From: Milton Aupperle <milton@outcastsoft.com>

Date: February 15, 2010 3:16:34 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Re: Mars: RGB vs L(IR)RGB

Hi Antonio;

For best results, you should be shooting your G2V stars at maximum altitude to minimize atmospheric extinction.

Primarily color shift is due to thickness of the atmosphere, which of course changes with altitude. Blue then green are absorbed the most with low altitude. Starizona:

http://starizona.com/acb/ccd/advtheorycolor.aspx

has a "Atmospheric Extinction" table  near the bottom of the page that is useful for seeing what effect it has in each case.

Once your above 45°, your within 5% of color balanced for R G and B, so it should be very easy to deal with.

HTH..

Milton J. Aupperle

On 15-Feb-10, at 2:07 PM, Antonio Agnesi wrote:

Thanks Milton,

I found your previous posts, and downloaded a list of G2V stars. Sometime I will give a try to this technique, but I need first a goto mount to locate these stars easily.

By the way, do you select a calibration star as the nearest to your shot field? I understand that its actual color shifts with altitude, and very likely with other unpredictable atmospheric factors...

For RGB calibration with my modified DSLR I took a shot (unsaturated) of a white sheet during a clear sunny day, then I measure the ratio R:G:B and correct the DSO channels accordingly: most often that brings me near the correct balance.

Antonio

-

Hi Antonio;

For LRGB filtered imaging, I color balance using a G2V star and shoot separate L R G B filtered images at the same exposure time.

I outlined the procedure I use for this to this list on November 19, 2009 and then again on December 8, 2009.

Yahoo is hopeless for searching, so you'll have to manually flip back to them to find them.

You can find lists of many G2V star easily with Google :

http://www.skymap.com/g2v.htm (list of 627 stars)

and

http://starizona.com/acb/ccd/advtheorycolor.aspx (near the bottom, plus a good description of how we perceive color)

or just searching with the phrase "G2V Star".

HTH..

Milton Aupperle