M20 - QHY8 - Calibration Question

[multiweb (Marc)]

Hi guys, here's two shots of M20. Flat/Bias and Bad Pixel Map applied.
The first one wasn't color corrected. It shows a fair bit of green tint to it:

http://www.multiweb.com.au/astro/m20_original.jpg

This one is the same shot but I used levels in photoshop to tone down the green. As soon as I did that it seems the rest of the colors fell into place.

http://www.multiweb.com.au/astro/m20_levelled.jpg

I processed the subs in Nebulosity. There is still a fair amount of vignetting and gradients in the pictures. Is any one using this program to stack and do you have any tips? I'm battling with the best way to do flats at the moment, exposure times, brightness etc... and getting rid of those gradients in my pictures. Also do you have a strong green channel in your exposures with the QHY8?

Thanks for any pointers.

[seeker372011 (Narayan)]

I have hardly -read one or two -shot in colour with the Qhy 8 but yes found the same green..actually much worse than yours

but if you read Craig Starks manual for Neb , this is apparently quite common

he suggests scaling the background to remove this green tint

[Alchemy (Clive)]

i have found my QHY8 has a bit of green in the images, having said that it can be removed so its not a big deal, i use deep sky stacker and then tinker in photoshop, i cant say vignetting has been an issue for me, as for color balace i think a better monitor would help me as i have trouble determining some colors......

flats i do are the t shirt variety, i expose at gain 50 and offset 117, i asume the gradients are from light pollution so i havent really had to deal with them . PM me if you want more details.

cheers clive.

[Tandum (Robin)]

All these colour ccd chips use a bayer matrix to capture images and a single 'pixel' has 2 x green, 1 x red and 1x blue elements in it. To get the real colour out of them you need to correct white balance. This is pretty easy for a DSLR by setting a custom white balance in the camera which most programs read, but prolly not so easy for a qhy8.

I'd suggest taking an image in daylight, of a colourful object, I used flowers with my modified canon, and importing that into your program of choice, correcting the mixing of colours in that program as you go, until it looks right. That should fix it.

Have you read this one :-
http://www.lightstorm.at/pdfs/QHY8_settings.pdf