Capture gain

[morls (Stephen)]

Hi,
Melbourne has been cloudy, to say the least, but last night I went out around 8pm and it was clear. I set up very quickly, aligned terribly and the seeing was pretty average, but I wanted to do my first run with the camera

I used sharpcap, and was able to get a feel for the initial workflow. My question here is, what sort of gain is best at this stage, i.e. capture before processing? I was imaging moon, jupiter and saturn, using the default auto-capture in sharpcap. I played around with gain and contrast, and was struck by the results at low gain (Saturn). The clarity was excellent compared to others at higher gain, but the brightness (luminosity?) was much lower. So, a dimmer image but sharp.

When using the live view and histogram, is it better to set gain and contrast such that the image here is as good as possible, or is it better to run at lower gain and add more information in processing? (by add more information I mean stack more images)

Thanks
Stephen

[xelasnave]

Well no one who knows what they are talking about is offering a reply, which is unfortunate as I too would like answers to your questions.
For me it's trial and error. I am not getting serious until I get things set up permanent...I have important issues to address like auto guiding, camera tilt and distance between flattened and sensor for example not to mention until permanent polar if off as I can't use pole master because a couple more trees have to go...I just push the gain as high as I can get away with simply so the result has nothing blown out...and I just that's all we can do until we get a reply to your questions here.
Alex

[morls (Stephen)]

Cheers Alex. It's something I'd like to understand better.

I've got a lot of experience working with digital audio and good practice there is to capture at -20 to -15 dB full-scale, where 0dB equals max and anything over is clipping. Because my system works at 24bit and 32bit this amount of headroom is still way above the noise floor, and leaves a lot of room for dynamics. I'm not sure how these principles translate to higher energy/shorter wavelengths. Maybe well-depth is the equivalent to audio dB?

I'm also wondering if gain is the crucial parameter in capture, and contrast is something that can be worked with in post-processing. I guess I need to go and do some reading, find out how contrast is captured. Once I get the filters happening is it just a case of optimising gain in each of the LGRB channels, and the contrast is tuned afterwards through balancing other parameters?


These questions are in relation to lunar/planetary, I'm sure deep-sky is something else altogether...

[xelasnave]

I don't think the contrast is for anything other than your monitor display and any images won't be effected no matter how you twiddle with the contrast.

The reason why I suggest to have a trial and error approach was because all that I read had me believing that to capture HA I needed long exposures ... Being limited to 30 seconds at the time I started using the camera I was not even going to try HA until I had auto guiding running...glad I played around..I use gain settings that I bet folk here would say can't be used.. like I go up to 450 on occassion with HA and regularly capture at 350 in RBG..I bet folk say that is so wrong..but it gets me there...I know what you are saying however..when you figure it out I would love to have you post some guide lines for folk like me who are getting started.
Good luck.
Alex

[morls (Stephen)]

Originally Posted by xelasnave

I don't think the contrast is for anything other than your monitor display and any images won't be effected no matter how you twiddle with the contrast.

I think I might be confusing contrast with exposure.
My knowledge of imaging is very very basic, so sorry for the confusion. I'm going to have to do some reading I think.

[The_bluester (Paul)]

I was doing some reading up recently and one thing that stood out was that the lowest worthwhile gain is unity, whatever that works out to be for your particular camera. Essentially unity gain means one captured photon should translate to one step ADU, less than that and you essentially have a non linear response to actual captured light. After that, exposure times will come in to it as a higher gain would more quickly bring signal up above the noise, but if you are using longer exposures then brighter stars will saturate pixels more quickly as well so it is a trade off and depends on what you are trying to capture.

If you are using a gain for instance where it takes 1.5 captured and converted photons to increase the ADU count my one, one photon captured will be an ADU count of zero, two will be ADU1, three will be ADU2, four will still be ADU2, five will be ADU3, six will be ADU4 etc.

Basically to capture the data most effectively you want at least one step in ADU per photon captured and converted.

[doppler (Rick)]

Gain is your iso, brightness is the shutter speed. The image will be sharper at lower gain but the trade of is shutter speed. With low gain you need to increase the brightness which lowers the shutter speed or frame rate and higher frame rate is needed to capture the still moments of atmospheric turbulence when imaging the moon and planets.
On a very steady night you can get away with low gain and low shutter speed, but it's always going to be trial and error to find the sweet spot on any given night.

Rick

[Nikolas (Nik)]

It would be really helpful if you told us a little more, firstly you did not mention which camera you are using and what sort of target you are after, ie deep sky? Planetary? Moon? cooled or non cooled camera?
so many variables.

[morls (Stephen)]

Sorry, I'm using ASI120MM-S, lunar and planetary. Very much a raw beginner in all matters photographic.

[Nikolas (Nik)]

What you need to do is experiment, firstly it's a mono camera so colour is not an issue unless you are using filter wheels. For Lunar gain is on the lowscale and exposure on the higher side and capture in AVI or SER to get lots of frames.
For planetary the gain should be around half or more whilst capturing as many avi/ser frames as possible, the frame rate of the video capture needs to be fairly high which is why exposure is quick and hence gain is higher for planetary, keep an eye on the screen for best results, if it looks ok on the screen then the stacking software will do the rest. it's all a balancing game

[morls (Stephen)]

Thanks Nik.