6 replies to this topic

[MikiSJ]

I just watched a very easy to follow video lecture on how to compute an exposure time using a CCD/CMOS sensor. It is very informative and like I said easy to follow if not a tad long.

 

https://www.youtube....h?v=3RH93UvP358 from March 2019

 

Moral of the story - you are taking too long of an exposure. Just stack your images (using SharpCap, maybe?).

 

Add: Maybe I should be keeping my D90.

Edited by MikiSJ, 03 March 2020 - 12:02 AM.

[Coconuts]

MikiSJ:  I and many others here have found Dr. Glover's lecture really informative.  Others have pointed out that it doesn't necessarily provide the "optimal" subexposure length, as it does the shortest, much below which you are beginning to add unacceptable levels of noise.  Once you have chosen the total integration time that you want for a given object, you definitely can break that up into subexposures set by his formula.  But there is no "harm" in going longer, it's just that in doing so you will need more precise tracking, perhaps a better mount, guiding where you might not have otherwise had to, etc.  Longer exposures also increase the risk of imaging time lost due to a cloud entering the field.  That is a major bugbear at my VI observing site, where small, rapidly moving clouds are the norm, and cloudless nights rare.  Satellites or planes can be handled with the right algorithm, but their odds also go up with exposure time.  So there are distinct trade-offs in going beyond his recommendation for subexposure lengths.  It certainly is valuable to know how long is long enough, and in many cases, that may well be shorter than tradition or habit might suggest.

 

His approach boils down to choosing subexposure lengths equal to (or longer than) ten times the square of the read noise divided by the sky electron rate.  Your camera specs will include the latter; while some of Dr. Glover's tables provide values for the latter, it's far better to use his very helpful web calculator:

 

http://tools.sharpcap.co.uk

 

Note that you can directly enter your specific values in each field, to a level of precision higher than that allowed by his increment and decrement buttons.

 

The above approach is aimed at faint, extended targets such as nebula.  Star clusters are another thing altogether, and there you may need to use shorter subs, to avoid saturating stars and losing their color information.  Subexposure lengths for these targets are best determined empirically.

 

All the best,

 

Kevin

[Allaboutastro]

With the new 60mp QHY600m camera, I'm finding that I don't want short exposures just because of the file sizes.   I have the equipment to do long ones, so there is a balance.   It's a good video and I use it in my teaching, but I think we make it harder than it has to be...if your stars become too saturated, you've gone too long.   If you see no stars in the unstretched image in PixInsight (or whatever) and your background counts aren't very high, then you probably haven't gone long enough.   And even if you DIDN'T go long enough, it's not a crime.  If I had a bad mount and needed short exposures, then I wouldn't care if I took a read noise hit.     

[DSOs4Me]

For me, I found it very helpful - Thanks for posting

[WebFoot]

I just watched a very easy to follow video lecture on how to compute an exposure time using a CCD/CMOS sensor. It is very informative and like I said easy to follow if not a tad long.

 

https://www.youtube....h?v=3RH93UvP358 from March 2019

 

Moral of the story - you are taking too long of an exposure. Just stack your images (using SharpCap, maybe?).

 

Add: Maybe I should be keeping my D90.

It's an interesting video, but your "moral of the story" is only correct for CMOS imaging in bright skies.  At the end of his lecture, he makes it clear that for CCD imaging in very dark skies, very long subexposures are the way to go.

 

Mark

Edited by WebFoot, 04 March 2020 - 06:46 PM.

[Coconuts]

Mark:  The formula is the same for both CMOS and CCD's.  It's just that most CCD's have significantly higher read noise, and the "optimal" (more accurately, "shortest with negligible added noise") subexposure length scales as the read noise squared.  The formula is also the same for dark or polluted skies; subexposure lengths scale inversely with sky brightness.  So the "moral of the CCD story" is that high read noise requires longer subexposures.

 

All the best,

 

Kevin

[WebFoot]

Mark:  The formula is the same for both CMOS and CCD's.  It's just that most CCD's have significantly higher read noise, and the "optimal" (more accurately, "shortest with negligible added noise") subexposure length scales as the read noise squared.  The formula is also the same for dark or polluted skies; subexposure lengths scale inversely with sky brightness.  So the "moral of the CCD story" is that high read noise requires longer subexposures.

 

All the best,

 

Kevin

Yes, I know all that; and also, that the darker the skies, the longer your subexposures should be. 

OP said, "Moral of the story - you are taking too long of an exposure. Just stack your images."  His "moral" is, at best, incomplete, and outright misleading for people using CCD cameras in dark sites (which is all I was saying).