This is something that I've always wondered about but never pursued until now because the phenomenon always calibrates out... But with my STXL 11002, and AG Optics 17" IDK, I can get the brightest part of my flat fields dead center in my luminance channel and clear (no filter at all) but the center of my flats are in different places with each filter. For instance, my red filter has the brightest spot way over on the RH side of the frame, and the OIII filter shows the brightest spot of the flat center and very high up in the frame. So today I decided to try and find out what was exactly causing this to be. I've tried making sure my filters sit in the carousel nice and flat so as not to be tilted, I've tried rotating the filters to see if the bright spot follows the rotation of the fileter, I've tried flipping the filter over to the opposite side so that if one side is thicker than the other it should reverse the position of the bright spot. None of those worked. The bright spot of the red filter still would be at the far RH side of the frame as well as the position of the bright spot for the other filters. So then I wonder if my light panel is putting out light in different wavelengths unevenly across the field. So I use the sky for flats and still no change in the position of the brightest spots on my flats. That leaves me wondering if my CCD chip has areas that are more sensitive to different wavelengths than other areas? Is this the main reason that images have different gradients in different filters in our subs? So I thought I would post up here to see what the experts think about this! Does anyone else have the same thing happen to their flats???? My broad band filters are Baaders and my narrow band filters are Astrodon. Dan
Hi Dan, It's on.. Of course you have pixel variations but that would not cause the hot spot to shift like that. Here is an article written by Alan concerning reflections and glints. http://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/ If you want to call me and chat I can offer some other ideas to try. Regards Tim
Hi Tim! The mic thing was just an attempt at humor. I'll definitely give you a call some time when I get back in the observatory and can bring up all my different flats I've taken with the various filters to look at. It is puzzling to me. I've even tried putting different filters in different carousel positions to make sure its not a problem with that particular position. I think my next test will be to put the camera on my FSQ 106 and see if the filters behave the same way. That should at least divide the system between the camera and telescope/imaging train. It's just plain weird that different filters put the hot spot in different places, and putting the same filter in different positions in the carousel yealds the same result for that filter. I know that article you mentioned by Alan! some of those images in there are mine! I had a high end focuser that I just installed and even though it was black anodized, it reflected light like crazy. Mine were the pics where you could see the vanes in the flat. He mentioned " was convinced he had a defective camera, and wanted a new STL-11K " which I didn't say.... I just called to see if I could get some opinions on what was going on to get this from a flat. He asked if he could use these pics in an article and I was happy to oblige.
