DSLR: How long between exposures?

[kirkster501]

When firing off multiple 2 to 3 minute exposures of a DSO, whats a typical "gap" between exposures to let the sensor cool down please? I was thinking the same time as the exposure i.e. 3 minute exposure, 3 minute gap, 3 minute exposure etc.... ????

Second Q if I may? Any rules of thumb about exposure length and number of exposures for different types of objects? I was thinking a Galaxy like M33 or NA nebula needs an hour at least made of 20 x 3 min exposure. Whast about clusters lkme M36 etc.

Thanks, Steve

[nmoushon]

There isn't really a rule of thumb for exposure length. It all depends on our target. If its a faint target then you'll want longer exposures. If its a bright object then you'll want short exposures so you don't over expose anything. If its a bright object but has faint nebula around it (like M42) then you'll want to do a combination of short and longer exposures. For cluster it depends on if its globular or open. If its globular the longer exposures will tend to burn out the core and you won't get to see the individual stars. ISO plays a lot into this whole thing as well.

[nmoushon]

As for gaps I tend to do about 10-40 sec depending on the outside temp. Also depends on exposure length. I definitely dont match it to exposure time. You can, it wont hurt anything, but tends to be a waist of time that you can spend on lights. The amount of noise you would reduce from a large gap can easily be made up with darks and flats and good processing.

[RikM]

I tested resting times using the sensor temp in APT. For 5 min exposures, I didn't get any further cooling benefit from longer rests than 20sec. When using dithering with the guiding I found a 25 sec pause to be optimal to let the guiding settle again, so that was my norm.

I generally matched exposure times to the brightness of the object I was after compared to the brightness of the sky background. My best DSLR images were all from about 35-60 exposures each of 5 - 8 min. I don't think you can have too much data. Even at those exposure times, I could have done with more. My skies are not great.

[JacobvonChorus]

Something I have noticed when imaging with my 1000D is that changing the battery which I do about 2/3 of the way through an imaging session cools the sensor by about 5 degrees. Otherwise the sensor temperature is stable all of the time.

Jacob

[RikM]

I tested resting times using the sensor temp in APT. For 5 min exposures, I didn't get any further cooling benefit from longer rests than 20sec.

What I meant to say was that with 5 sec pauses the sensor would slowly heat up as the session wore on, same with 10 sec and 15 sec pauses. Once I got to 20 sec pauses the sensor temp stayed constant, so going to 30sec, 60 sec pauses didn't make any further difference.

[ronaldo999]

I agree about 20 sec pauses, i also noticed as the temp drops the time it takes to write data to the sd card slightly increases. The data light can be active for about 5 seconds so you would need more than this anyway to allow the image captured to be written.

[Bizibilder]

I use a 1000D for imaging and have not noticed any artifacts in my subs when taking 5 min exposures with a 5 second gap between them, similarly inspection of my darks shows no noticeable difference between the first and last of a sequence of 30x5min again with 5 sec gaps.

The main reason that i use gaps at all is to allow time for the data to download - Canon utilities "progress bar" shows that it takes around 1-2 seconds for a RAW+JPEG combination to fully download.

[Luke]

Years ago, I thought I read somewhere that you should leave as long a gap as your exposure, so a five minute gap between each five minute exposure. So for months, I did that. Then one way or another, probably an SGL post (thank you, SGL!) I found out lots of people were getting good results with a much, much shorter gap! I tried with something like a 10 second gap between shots and it seemed to make no difference at all being so much shorter other than I got a lot more data! I still wince at the thought of all the data I lost!!

I have a 450D. I think my misus has the 1100D and only uses a short gap too. I can't remember if some older cameras might have had an issue with lamp glow or something like that and might have needed longer, but I might be barking up the wrong tree there?

[Chris]

I leave 10 second gaps between 2 minute exposures and I don't get any noticeable build up of noise from the start to the end of an imaging session.

[kirkster501]

Thanks guys for all your responses - sounds like you have saved me a shed load of time. I am going with 20secs to start with.

[melsky]

Interesting, when I'm shooting with the Astrotrac I only have two hours before the arm has to be rewound so I'm only using a 1 second gap ( plus 2 seconds for the mirror lock up) between frames on a modded 600D, works OK for me.

[sgazer]

I also use about 20s. I think by leaving a gap the same length as the exposure, you are effectively wasting half your subs over the same time period. I think it would be more beneficial to have double the subs or a longer exposure time than a long cool down time between subs.

[sgazer]

as for the exposure time, it really depends on the target, light pollution and the speed of your scope/lens. I found for most DSOs, using a UHC filter and f5 scope, 3mins @ ISO800 was fine. For M42 it had to be shorter not to blow out the core and occassionally pushed it to 5mins if tracking was going well and for a dim DSO. Total time was as long as possible, but typically 1-2 hours.