m81 m82

[alacant]

Hi everyone. I had around 90 minutes before moon rise yesterday and thought I'd post this as a heads-up for imaging near the pole. I started with big swings in dec before I realised that the tube balance was poor; it seems critical around this point. A re-balance with the tube in more or less the imaging position got the guiding back to something respectable. I know I'm gonna need longer on target but any advice on -especially- the colour balance most gratefully received. HTH, clear skies and thanks for looking.

[vlaiv]

I did this combination of targets (almost the exact scale) a while a go and found following useful for color balance:

Find a star, either G2V or star with color index close to 0.625 and do color correction based on that. If star is G2V - nothing else is needed, for other "unknown" stars, one will likely need to tweak color balance a bit.

I marked the star I used for color balance in screen shot.

For color balancing on a star this is procedure: Separate into r,g and b channel, subtract median of background so it is close to 0, take small region around the star and do "sum" of pixel values. Take one reference value (for example green) and divide R and B images with ratios G sum value / R sum value, G sum value / B sum value respectively. What this does is effectively setting total light per R,G and B channel for that star to equal (G2V star should have white light - and that is same amount R, G and B light coming from it).