Best OTA for EAA in heavy light pollution

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Strykyr22
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Joined: Tue Jun 01, 2021 4:24 am

Best OTA for EAA in heavy light pollution

#1

Post by Strykyr22 »

Hi everyone! I'm new to EAA and am currently using a C6 evolution with focal reducer and asi385mc camera. I also live in a bortle 9 city. Sharpcap is an amazing program. I absolutely love it.

I have read all your articles on optimal exposure times and found the math very interesting.

Here is a question that I haven't found a satisfactory answer for: How does the f ratio of a scope affect EAA? How does it change optimal exposures? Does heavy light pollution favor fast scopes or slow ones?

In my case I am thinking about getting either a rasa 8 focal ratio 400mm f/2 or a 73mm refractor f/5.9. I suspect the rasa will be better but I would like to be sure. Also either one would be used on my evolution alt AZ mount and exposures should be limited to 20s max. Seeing and transparency are often good or above average at my location.

Thank you for your help!
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Re: Best OTA for EAA in heavy light pollution

#2

Post by admin »

Hi,

an interesting question - heavy light pollution already pushes down the length of sub-frames that you need to take - a fast scope has the same effect. I'm not sure that is necessarily a good thing as the amount of light pollution build up per pixel per second could be very large for a fast scope under poor skies. What then becomes important is not the minimum exposure, but the maximum... If you are starting to get 40% or more histogram peaks due to light pollution in relatively short exposures then you actually have a limit to how long your exposures can be without getting whiteout frames.

Putting bortle 9 and F/2 into the sky background brightness estimator at http://tools.sharpcap.co.uk gives a sky brightness of ~210 e/pixel/s, which would fully saturate your 385MC at minimum gain in about 90s, and give a maximum workable exposure of 30-40s.

Personally, I think I would go with the refractor because it is low maintenance, but other opinions may differ - we have several imagers on here who image from heavily light polluted skies - hopefully one of them will contribute.

cheers,

Robin
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