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Re: Picking the correct exposure for Deep Sky

Posted: Sat Nov 28, 2020 10:16 pm
by Fabioibirru
Dear Robin,

I'm glad to you about this article and other technical information about this topic.

Then I spent little time to translate your article into my native language because I found lot of people that use the old "GrandMother recipe" and do not belive that short exposures could lead to good results.
I'm a chemist and for me is unbelivable that such information are not a collective domain.
Anyway this is my little contribute to spread it also to non technical people.

I add to original article just 3 pictures taken from Smart Histogram part of SharpCap manual.

Could I share it to Astrophotographic italian community?

Best Regards


Re: Picking the correct exposure for Deep Sky

Posted: Sun Nov 29, 2020 9:50 pm
by admin
Hi Fabio,

thanks for taking the time to translate the article – I'm very grateful, and of course you are welcome to share it with the Italian astronomy community :-)

Cheers, and clear skies, Robin

Re: Picking the correct exposure for Deep Sky

Posted: Fri Jan 08, 2021 10:12 pm
by John Jennings
I'm using the Sharpcap online Sky Noise calculator to compute optimal Sub times for my CMOS QHY268C camera with a couple of different OSC filters. One is the Optolong LProExtreme which has (2) 7nm band passes.

My questions are:

(1) Does that mean I can a use a 14nm bandpass setting for mono in the calculator and then divide by 3 for the final Skynoise calculation?

(2) I estimate my Hutech LPSP3 broadband light pollution filter has about a total bandpass of 215nm if I add all the half widths of the bandpass notches. Can I use this simple technique to estimate the Sky Noise with different filters and a color CMOS camera?

(3) Is it possible to place any imaging filter in front of my portable SQM meter to get a reading to use in the online Skynoise calculator for the MPAS without utilizing the filter settings in the calculator?

These filters and the new third generation CMOS color cameras are some of the latest tools for imaging in the suburbs and most everyone is experimenting with different filters and sub lengths. I created a simple spreadsheet which is extremely illuminating based on you tutorial, but the big unknown for me is how to account for these filters in the calculation. My imaging sky ranges from 8.2 MPSAS east of the meridian to 7.8 west of the meridian as measured by my SQM meter. My current strategy is to split my automated imaging runs into multiple segments.

John J.

Re: Picking the correct exposure for Deep Sky

Posted: Sat Jan 09, 2021 10:19 pm
by admin

hmm, interesting questions :)

1) Assuming that the two narrow band windows are each within the range of one colour of the RGB sensor (but in different colours), then I think you would treat this as a mono camera with 7nm filter. Imagine there is a 7nm window in the red region - light that passes through that will (almost) all pass the red filter and be counted by the red pixels (and practically none on the G/B pixels). The same if the second window is in the G region.

2.) Here I would find the colour (R/G/B) with the least bandpass and use that in the mono setting - for instance if there is a bandpass of 40nm in the blue then use that figure in mono setting, since that is what the blue pixels will see (and they will be getting less light than the colours with higher bandpasses).

3) Yes, pretty close to equivalent in the case of a mono sensor or a single band filter. More complex with multi-band filters.



Re: Picking the correct exposure for Deep Sky

Posted: Sun Jan 10, 2021 9:30 pm
by oopfan
May I suggest a Plan B?

If you know your Bias level (mine is 200 ADU) then increase the exposure until dead space begins reading higher than Bias. How much higher? It depends on your camera's Read Noise, but a good target would be 100 ADU above Bias for a CMOS camera, or 200+ ADU for CCD.


Re: Picking the correct exposure for Deep Sky

Posted: Mon Jan 11, 2021 8:11 pm
by elizaaa
Simple constellation shots with a camera on a fixed tripod usually run from about 10 to 30 seconds. Piggyback shots of wide fields can run about 5 to 10 minutes. Prime focus shots through a telescope can run 30 to 90 minutes. This discussion will primarily deal with long exposures for deep-sky astrophotography.