Sub Exposure Calculation

Discussion of using SharpCap for Deep Sky Imaging
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roboscience
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Joined: Mon Nov 23, 2020 6:08 am

Sub Exposure Calculation

#1

Post by roboscience »

Hello,

After many hours ( MANY!! ) of trying to get my astro imagining setup going I finally have a working situation. Right now I am trying to get my sub exposures dialed in, I spent 3 hours this morning watching videos of the author of SharpCap and I decided to make the purchase of a pro license in order to get some figures to start making decent subs. I live in Seattle WA and my main issue is I can not use my telescope right but for a few hours every couple of weeks due to the weather out here.

Long story short is I am trying to get some calculations for my 750mm F5 newt with an ASI178MC, I purchased the pro license and used the tools.sharpcap.uk to get my e/pixel/s value ( 3.93 ) but when I go to plug these values into the brain option of the histogram section I can only use values of 5 and 10, there is no direct entry field. I can not uncover my telescope and hook up the camera to calibrate as the software wants me to because the weather is constantly raining.

Every minute I can work on this while the weather is bad buys me 30 when I finally get to shoot light frames.

Please advise.

PS... thank you for all the hard work you have done with this software setup.
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admin
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Re: Sub Exposure Calculation

#2

Post by admin »

Hi,

yes I have a to-do item to improve the entry of the sky brightness so that you can type in your own value in a future update to SharpCap.

Right now, you have an estimated brightness of 4 e/pix/s. Remember that that's only an estimate – given the inaccuracy in things like camera manufacturer claims of QE and so on, I'd say that the likely range of values was probably between about 3 and 5 e/pix/s (and even that may be a bit on the optimistic side in terms of how wide the range is).

If you look at the figures as you pick different values you will see that the exposure suggested for 5 e/pix/s is substantially less than the one for 2 e/pix/s. Your final answer once you measure the sky brightness will probably be somewhere within that range – quite likely closer to the shorter exposure suggested for 5 e/pix/s.

Cheers, Robin
roboscience
Posts: 3
Joined: Mon Nov 23, 2020 6:08 am

Re: Sub Exposure Calculation

#3

Post by roboscience »

Thanks Robin;

Seeing that these short exposure times are very short and image transferring may become an issue do you have any recommendations for extending the exposure time? I am trying to not buy every whirly gig on the planet for this setup right now and image with what I have at first to help with the learning curve. What would the effect of cutting the gain in half be?
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Re: Sub Exposure Calculation

#4

Post by admin »

Hi,

SharpCap will suggest the optimum exposure length (more accurately the exposure length that you should treat as a minimum) for all the different gain values in the graph below the main area of the brain window. See https://docs.sharpcap.co.uk/3.2/#The%20 ... n%20Window

You can always use a longer exposure than SharpCap recommends if you want – reasons that you might do this are that you are concerned about the amount of data that you will have to process if you take very short exposures or perhaps that with short exposures the stacking software has difficulty finding enough stars to align with. Just remember that you won't be gaining in image quality in the final stacked image once you go past SharpCap's recommendation. What you particularly want to avoid is going past SharpCap's recommended exposure by such a large amount that it then makes you upgrade your system (for example add auto guiding to allow longer exposures or upgrade the mount). You won't be improving your image quality, you will be spending more money if you do that!

Cheers, Robin
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