SharpCap Forums

Hello,

I just used my new Optolong L-Ultimate filter for the first time and decided to use the smart histogram to figure out the best exposure time.

I am using an ASI294MC Pro in Bortle 5 skies with an f/10 scope.

I set the min exposure to 1 minute and the max to 10 minutes. The results came back that it was unable to calculate an exposure for that length of time. But it did suggest a gain of 225 (read noise of about 1.4e) and brightness of 4 with a sky brightness of 0.02. When I adjusted the max exposure time to 20 minutes, it produced a result. Somewhere around 20 minutes. I don't think I can get good 20 minute subs. I ended up using 10 minute subs at 225 gain and brightness of 4. I haven't processed them yet, but the live stack image looked promising.

I understand the math and stuff behind the SH but how would adjusting my exposure time lower affect my subs and the final stack? I believe what will happen is that the read noise will be more significant in each sub but I would be taking twice as many so would that even things out? Is 18x600s going to work out the same (or close) as 9x1200s?

Thanks,

Keith

P.S. - Robin, my astronomy club posted a picture of mine on Facebook and you shared it. Thanks for that! It made my day...

Hi Keith,

the end result of using the 10 minute subs rather then the recommended 20 minutes will be that the final stack is a bit noisier than it could have been with the longer sub-exposures. Of course, that has to trade off with the other considerations like tracking/guiding being able to cope with the longer exposures or how much is lost when a satellite runs through a particular sub-frame. There's not much point in having a perfectly low noise image if all the stars are trailing because the 20 minute subs were just too long...

Mathematically, you will have the read noise in each frame as a more significant fraction of the shot noise (maybe 50-60% of the shot noise, rather than the optimal of <30%). That means that even with the larger number of subs, the final noise value comes out higher. The actual effect on the final noise level is not too large - perhaps 20% or so above the absolute minimum (which would be a single 3 hour sub!), and maybe 10% more than the 20 minute exposure would get you. If you start going shorter (halving exposure again), then the extra noise really begins to build.

I guess your image was the M51 from 8th May - a nice image showing good detail, and it's always nice to see a bit of explanation either of the target or of how the image was captured too.

cheers,

Robin