Longest useful subs in light polluted skies

[RikM]

I posted this in Getting Started, rather than imaging discussion because it is a question I should have asked when starting out rather than jumping in with both feet.

There are plenty of calculators to work out the minimum useful sub length to avoid read noise having a significant contribution (about 2min for me) but how do you work out the maximum useful sub length? Can you just keep increasing it until your stars saturate? Is there a sensible limit to background ADU value that you should keep below?

I tend to go for 5 - 10 min luminance subs and aim to get the max pixel value to about 60k ADU. My background is normally about 5-6k ADU. Is there any reason why I shouldn't go for 15min subs (provided I don't hit 65k ADU on the bright stars)? This should give me better signal to noise and a deeper view of the faint stuff.

[glowingturnip]

as far as I understand the maths of it, it's only the read noise that depends on the number of frames taken, every other source is linear with the total amount of exposure time, hence so long as your guiding is upto it, then fewer longer subs will reduce your read noise without affecting anything else.

That's the theory anyway, my DSLR has quite an amp-glow problem, and I'd be pretty certain that that's not linear but increases with the time for each shot (and for total exposure time), so that would give me a maximum sub length.  Also i'm not sure I'd trust my guiding for too long a period either.

[RikM]

So to get good separation of the faint areas of the object from the background light pollution it shouldn't really matter if I go for 48x 7.5min or 24x 15min. Once I am past my read noise limit (2min) it only depends on total integration time?

What would be the problems with using 15min subs in light polluted skies? (I know my guiding is up to it. I'm only at 750mm FL and I can do 30min in Ha without much trouble.)

[lensman57]

I posted this in Getting Started, rather than imaging discussion because it is a question I should have asked when starting out rather than jumping in with both feet.

There are plenty of calculators to work out the minimum useful sub length to avoid read noise having a significant contribution (about 2min for me) but how do you work out the maximum useful sub length? Can you just keep increasing it until your stars saturate? Is there a sensible limit to background ADU value that you should keep below?

I tend to go for 5 - 10 min luminance subs and aim to get the max pixel value to about 60k ADU. My background is normally about 5-6k ADU. Is there any reason why I shouldn't go for 15min subs (provided I don't hit 65k ADU on the bright stars)? This should give me better signal to noise and a deeper view of the faint stuff.

On the very rare clear, moonless nights and an OSC CCD and an IDAS D1 or P2 filter I have managed up to 1200s per sub with a background value of about 3600 ADU but this is very rare for my location not far off the airport. I normally stick to 600s but I have had the odd disaster even 300s subs. These have been with Apo triplets @ F4.8 or 150 PDS @ F5.

Regards,

A.G