how do you find the perfect imaging time for subs lrgb

[iwols]

ive always imaged at 300 secs per sub but how do you know what is the perfect length for your subs please

[vlaiv]

Perfect length for subs depends on magnitude of read noise compared to other noise sources (LP noise, thermal noise or target shot noise - depends which one is the strongest) and how much of it will you tolerate impacting your final image. It also depends on quality of your guiding, how often do you have ruined frames and your storage and processing capacities.

Here are some rules:

- you want your read noise to be at least x5 lower than highest noise component in your subs (usually LP noise when shooting faint targets with cooled camera)

- processing algorithms favor more data, so shorter subs

- storage and processing capacity likes working with less data so longer subs

- unforeseen circumstances mean more lost data with long subs (you loose 10 minutes of data for one ruined 10 minute sub, and only 1 minute of data for one ruined 1 minute sub).

[Clarkey]

It's worth looking at this video by Dr Robin Glover - Sharpcap creator.

https://www.google.com/search?q=dr+robin+glover+astrophotography&oq=dr&aqs=chrome.5.69i60l2j69i57j69i61j69i60j35i39l2j46i199i433i465i512.2130j0j4&client=tablet-android-asus-tpin&sourceid=chrome-mobile&ie=UTF-8#

Gives quite a good run through of sub length selection.

[Elp]

Best thing to do is run a few previews and try, as @vlaiv has said it depends on the circumstances at the time and what you're imaging. The Dr Robin Glover video above is highly recommended. As a general rule I use nebulae 2-3 mins, galaxies 1-2 mins imaging mono cooled, if light pollution is an issue dial the times back. With the preview images you can saturate the brightness and contrast to see how bad any unwanted signal is.