Can someone from DL give me the distance between the 16803 sensor chamber cover and the first filter surface when using the FW7? I'm having some very large halos on bright stars and am trying to determine the location of the reflection based on size and distance. I have the specification of the total distance to the top of the FW but not to the filter.
Chamber window to front face of camera (w/o adapter) is 0.60". Filter carousel mounting plane to back of filter wheel is 0.52".
Thank you Doug. Have you ever heard of the chamber window anti-reflection coatings going bad? I seem to be experiencing some large halos and it looks like it's from the chamber and filter reflevtion.
Those coatings are pretty tough. Perhaps you are seeing pupil ghosts. Do you have a focal reducer or flattener optics near the focal plane? I’d like to see images of your reflections.
I've been working with Roland C. of AP. I calculated the diameter as 6.9 mm. His thoughts are. "I just did a ghost analysis with all the components including the field flattener, filters and cover glass. The reflections between filter and cover glass will produce a halo of about 6.8 mm diameter. The reflection between filter and field flattener produces a 33mm halo that does not encircle the star and is off the edge of the field unless the star is exactly in the center of the field." Here are the images, blue being the worse. www.astroimager.net/FilterTest_Blue_300s_-30degC_000007219.jpg www.astroimager.net/FilterTest_Open Slot_300s_-30degC_000007227.jpg www.astroimager.net/FilterTest_Luminance_300s_-30degC_000007226.jpg www.astroimager.net/FilterTest_Baader OIII 8nm_300s_-30degC_000007224.jpg www.astroimager.net/FilterTest_Red_300s_-30degC_000007222.jpg www.astroimager.net/FilterTest_Green_300s_-30degC_000007223.jpg
So there you have it - reflections between the filter and cover glass. You have to realize that a bright star like that is an incredibly aggressive test. Even if the optical surfaces only reflect a tiny percentage, the star is so bright it's going to show up.
I really doubt it would change anything. This is more a matter of the relative spacings, I would think. That's generally the case with pupil ghosts. "Cover glass" usually refers to the sensor's window, not the chamber window. Please confirm which one Roland is talking about.
Hi again Doug. I tried a short test on M45 tonight and had bad results. http://www.astroimager.net/M45-160-FilterTest-300s_Blue.jpg This is just a 5 minute exposure with a blue filter. I'm certain that I must have M45 many times with even longer exposures without this result. Is it even remotely possible that something happened to the chamber cover? I'm really bummed that this is happening. Bill Lynch is checking about replacing the cover but I would really like to be certain that is the problem before I do that. Any advice is appreciated. Jim
The optical coatings are working as designed. You're doing a deep exposure of a 2.9 magnitude star. Even if there's the slightest reflection (and there will be a slightest reflection) it is going to show up. The only option is to somehow change the optical spacings. This will put the pupil ghost so far out of focus that it won't show up.
Change the spacing between the filter and the chamber? The spacing between the FF'r and the sensor is fixed and according to Roland the FF'r is not the issue. I've shot M45 many times with other cameras and never had this problem. Thanks for working with me on this. Jim
You could try that... would need some kind of spacer. Different cameras have their windows and filters in different positions. Different optical systems have differing pupil ghost characteristics. It seems you were “unlucky”.
