Exposures?

[GrampyG]

Hi

 

Bearing in mind that there are a huge number of variables in astrophotography, are there any rough guides as to reasonable exposures for different objects. Just thought I'd ask.

[vlaiv]

Not related to object, but there is rule that covers it generally:

Expose until sky background noise (light pollution noise) is significantly higher than read noise (meaning of significantly higher is open to interpretation here).

That is rather general rule - but it works rather well for all use case scenarios.

More general rule is - expose until any time dependent noise is significantly larger than read noise - most people use cooled cameras, so thermal noise is not going to be first to be significantly larger than read noise. Likewise, most people image faint targets - so target shot noise is not going to dominate read noise - but light pollution noise is ever present and for most people presents source of noise that will first swamp read noise.

You want LP noise to be x3-5 larger than read noise - and you don't have to expose for longer.

[MartinB]

A very good question!  The answer will depend on the characteristics of your camera, especially it's read noise, along with your sky conditions.  The aim is for the the read noise to be swamped by background sky brightness signal,  Below is a resource which I have found very useful.

https://www.ccdware.com/resources/subexposure.html

It isn't necessarily detrimental going beyond  the recommended time, this is really just a minimum time for which to aim.  

The tool needs you to have the necessary info regarding your camera.  

There are some targets which contain both very bright and very dark elements e.g. M42.  Imagers sometimes use some short subs for the bright areas and use these to present the bright central area.  In this case the read noise is less of an issue since none of the bright signal is likely to be read noise limited.

[GrampyG]

Thank you for replies

 

Most useful!

[rickwayne]

If you'd like a quantitative but VERY pragmatic discussion, video search for "Robin Glover exposure times and temperatures".