I have a 18 year old STL 1001E, recently checked out and cleaned by Bill Lynch. Bill also repaired my non- functioning internal filter wheel. When the camera returned, I replaced the RGB filters with SII, OIII, and Ha filters from Diffraction, Ltd. I recently took an image of the Veil Nebula and got this image with a donut (for a lack of a better description) around the bright star. All the filters have this donut. Luminance does not. My process software doesn't remove it. Any thoughts? Thank you, Jim
Hi Jim, the size of the "donut" has me stumped - it's not close to the CCD. And its a "double donut" - two distinct regions. I also notice something around a few of the brighter stars, going off to the right. What's in your optical train? eg CDK17 - ..?.. - Filter wheel - STL1001E Is there some kind of focal reducer/corrector? Maybe bounce this off the Planewave guys and see if they have an idea.
Looks like some sort of reflection, possibly a pupil ghost. What telescope do you have the camera attached to? Are there any refractive optics in the system close to the camera?
Hello Colin and Doug. Thanks for the quick response. My optical train has not changed. PlaneWave CDK 17 directly to camera. Camera internal filter wheel and no reducers. The only thought I could think up was perhaps I have mounted all three new filters upside down? I don't know if that would create this artifact. I will send to Bill Dean at PlaneWave. Thanks for the suggestion. Jim
Are these unmounted glass filters? One side is usually shinier, and you want that side away from the sensor, or you may get reflections. If they are mounted, there's nothing you can do about the orientation.
CDK's do have refractive elements in front of the focal plane. So the question is, do any have a center of their radius that is close to the position of the filters? Dichroic filters do work by reflecting light away from the sensor, so if it hit a concave element facing towards the camera (or a convex element facing away) it could possibly reflect out-of-focus light back into the camera.
Bill Dean at PlaneWave says it's very unlikely that the optics of the telescope are causing this artifact. Nonetheless, I sent him an image. He thinks it is probably caused by a reflection between the filter and the cover of the sensor. That leads me to think the filters are mounted backwards. He was somewhat uncertain that could cause the problem, buy worth a try. Will have to wait until thunderstorms and smoke leave our usually dark skies. Jim
