I had not really given much thought to how long my subs should be. I figured it didn’t really matter much. That’s until I started to come across some resources that not only did it matter, but that you could also figure out what the ideal sub-exposures are!
Method 1
The first video I found was What exposure time should I use?! Let’s answer that!.
This method recommended was to look at the histogram and the average ADU value, and just get “significantly more” than that value with your light frames. 10s? 30s? As long as it is higher than the bias value, then you are set.
This at least showed me that I was probably taking subs that were too long, but I was hoping for something a little more accurate.
Method 2
The next video I stumbled on was Deep Sky Astrophotography With CMOS Cameras by Dr Robin Glover. This was a recording of a presentation he did where he goes into depth about how to calculate the ideal sub-exposure time. I recommend everyone go check this video out.
It was eye-opening for me because I always though that I should just take longer and longer exposures to get the most light from deep-sky objects as I could. He addressed that and explains why that doesn’t work.
I tried to follow along and do all the math he laid out, but I ended up with a value of 42 minute exposures. That seems wrong. Re-doing the math I got 12 minutes. Still seems high. If you are really curious, think it broke down when I was trying to account for the filter and include a multiplier for that that made it go from 15 seconds to 12 minutes. It also uses a value from a tool on the Sharpcap website, which did not let me set the filter because the drop-down was not working.
It did say that for darker skies you will want to take longer exposures. I’ll remember that for when I get away from my suburban backyard.
Method 3
Now my interest was piqued. There must be something more precise than “just more than bias.” And the formula in the previous video seemed to be missing something.
I knew that some software like NINA and SharpCap would calculate the ideal exposure for you, so I went looking for how to do that in ASIAir. That got me to a blog post titled ASIAir Exposure Calculation. The author explained the goal of this exercise fairly well:
In astro imaging we want to preserve as much dynamic range as possible but expose the image high enough so we can get our skyglow far enough above the bias noise that we can absolutely send that noise to black in our final image and be able to process the fine detail higher above it.
Like Method 1, this suggests taking the average ADU value from your bias frame and then adding 1200 for color cameras (like I have). OK, this is simple, and I can use some trial and error to get that.
Method 4
Finally I got a forum post on CloudyNights. Here was a formula I could use that I was able to add my own values to.
The bit depth, read noise (e-) and Gain (e-/ADU) I was able to get from the manufacturer website. I still am not sure how to calculate the offset ADU, but I found another forum post that had my camera and set the offset to 30 for this formula when the bias average was about the same.
This ended up giving me a minimum exposure time of 163 seconds and a target exposure time of 264 seconds.
When I tried that, my average values in my light frame were just about 1200 above my bias, so using “method 3” as a sanity check, it seemed to work.
The only problem with this, is that it does not take into account the actual light pollution noise, which “method 2” deals with.
Summary
I think there is a exact optimum exposure time. I do not think I know what that is just yet. Sorry I do not have the answer… yet.
I think I did learn that I do not need to take really long exposures. Lots of short exposures are probably better. I’m going to stick to 3-5 minutes until I can really figure out the equations.