always take my time to get a good polar alignment. 

Do you have any advice/tips to know if I can keep increasing my exposure time? I read that my histogram should be 1/4 or 1/3 away from the left side. In my images, so far, it was always separated from the left, but never 1/4 separated. Should I increase my exposure time if that is the case?

Thank you for the replies.

Sorry man, just read your reply. At the moment I don't have my HD with me. As soon as I can I will show you how the histogram looks in one of my subs. It definitely has to be separated from the left side (if not you'd be clipping the black). When you take a sub, and open its histograms you'd see values expressed in ADU (for my camera the range is 0-65000) just be sure the lowest values are not 0. If you should be stuck on such values this is something you can adjust by using the offset. Anyway you could also take a look at some images here on astrobin and check what the histogram looks like...just consider that the histogram you'd see is referred to a final image so it has been modified, but you could get an idea.

Gaetano R. Cassarino:
I read that my histogram should be 1/4 or 1/3 away from the left side

I would think that would be wildly excessive. You just want to make sure that the left side of the histogram isn't clipped. If it is, add offset until it isn't. I run both of my 585's at zero offset. You have to remember that adding offset reduces dynamic range which matters on a 585's. You don't want to do it if it's not needed. There's a lot of factors that come into the choice of exposure time but it's not like daylight photography. You won't have a different exposure time for every object you photograph and depending on your conditions you might only have one with very few exceptions. Total integration time is much more important and even there it's pretty simple, as long as possible.

Just a quick update for everyone that took the time to respond and help me out.

I went to some darker skies for the weekend (Bortle 4) and did some testing. Sticking to gain 252 seems to work best for me. I took some test exposures (30s, 45s, 60s, 90s, 120s) with the Antlia Quad Band filter. From my testing 90s works the best with the filter and the darker skies. 

On the histogram, my minimum values were around 1300-1400, depending on the target Im shooting. The stars didnt seem bloated. When I took the gain down to 0, the exposures were noticeably more noisy, although I could get a longer exposure time without blowing out the stars.

All that being said Im pretty happy imaging at 252 gain with the camera and default offset (I read it is 50 on the ASIAir, but some sources said it was 0).

When I get back to my Bortle 8 balcony I will redo the tests there, but I think i will stay at 60s exposures for my balcony, and up it to 90s when Im at darker skies. 

Once again, thanks to everyone that replied and gave advice. You really helped me understand my camera much better. I will be uploading the final Pelican Nebula image to my gallery soon, so feel free to give it a look


Clear skies everyone!
ogchicken

Congratulations on the thorough testing.  I look forward to seeing your Pelican Nebula!

Regards
Neil

I just uploaded the image

Thank you very much for the kind words!

I really enjoy the ASI585MC and MM cameras and have been using them often lately, mostly with f/4.9 and f/2.2 optics but on occasion at f/7.

I run mine almost exclusively at gain 252 and offset 20. I originally ran offset at 15 but on some occasions this caused zero valued pixels at f/7. But my skies are darker than yours.

The 252 setting, although limiting dynamic range to about 11 stops, does the best job of recording dimmer dust structures etc. One would have to set gain down to about 125 or lower to get back up to 11 stops, but this is lower than unity gain. Imaging the brighter nebulae this way is fine, even down to gain 0 where the dynamic range is around 13, but realize that you are discarding true signal photons when imaging less than unity gain. This seems counter productive to maximizing SNR per unit of time on the sky.

As a starting point once focused on the target of the evening I run a series of test exposure times (for each filter I'll be using), to find what duration yields a noise floor of around 500 to 1000, and does not saturate more than a couple thousand pixels at the brightest stars.

Also, lower gains and more exposure time per sub puts more demand on tracking and guiding, whereas more short duration subs at 252 yields a higher percentage of stackable subs, and cuts down discards of longer exposure subs spoiled by clouds, aircraft etc. Many short exposures facilitate dithering to reduce the stack's noise floor and increase its spatial resolution if drizzle stacked.

It is simple thereafter to increase the realized dynamic range and SNR of the final stack by also imaging at shorter exposure times that that first setting, so you'll have bright stars without blown out cores. Or create a "stars" stack and a "dims" stack, discarding the stars from the latter stack in post processing. It is very common at f/2.2 to require very short exposures, 5-10 sec!

The camera has such low readout noise at 252 that individual broadband exposures longer than about a minute are not really necessary to capture dark details in my experience. For narrowband I've rarely exceeded 5 minutes and more often it's about a minute for Ha and two or three for other filters.

No matter what camera or optic I am using, I follow much the same strategy, albeit with whatever gain setting vs. read noise is best for that particular camera, and whatever exposure time yields a noise floor minimum around 500-1000 and less than a few thousand saturated pixels in the brights, with whatever optic is on the mount and whatever sky brightness I happen to be dealing with that evening. The same general methodology applies if binning.

I like the strategy you are using. For my last two images the noise floor minimum was around 1800-2300. I will try lowering my exposure time to reach the 500-1000 you suggested.

Since I’m new to astrophotography I haven’t yet gotten into dithering and drizzling.
I have a pretty well sampled setup with my RedCat71. I’m at 1.71’’ per pixel, with mostly average seeing at 2.5’’.
From what I’ve seen people mostly recommend drizzling if you’re oversampled. But there are also people who just always drizzle, no matter what.
I think I’m going to stick to what I’ doing now for a little bit longer. Just to get a better understanding of everything and then dive deeper into dithering and drizzling. 

When I started thinking about getting into astrophotography I knew it wouldn’t be straightforward, but it’s even more complex and interesting than I hoped for. I’m glad i started the hobby.

Thank you for the reply.
Clear Skies!