Optimal sub exposure time

[konstantinos75]

Hi

What is the optimal sub exposure time when you shoot with a CCD camera? This exposure time should preserve star colors and not oversaturate your image.

There's in an empirical astrophotographers rule which states that you should shoot your target so that the average ADU value in the background is 2500-3000 per filter L,R,G,B. If you go more than 3000 ADU then it is useless since you collect more light pollution photons instead of deep sky object signal photons. So you oversaturate your image with light pollution noise. On the other hand, there are a lot of astrophotographers who don't follow this rule, and shoot 5-10 min exposure from light polluted areas per L,R,G,B filter and get 5.000-15.000 ADU in the background.

I normally shoot from a light polluted area and I have seen, on a moonless night, that when shooting with a R filter, for 1 minute exposure bin 1x1 I get 1648 ADU in the background. In a dark sky location, to get this ADU value with R filter I need to take a 5 minute exposure bin 1x1.

When I put these values (1648 ADU - 1 minute) in CCDware exposure calculator (http://www.ccdware.com/resources/subexposure.cfm) I get a recommendation for optimal subexposure time to be 0.4 minute.

When I put these values (1648 ADU - 1 minute) in Starizona ideal exposure calculator (http://starizona.com/acb/ccd/calc_ideal.aspx) I get a recommendation for optimal subexposure time to be 0.61 minute.

These exposure values look too short for me…

What is your experience regarding the optimal sub exposure time? Do you follow an empirical rule? Do you use a CCD exposure calculator?

Let's have a short discussion here

Thanks, Konstantinos

www.albireo.gr

[sharkmelley]

Which camera did you select in the exposure calculators?  Exposure length should be long enough such that the noise from sky background plus dark current swamps the read noise.  Some sensors have high read noise figures (e.g. 12e), others are quite low (e.g. 5e) and newer DSLRs have extremely low figures (<2e).

In any case if you are collecting 1648 ADU in one minute then you would certainly be swamping the read noise.  You can certainly take longer exposures if you wish but avoid saturating the brighter parts of the image.  However, once you are swamping the read noise in your subs then longer exposures won't improve the overall signal to noise ratio in the final image.

Mark