LRGB Exposure times

[alistairsam]

Hi,

I'm new to LRGB imaging and wanted to know how you calculate exposure lengths for each of the channels.

I do know binning RGB 2x2 is fairly common and the exposure would be 1/4th of L as 4 pixels are used instead of 1, but is there variation between R,G,B depending on the object?

I had a read of excalibrator and how it uses measurements from known databases. but is this for the final processing part?
any good sources / guides on the basics?

Thanks
Alistair

[Joshua Bunn]

Dont think there is much of a variation in B,G,B between object. But there is a slight variation depending on imaging altitude where Blue has a higher Atmospheric Extinction factor followed by green, http://starizona.com/acb/ccd/advtheorycolor.aspx

I think the various weights of the filters is more to do with the characteristics of your optical path, ccd filters and Quantum Efficiency of the camera chip.

CCDstack has an automatic color calibration feature, I use G2V star calibration, but should try CCDstack to compare. I'm much a beginner in this area and would be interested to hear others replies as well.

Josh

[alistairsam]

Thanks
That's a good article.
so from this, there are two methods?
pre-calculate your ratios depending on the altitude of the object, your ccd, and set your exposure times to match
else, take equal length exposures, measure the value of a known star and calibrate with software?
how do you use the G2v calibration?
is this possible with DSS or Maxim?

Alistair

[alistairsam]

found a nice tutorial here
http://www.astrodon.com/Orphan/g2v_tutorial/

[Joshua Bunn]

I dont know if the G2V method is the best, its probably not, but then again its probably OK too?
Take your test RGB exposures on a G2V star at the zenith, then there are no extinction factors to think about. Calculate your weights based on the adu readings. When you take your image, compensate for the atmospheric extinction factors for each filter from the table, multiply by those factors as per the article. Then there is just a loss of general light reaching the scope when you get low in the horizon that you may want to compensate for, mentioned last in this article.

Josh