G2V callibration - what am I doing wrong?

[Dangerous-Dave]

Hi guys, I have recently got my Pod observatory up and running and been trying some G2V callibration with a view to doing some serious LRGB work. I have followed the tutorial on the Astrodon website but I keep getting results with a colour cast.

I am using a Vixen R200SS photo newtonian with dedicated coma corrector, Astronomik CLS filter (screwed to the inside of the corrector using a 52mm:48mm step ring), 2" Astronomik Typ2c LRGB filters and a Starlight Xpress SXVF-H16 CCD. I am controlling the mount (EQ6 pro) with Skymap Pro 10 and using John Moore's G2V data catalogue downloaded from the Skymap website to pick and slew to G2V stars.

I carried out the callibration twice, on two separate G2V stars within a few degrees of Arcturus in Bootes, then made RGB composites of Arcturus using the weighted exposures to test the outcome. The first time I captured a single set of 5 second subs (2x2 binned) for each colour, using the information window in Maxim to record the 'intensity' of the star then divide green by red and then green by blue to get the weightings (using green = 1.0). The second time I took 5x10s subs (2x2 binned) for each colour and used the average intensity from each set to calculate the weightings.

The first time, I arrived at weightings of Red: 1.60, Green 1.00, Blue 0.90, the second time it was Red: 1.30, Green: 1.00, Blue 0.90. I applied these weightings to the test exposures of Arcturus and the first resulted in a white star on a slightly green background, with the second showing Arcturus in its familiar orange hue but having a background with a similar, orange cast to it. I also tested the second set of weightings by taking a set of images of M64, using Green = 300secs, Red = 390s, Blue = 270s and got a colour image with a cyan cast, which I would expect to see from an un-weighted shot using the CLS filter.

I am not doing anything 'clever' in Maxim, like checking any auto balance boxes, or adjusting the colour weightings in the stacking dialogue, just loading in all the subs, measuring, aligning them and carrying out an SDMask stack. No calibration frames were used.

I was very careful to ensure I was measuring the right star: I centred and sync'd on Arcturus before slewing to the G2V stars, then superimposed the CCD field of view in Skymap and carefully checked the asterisms to make sure I had the right one. I don't suppose it's not impossible I got the wrong star, but it seems highly unlikely given they were both the brightest stars in the field of view and the asterisms matched up.

One thing that occurred to me is I used stars at 50 degrees and 60 degrees altitude for the calibration, as opposed to the 80+ degrees recommended in the Astrodon tutorial, which was due to practical considerations (the primary dome of the Pod obscures the zenith) and wanting to pick something fairly close to the test object (Arcturus). Surely atmospheric extinction couldn't have such a pronounced effect? I don't live in an area of particularly bad light pollution and the transparency was good. Alternatively, could the light pollution filter be messing things up? I assumed blue and green intensities would be higher relative to red as a result of the filter and the G2V callibration would correct for this. Is this incorrect?

I would appreciate any pointers.

Are there any alternative ways to colour callibrate your scope/filter/CCD setup?

[martin_h]

I use this routine..........

Starizona's Guide to CCD Imaging

[RobH]

Hello Dave.

I tried G2V calibration a couple of years back and had awful results.

Your results may well be caused by a combination of atmospheric extinction, light pollution, and the differing sensitivity of the camera that you're trying to correct anyway.

There was a discussion about this subject on here a while ago, and several folks were going to try different methods for setting the white balance, but nothing came of it.

I do remember someone pointing out that photographers use a white card for setting the whitepoint, which struck me as the best way to do it...use a white sheeet of paper illuminated by sunlight. (getting the exposures short enough might be tricky though, but a severe aperture mask would probably work.)

Rob

[Dangerous-Dave]

Thanks guys. Some very useful info there.

I have just been taking some terrestrial shots of the sheep fields across the valley and adjusting the weightings by eye, but they are varying from 1.85/1.00/0.90 to 1.70/1.00/1.20. From reading that Starizona page it sounds like extinction could render these not altogether useful anyway.

[davew]

Hi Dave,

Have a look at this -

Photoshop Plug-in: WhiteCal | Deep Sky Colors - Astrophotography by Rogelio Bernal Andreo

I have known it work but make quite sure the G2V star you use isn't saturated !

Dave.

[George]

Thanks for the link Dave looks interesting

[Dangerous-Dave]

Hi Dave,

Have a look at this -

Photoshop Plug-in: WhiteCal | Deep Sky Colors - Astrophotography by Rogelio Bernal Andreo

I have known it work but make quite sure the G2V star you use isn't saturated !

Dave.

Many thanks for that Dave.

[Uranium235]

Did you apply an extinction factor to your calculations? I remember seeing a table for it somewhere.

Ahh.. here it is:

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

lol... just noticed martin beat me to it!

[martin_h]