Question for Mark re DSLR

[Carole]

Hi Mark,

I am putting this question here so others using DSLR can benefit from your reply.

I have been getting feedback on other forum's regarding taking subs for DSLR longer than 5 minutes.  I remember you saying that there is no benefit to this but can't remember the reason why. 

Was it because of noise?
If not, what was the reasoning?

Thanks
Carole

[MarkS]

Unless you use narrowband filters the limiting factor is the background levels resulting from light pollution and the thermal noise (dark current) from the CCD.  Both of these increase in proportion to the exposure length.  As long as the noise from this background level is sufficient to swamp the CCD read noise then there is nothing to be gained from longer exposures.

Or to put it another way, it is worth using longer exposures if you are using a cooled CCD with narrowband filters.

Mark

[Carole]

Thanks Mark, I had a feeling it was something along these lines.

Carole