Hi all; I was wondering if anyone here has had this before, and if not, Doug George, can you share an opinion? What I have is my bright spot on my flats being located in different places when taking flat frames with the different filters. The filters are Astrodon Gen 3 i series. What I experience is the luminance flats having the brightest spot at the center of the frame. The Red filter puts my bright spot at the extreme RH side of the frame and it's not exactly a round spot, most of the RH middle side is brighter than the top of the frame and bottom of the frame on the RH side while the LH side is the normal darkness drop off. On my Blue filter, the spot is mostly centered like my luminance. The Green filter produces the brightest part being the center of the frame and from the center up to the top of the frame. Again, like the red filter, its like the bright spot is smeared all the way to the edge leaving the RH and LH sides of the frame with a normal drop off of brightness. So what I have done is rotate my filters in their cell to see if the problem follows the orientation of the filters. No change. I've tried putting the filters into different positions on the carousel. Still no difference. I've also tried rotating the camera at the anomaly stays with the camera frame. The only thing I can think of is that my ccd chip is more sensitive to different frequencies of light in one area of the chip. The RH side of the chip being more sensitive to the red end of the spectrum and the top part of the chip being more sensitive to the green part of the spectrum. It wasn't always this way. I don't remember having this problem when it was new but was kind of one of those things that just creeps up on you as time goes by. As it pertains to corrected images, I think the gradients will cancel out, but I think that I'm not getting true color representation in those areas after calibration... What do you think? Any suggestions are welcome. Dan
Doug, Here is the link to dropbox, and I'll also include a quick JPG here so that those who don't want to download 160mb of files to look at. The master flats (on Dropbox) were 20 each of LUM, R,G, and B and made into master flats in MaxIm. You can see the anomaly well in the medium stretch. For the jpg pic included here, these were all stretched with the same parameters in PixInsight to show the same thing. Dropbox link: https://www.dropbox.com/sh/9vc5f6jh5ajrcou/AABDcB3LjaJZ0yB47eO1aiW1a?dl=0 P.S. I'm having trouble uploading the jpg file here in the observatory. I'll try posting it from the house here in a bit.
Here is the quick N dirty JPG. The top left is the red channel, the top right is the LUM, the bottom Left is the green and the bottom Right is the blue channel. All with the same stretch applied in PI.
The KAI-11002 data sheets do not list the bright field global non-uniformity for some reason. If you look for example at the KAI-08051, the spec is maximum 5% RMS. Much of the variation present is vignetting within the optical system. So what I did was I flat-fielded your red, green, and blue frames using your luminance frame. That is an attempt to show only the variation in flat field uniformity versus wavelength. Yes, there is some differences. That is not totally unexpected; CCD sensors do exhibit flat-fielding variations with wavelength. It's a big reason why we do flat fields in each wavelength of interest (not just dust on the filters!). The variation seen here does not exceed the expected limits.
