Picking the correct exposure for Deep Sky

A place to post guides, instructions and HOWTO documents on the use of SharpCap and Astrophotography in general
Forum rules
One tutorial per topic please.
Please only post content that you have created in this form.
Tutorials should be either in PDF format as an attachment or written in the first post of the topic. Links to video tutorials you created are also acceptable.
User avatar
admin
Site Admin
Posts: 4976
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Picking the correct exposure for Deep Sky

Post by admin » Sun Feb 16, 2020 3:43 pm

Hi,

Yes, you're welcome to translate this content and share it. I would be very grateful if you could make sure that your translated version includes a link to the original content and credits me as the author.

Thanks, Robin

Landhiker
Posts: 2
Joined: Sun Feb 16, 2020 9:11 am

Re: Picking the correct exposure for Deep Sky

Post by Landhiker » Mon Feb 17, 2020 1:10 am

Thank you very much Dr Robin!
Sure I will include a link to this thread and credit you as the author.
Thank you again!

mwg3656
Posts: 4
Joined: Mon Aug 05, 2019 3:49 pm

Re: Picking the correct exposure for Deep Sky

Post by mwg3656 » Sun Mar 15, 2020 9:36 pm

Hi Robin,

I have been a user for 2 years and I found your noise analysis (previous posts) helpful and the March 9, 2019 video. I would like to see more of the math behind the calculations from the table in the video.

I run the sensor analysis and divide Full Well Depth by Read Noise and usually pick the point where the sensor changes gain amplifiers. What I find when discussing with individuals is 1) too long of exposures with an outside source (airplane lights, satellites, tracking issues) causing them to unwanted artifacts or 2) too much gain (poor SNR, over exposed), because the ability to see an image in real time exciting.

Equipment:
RASA 11" f/2.2
ZWO 6200 color (Read noise 1.53)
Optolong L-enhanced filter (maybe medium band filter)
Typical settings gain 100 and exposure time 30 to 120sec depending on object

I usually do not image for fixed time, but until I see a "smooth" histogram in front of the major peak and low ripple in the end of the histogram. My judgement based on observing the histogram develop.

My question is calculating the optimum number (10% or 5% noise) frames to stack?

thanks,
Mark

User avatar
admin
Site Admin
Posts: 4976
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Picking the correct exposure for Deep Sky

Post by admin » Sun Mar 15, 2020 9:57 pm

Hi Mark,

I think you can still use the calculation results even though you don't image for a fixed time – the calculations are trying to optimise to give you the best possible output image in a certain amount of time (for instance one hour). The same settings are also going to be optimal for getting to a particular image quality in the shortest time possible. If you run the calculations either in SharpCap or in a spreadsheet calculator, you will notice that the recommended gain, exposure and offset values don't change if you change the total imaging time.

Hope that makes sense, Robin

mwg3656
Posts: 4
Joined: Mon Aug 05, 2019 3:49 pm

Re: Picking the correct exposure for Deep Sky

Post by mwg3656 » Mon Mar 16, 2020 2:48 pm

Hi Robin,

Thanks, I think I understand. I like to plan my astrophotography sessions and would be interested in spreadsheet to calculation the number of frames for each object before imaging. Where do I find the spreadsheet?

thanks in advance,
Mark

User avatar
admin
Site Admin
Posts: 4976
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Picking the correct exposure for Deep Sky

Post by admin » Mon Mar 16, 2020 6:40 pm

Hi Mark,

As far as I'm aware there isn't a spreadsheet readily available with these calculations in (unless somebody has followed the discussion above and put the calculations into their own spreadsheet). I was just commenting on the possibility of making that sort of spreadsheet if you felt inclined.

Cheers, Robin

User avatar
oopfan
Posts: 734
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: Picking the correct exposure for Deep Sky

Post by oopfan » Tue Mar 17, 2020 12:52 am

Mark,

You can check out my site here: https://snrcalc.now.sh/

The code is open source and available at GitHub (scroll down to the footer for a link). You can create a spreadsheet if you want from the formulae in calculation.service in the core/services folder.

It takes minutes to read the general information on the home page, followed by the detailed information at the Help link.

I used the site to help plan the effort behind these DSOs:
https://astrotuna.com/m81-bodes-galaxy-in-11-6-hours/
https://astrotuna.com/star-clusters-m35-ngc-2158/

All of my work is with a mono cam using LRGB filters. I use the site's calculator to get a feel for the amount of time needed to capture the DSO with the "L" filter. I shoot for SNR 15-20. To pick up the color I use another calculator at a different site that tells me how much RGB to capture. Generally speaking if I spend "X" time capturing "L" frames then I will spend "X" time capturing R+G+B frames. The RGB frames add to the total SNR but you get your biggest boost in SNR from capturing "L".

I don't have any feedback from people using OSC cams. If that is your camera then I can only recommend that you start somewhere. Start with a target SNR of 15 and then go out and capture those number of frames. Process it and decide if that meets your expectations. If not, capture more and add it to your inventory. You'll eventually get to a point where you are pleased with the result. The last important step is to plug the total time you invested in the DSO into the calculator to see what your total SNR is. If it says SNR 'X' then use 'X' as the target for your next DSO effort.

I don't LiveStack. I capture individual frames and process them in APP. If you do LiveStack then you can look into a technique that Minos developed for saving your LiveStack results every so often and then stack those stacks in APP. I think that is what Minos does.

Good luck,
Brian

Post Reply