I've been doing astrophotography for about 5 years now with my modified Canon 40D. Recently I upgraded and I received a new STF-8300m a few weeks ago. We had clear skies for the last few nights so I took a series of shots with the new camera. There seems to be some type of reflection or artifact in the center portion of the images. The more starlight there is the more intense it seems to be. I was not able to completely clean up the image using Pixinsight. I took the flats in the early morning before the sun rose and it's quite prominent. The camera is attached to a Williams Optics 90mm Megretz with a 0.80x reducer. I removed the reducer and the 'reflection' is still there. I took similar shots with my Canon 40D and the reflection is gone. No filters where used with the SBIG camera since I don't have a filter wheel yet. The Canon had a CLS filter . Since this is my first time using a CCD camera with my scope I'm hoping it's just pilot error on my part and someone out there can point me to how to fix this. I have four .jpg images that show the issue on Flicker. I stretched the raw images and then saved them as .jpg files. The raw files can be uploaded to if that would be helpful. STF-8300m, FLAT, 0.80x reducer, image taken in early morning prior to sunrise scope pointed to west sky with t-shirt over scope. It almost looks like someone is shining a flashlight in the center of the image. https://www.flickr.com/photos/38999112@N02/15990783279/in/set-72157649661451647/ Canon 40D, FLAT, 0.80x reducer. Same setup as above taken the following morning. https://www.flickr.com/photos/38999112@N02/16151318576/in/set-72157649661451647/ STF-8300m, Lovejoy, 30 sec, 0.80 reducer Notice the bright ring at the center of the image. I took about 90 minutes of 30 second subs. The ring stays exactly in the same spot as Lovejoy moved from the NE to the SW corners of the frame. https://www.flickr.com/photos/38999112@N02/16151039966/in/set-72157649661451647/ Canon 40D, Lovejoy, 30 sec, 0.80 reducer This was taken the following night. No ring is present. https://www.flickr.com/photos/38999112@N02/16174965681/in/set-72157649661451647/ If any other types of images are needed to help diagnose this issue please let me know and I'll get them. Thanks for any assistance. -Bill
I've seen something like that before. It turned out to be reflections off the inside of the focuser tubing. The focuser tubing was of course anodized black, but most anodizing is actually completely transparent in infrared, so the inside of the tubing looks like a mirror in IR. You can't see the reflection with your eye, and since the Canon has an internal IR cutoff filter it won't see it either. A monochrome CCD camera on the other hand is extremely sensitive in IR. There are two simple solutions you can try. One is to put a little baffle inside the focuser tub to block the reflections (that's what I did when this happened to me). The other (and probably better/simpler) solution is to apply black spray paint to the inside of the focuser tubing. Unlike anodizing black paint really is black in IR.
IR rejection will help but you need to darken all the surfaces in the OTA. One way I have of telling what is reflective is to put a 100 watt bulb just outside the scopes view. remove the camera and put your eye in the cameras position, you will be shocked at the amount of reflective surfaces. If you flat black most of them your contrast will shoot up and reflections will disappear. You can also use flat black cork as well. Cheers Tim
Thanks for all the comments. IR reflection appears to be the cause. I'm able to remove the back end of the scope (focuser tube) so I'll be hitting that with flat black paint. I'm not sure I want to tackle the other half of the tube with the optics. Once I get my narrow band filters I'll take another look to see if there if more attention will be needed. This got me wondering, is there a spec for internal IR reflectivity on telescopes? Seems like a good quality scope should have a flat black coating on top of any anodozation. Thanks again. -Bill
Bill, There is not an official spec in these telescopes. You can also blacken the edges of your filters as well. Some brands already come with the edges blackened. Every bit helps.
