Sub Length Limit due to LP

[adamsp123]

I was wondering if any of you have any thoughts on what limit you place on sub length due to LP with regards to the ADU reading you get in the dark area of the subs.

To explain, if I take subs of 5 minutes and have background levels of 2000 ADU measured with Maxim would I be wasting my time trying to go for 10 mins, this would be on targets such as M101 for example.

Camera is 16 bit.

If on a good night I wa getting 500 ADU for the five minutes again would a increase in sub length realy benefit me over getting more subs at the original 5 minutes?

cheers

[James]

I must admit I'm quite interested in people's take on this too (especially now that I'm dabbling in imaging galaxies).

James

[adamsp123]

James, as there have been no responses I have done a bit more digging and I came across this.... Starizona's Guide to CCD Imaging now at the bottom of the page is a calculator, my Atik is not listed so I used the QHY9 for the camera as it uses the same chip.

So using that, plus 5 min test exposures recently and then Maxim to get ADU background readouts I get results between 5 to 10 mins exposure for 5% depending on my sky conditions.

HTH

[Sparrow]

I'm afraid I don't go as technical as ADU's but in heavily light polluted Chester I generally find that about 6-7 minutes is the limit at ISO800. On exceptionally clear nights I have managed to go up to 15 minutes, but I judge it by the brightness of the sub visually. From experience I now tend to know when the brightness of the image is reaching its limit exposure wise before it becomes washed out.

When I start an imaging session I generally start with 5 minute subs and gradually increase in minutes until it begins to look washed out.

Regards

John