Ok, so this is just my personal experiment, hoping to help out some people who just can't get a good enough image, regardless how good they guide or what post processing they do.
If your image ends up noisy, even after plenty of subs, read on.
First thing first, to help yourself just get Sharpcap Pro and familiarize yourself with the software. I started out my astro-journey using Sharpcap Pro, after I spent time doing plenty of visual observing only.
My first stop before I image (unless I use the Asiair) is still Sharpcap Pro- doing it for first time focusing, sky brightness measurements and other stuff.
So, the first thing I always do when I get a new camera is do a sensor analysis. It can take some time and some practice to get the right amount of light and so on, but it's so worth it.
Here is what I'm taking about, related to the title:
I see people just copying what others do, regardless of which camera or what their sky conditions or what filter is used. While that could be a good starting point, but that won't guarantee that you gonna end up with a good image regardless.
Let me show you a demonstration with my QHY183C camera. I'm doing this during daytime so the exposures are tiny, but it's the same thing at night with longer exposures.
The general consesus for this camera is "use gain 11 and offset 30"- Yeah ok.. except the fact that the 183C sensor doesn't behave like the 533 or the 2600. While it has a "unity" gain, that doesn't matter, because unlike the 533 or 2600, there is no low gain conversion versus high gain conversion stuff happening, there is no "step" in the analysis, the camera doesn't gain back dynamic range, neither the noise have any sort of "step-down" , but it's a gradual decrease.
Ok, so regardless, see the picture below what the camera shows "when the histogram is a bit to the right and the histogram bump is a nice line".
Oh cool, right?
I'm not over exposing and my histogram is not "hanging off" the left edge, must be good settings! NOPE.
The correct would be this one, but let me get to it.
Just for a demo, the Green bar on top is the "good" signal to noise ratio area, the red and orange are the not so good area. It's dominated by more noise than signal. Your histogram bump should be past the red and orange area.
Look carefully what the smart histogram is showing you.
I made 2 more screen captures about the red and the orange area of the histogram what it represents. Please ignore the bottom green/light green bar, that's for the 8Bit information only, not the 16, we don't care about that right now.
Read the the pop up note from Sharpcap: That's the red area.
So basically having your histogram bump in either of those areas would mean, that you are capturing more noise than signal. You're wasting a lot of your time and pumping out subs that have the noise dominate over the signal and you can stack the heck out of them, you will get an image, but you could have gotten a better image if you would have followed the recommendation of the software.
Let's say I'm a free thinker and don't care about what anything tells me and instead of increasing the exposure as recommended, I increase the offset.
Look what happens.
Did that fix the problem?
I just increased the area (cutting down further on the dynamic range) that will be dominated by noise mostly. The histogram bump moved to the right and without seeing this smart histogram, I would assume I fixed my bottom noise issues, but I did not, I just reduced the capability of the camera even more. My histogram bump is still sitting in the orange zone (which is dominated by noise and not signal).
I'm gonna be a free thinker again, and this time I'm gonna increase the gain! That will fix it!
Here is what happens, after bumping up the gain:
Dit that fix my noise dominated image?
Nope! The majority of the signal (mostly the faint) is still sitting in the red and orange, the gain only amplified the little signal and majority of the noise- hence the gain is called amplification, but does not increase photon input)
So, let's listen to Sharpcap pro, and instead of messing with the gain and the offset, let's do a sky analysis. Again, I'm doing this during day time and the exposures are very short, but it's the same deal at night, except the exposures gonna be longer and the recommendations will be those longer exposures . So, it's just the matter of scale and brightness, but it's the same thing.
I do the sharpcap sky analysis (my garage wall inside), and I want to go for max dynamic range. I want low noise, so I chose 1%. (5% or 10% would be more realistic for this camera, but for the sake of demo I chose 1)
Note, that it recommends me to drop the offset to 12, drop the gain to 0 and increase the exposure. This will vary for everyone, these numbers (again daytime here) will be different for everyone and every camera. Even cameras of the same model and brand will vary to a certain number and only after local - at your home - sensor analysis can tell you the right numbers at that particular sky brightness you happen to have - wherever you are imaging.
(Black level = Offset)
See what happens when I apply the settings,
Take a note of the histogram bump(s), since I'm using a colored camera. Note that they moved out of the red and orange zone, the majority of signal is outside the camera noise level. This is the optimal gain and offset and exposure length to capture the DSO I'm going after (in this case, my garage wall inside)
Now the faint stuff is no longer buried in the noise, the dynamic range will be the max without clipping the darks.
BTW, I stopped following this idea that either NINA or whatever else telling me how many stars are over exposed. NINA shows a unlinked stretched image and the histogram is showing of an average of 3 colors if it's an OSC camera. While it's cool to have it, it's not telling you the whole story.
And if you got a camera with an amp glow, then forget about counting your overexposed pixels, because the amp glow is also calculated in and will mislead you for sure, whether you are under or over exposing. Stars peaking through an unstretched image is still fine, as long ast they are super bright to begin with.
BTW, modern cameras like the 533 and 2600 are much much better when it comes to signal to noise ratio so yes, you can do a lot shorter exposures than needed, but there is a limit there too how short.
If you disagree with everything seen here, please contact the person behind Sharpcap Pro- Dr. Robin Glover- over the sharpcap forums and tell him he is wrong.
Edited by unimatrix0, 03 October 2023 - 01:27 PM.