SQM/Bortle relation and optimal sub-exposure.
Posted: Sun May 02, 2021 9:47 pm
Hello, I am new to this forum but not to sharpcap. Greetings to everybody!
Recently I saw a video of Dr. Glover where he mentioned tools.sharpcap.co.uk calculator. In the javascript I found the formula to calculate the read noise of sky to embed in my excel sheet.
I have the possibility to reach by car a 21.77 SQM (at zenith of course) location. Lightpollutionmap.info declares a correspondance to Bortle 3 for that area (see attachments). tools.sharpcap gives me Bortle number 1.4.
What is the correct tools I should trust ? Why this discrepancy ?
Furthermore, for my camera ASI294MC, QE about 70%, px size 4.63 um, for that SQM I get an electron flux 0.28. Applying the well known formula 10*R^2/P (and using 1.75 e/s/px at unity gain 120 of my camera) I get a reasonable value of 110s for sub-exposure; my understanding (maybe incorrect) is that using a narrowband (like l-eXtreme) I should multiply by 25, which is about 2700s. But in astrobin I can find definitely acceptable pictures with subs not exceeding the typical 600s used for narrowband (with OSC). How can I interpret these numbers (theory vs. practice) ?
Thank you,
A.
Recently I saw a video of Dr. Glover where he mentioned tools.sharpcap.co.uk calculator. In the javascript I found the formula to calculate the read noise of sky to embed in my excel sheet.
I have the possibility to reach by car a 21.77 SQM (at zenith of course) location. Lightpollutionmap.info declares a correspondance to Bortle 3 for that area (see attachments). tools.sharpcap gives me Bortle number 1.4.
What is the correct tools I should trust ? Why this discrepancy ?
Furthermore, for my camera ASI294MC, QE about 70%, px size 4.63 um, for that SQM I get an electron flux 0.28. Applying the well known formula 10*R^2/P (and using 1.75 e/s/px at unity gain 120 of my camera) I get a reasonable value of 110s for sub-exposure; my understanding (maybe incorrect) is that using a narrowband (like l-eXtreme) I should multiply by 25, which is about 2700s. But in astrobin I can find definitely acceptable pictures with subs not exceeding the typical 600s used for narrowband (with OSC). How can I interpret these numbers (theory vs. practice) ?
Thank you,
A.