Here is a recent photo of M31 taken through my new STXL16200 and a Tele-Vue NP101-is. I noticed when I did the LRGB combine that I had to weight the blue quite a lot (1.25X) to avoid getting a really yellow image. Is this typical for the STXL16200 ? The stack is 4X600 sec L and 3 X 600 Sec each for R, G, and B. There is some slight elongation of the stars in the corners. Do you think that the large field corrector that TeleVue offers for the NP101-is will eliminate that ?
Color balance isn't right; the spiral arms look green. The cores of most galaxies are reddish, and the spiral arms are bluish. M31 looks a bit on the reddish side, but you don't usually see a lot of blue except in the outer spiral arms, and even then I think the colors are exaggerated in most photos. The Wikipedia article on M31 has an image with reasonable color. To get your color balance right, try imaging a G2V star. See https://diffractionlimited.com/help/maximdl/Color_Balance.htm Can you upload your FITS LRGB stack, prior to any color balancing or processing (other than calibration)? I could have a go at adjusting it.
Here they are: https://www.dropbox.com/s/ogias7jwecv4p5u/M31, 181201, 600 Sec, Luminance #1.FIT?dl=0 https://www.dropbox.com/s/jb5hjefr8rh29jj/M31, 181201, 600 Sec, Luminance #2.FIT?dl=0 https://www.dropbox.com/s/50mjzlt86kx22f2/M31, 181201, 600 Sec, Luminance #3.FIT?dl=0 https://www.dropbox.com/s/qc8zzgga2i9r95q/M31, 181201, 600 Sec, Luminance #4.FIT?dl=0 https://www.dropbox.com/s/rh4ec6y1aej822d/M31, 181201, 600 Sec, Red #1.FIT?dl=0 https://www.dropbox.com/s/27rb70hzfph6ubq/M31, 181201, 600 Sec, Red #2.FIT?dl=0 https://www.dropbox.com/s/v4jr8kkeag4cbto/M31, 181201, 600 Sec, Red #3.FIT?dl=0 https://www.dropbox.com/s/bt9m5pjaizm6sge/M31, 181201, 600 Sec, Green #1.FIT?dl=0 https://www.dropbox.com/s/ta9yj2hos07bd38/M31, 181201, 600 Sec, Green #2.FIT?dl=0 https://www.dropbox.com/s/dav1m2p5c6e5mbl/M31, 181201, 600 Sec, Green #3.FIT?dl=0 https://www.dropbox.com/s/orf8so4g6i4plnn/M31, 181201, 600 Sec, Blue #1.FIT?dl=0 https://www.dropbox.com/s/d6bsza092h1z0hz/M31, 181201, 600 Sec, Blue #2.FIT?dl=0 https://www.dropbox.com/s/c5xncy0jlxwsfsr/M31, 181201, 600 Sec, Blue #3.FIT?dl=0 https://www.dropbox.com/s/0o81qjdrr1zb6cw/Dark Frame1, 181201, 600 Sec, Camera -35 Deg C.FIT?dl=0 https://www.dropbox.com/s/dx26f0tm21zdq51/Dark Frame2, 181201, 600 Sec, Camera -35 Deg C.FIT?dl=0 https://www.dropbox.com/s/eyn5i22fd65b130/Dark Frame3, 181201, 600 Sec, Camera -35 Deg C.FIT?dl=0
Mike, first off, looks like that camera is great match to your scope - good choice, nice wide field ! There are a few factors affecting the color: - the KAF-16200 chip is a little less responsive in blue, so it might need a boost of say 1.2x - the transmission of the filters varies a bit, so that has an impact as well, so you have to adjust for the filters too. Doug's suggestions are good - some useful stuff in the help. The G2 V star trick works for this sort of thing. The field flattener should help clean up the elongated stars in the corner. Cropping the image a bit would help - gets rid of the differences in position of the exposures, cleans up the junk at the edges, and if you crop even more, gets rid of the elongated stars. Please share some of your images in the astrophoto section - I think you're doing good stuff, and it will only get better.
Thanks for the comments Doug & Colin. I am still very much in my learning phase, but I am finally starting to get some results that are worth sharing. I will try the color balancing on a G2 V star next time I am out. The flattener is on my short list of upgrades. On M31, I tried restacking with the color channels R=0.8, G=0.8, B=1.3 and cropped it a bit. I got a better looking result.
That is a lot better! I notice a little bit of greenish gradient diagonally from top left to bottom right. I just took your JPEG and did an "Auto Remove Gradient" on it:
Nice improvement Mike. Looks more "natural". Now just get a really really big scope to resolve stars in there. Say 200" or so...
Thanks again Doug. I have never tried (or known to use) the Auto Remove Gradient function. That is a great tip.
I have also found this program very useful for getting your color balance correct. http://bf-astro.com/excalibrator/excalibrator.htm
