Exposure times

[Richard N]

I suspect this is complicated but is there a method to calculate the likely exposure times needed to see faint stars/objects of known magnitudes? 

[PeterC65]

You've posted this in the EAA forum so I'm assuming you're not doing AP.

For EAA, where you're generally wanting a fairly quick result, it's more a case of how faint can you see within the context of a fairly short exposure time.

The maximum exposure time is limited to about 30s if you're using an AZ mount. You can go much longer with an EQ mount but then it becomes more like AP timescales. I never go beyond 15s, but I sometimes stack for 15 minutes. Longer stacks seem to reduce the noise and bring out more detail but I'm finding that longer individual frame exposures are better for seeing faint objects, even if the total exposure time is the same.

The same things apply as for visual, so more aperture helps, and narrowband filters help but only with emission nebulae.

If you use SharpCap and get it to analyse your camera it can work out for you what the best gain and exposure time should be, but these are dependant on the camera and conditions rather than on the object magnitude. Personally I always use the same gain (x400 for my camera) and exposure times of between 4s and 15s. 4s is my default but I increase it if I'm using a narrowband filter or if the object is very faint. This is for DSOs, the Moon and Planets are another story.

In practice I find that how faint I can see is greatly dependent on the type of object. So I've been able to see magnitude 17 stars but struggle with some magnitude 9 reflexion nebulae.

 

[Richard N]

Thanks. Yes my query was for EAA.  I was looking for a mathematical approach but possibly there are too many variables to make it worthwhile. 

[PeterC65]

I was looking at the camera analysis feature in SharpCap today and there is a graph that shows Faintest Detectable Object against Exposure Time but the faintness scale is in electrons per pixel which doesn't easily relate to object magnitude. Here is a screen shot of what it shows you from the SharpCap User Manual ...

The graph depends only on the camera but I expect the connection between electrons per pixel and object magnitude depends on the scope and the object type.

 

[Martin Meredith]

Check out this thread from here for one approach:

 

 

[PeterC65]

I came across this tool yesterday. It goes some way to answering your question I think, letting you determine what is preventing you from seeing detail, whether it is the seeing (it usually is) or the camera or the optics.