Maximum expsoure time with dslr camera

[Gustav Bertelsen]

Hi SGL

I've previously heard that the maximum exposure you should use with a dslr camera is 5 minute subs because beyond 5 min the images should supposedly get too noisy. However Im doubting if that is true. Im using an eos 1100d and an astronomik 12 nm ha filter so the sky doesn't get blown out. So it's purely a noise question. But im still wondering if there is any "maximum exposure time" for a dslr camera and is it beneficial to reach for longer than 5 minuite subs on a dslr? So I would like to hear others opinions and experiences.

Thanks in advance:)

[symmetal]

As long as the target you're imaging doesn't start getting clipped the signal to noise (S/N) ratio of the image will keep increasing the longer the exposure time so in that respect the longer the exposure the better. If you double the exposure the signal accumulated will double but the target noise will only increase by the square root of this, in this case 1.41 times.

With normal RGB imaging the sky background level increases the longer you expose, so as the sky background peak in your camera histogram gets further to the right the images lose contrast and dynamic range. Clipped stars also get more bloated the longer the exposure. I think this increasing sky background level is what makes you think the images get more 'noisy' over 5 mins.

However with narrowband imaging the bulk of the sky background 'signal' doesn't get through the Ha filter so you can expose for a lot longer before the sky background peak on the camera histogram gets too far to the right. In fact you may have to expose for 30 to 60 mins or more, depending on your light pollution level, before the sky background level is excessive. This is longer than most practical exposures so for narrowband expose for as long as you can if your guiding allows it. Very long exposures have a greater chance of being ruined by aircraft passing through etc. so it's often better to use a series of shorter subs. Star bloating is also less of an issue with narrowband as the bulk of the starlight is blocked.

the 12nm astronomik filter is a wider pass bandwidth than the 7nm or 3nm ones used with dedicated astro cameras, so the sky background level will increase faster compared to using the narrower ones but I would have thought 10 to 20 min exposures would be worth trying if you can manage it.

I don't do DSLR imaging so someone who does narrowband with DSLRs can give a more experienced answer. For narrowband however it's generally the longer exposure the better within practical limits. 

Alan

Edit: I forgot about the dark current which is a significant source of noise in DSLRs. I did say I don't do DSLR imaging.  The noise from the dark current is the square root of the dark current * exposure time, and the total noise from dark current is the same whether you take 1 eight minute exposure or 4 two min exposures. I believe the read noise quickly becomes insignificant compared to the noise from dark current so an advantage of taking long exposures compared to more short ones to reduce read noise is not really important.

vlaiv needs to confirm or deny the last paragraph so don't take my word for it yet. 😁

Edited April 13, 2020 by symmetal

[michael8554]

I can generalise:

The longer the exposure the more noise

The hotter the weather the more noise

Most DSLR's, not all, higher the ISO, the more noise.

So you should do tests on your camera to find the sweet spot of exposure and ISO.

Dithering or  Darks can reduce noise on the stacked results.

Michael

[Gustav Bertelsen]

Thanks for the very helpful replies:)