I am seeing edge artifacts on my narrowband flats, which are impacting my ability to calibrate the narrowband frames (see images). I am using Chroma 3nm 3mm-thick filters. In corresponding with the Chroma technical staff, they suggest this is likely a light leak issue, which lead to an issue where the flats will not be able to properly calibrate the light images. I was cautious not to put too much pressure on the screws for the filter wheel when setting up ("Too Much" is not particularly quantitative) as per the instructions. Has anyone seen this? Is there a remedy? Are masks available for the filter wheel (This was suggested by Chroma)?
We do try to provide as much unobstructed filter area as possible, without light leaking through. Sometimes you can get reflections off the edges of the filters, if they are not blackened and the light is coming in steeply enough to hit them. I'd suggest trying to add a little bit of baffling in front of the filter wheel. You could try making a cardboard baffle first, to see if it works. Then maybe 3D print something more permanent (if you have a printer or know someone who does).
Hi Doug. My setups unfortunately hav only 2-4 mm of space to put in any baffles. At the behest of Chroma I have spoken with Joel Short at buckeyestargazer.net. The sell filter masks for many astronomy cameras. He is willing to fabricate the masks, but would prefer to have an engineering drawing for the filter wheel (the part that holds the filter). Is it possible to get the drawing?
Hi Doug, Joel had a couple more questions, see red lines in the image below. He is concerned about the holes for the screws that hold the filters 1. Are they centered vertically on the filter openings? 2. What is the center-to-center distance (Horizontally) between the holes. If you could get this information, it would spare me the trouble of taking my filter wheel apart to measure. Thanks.
Thanks for the reply. Sorry to relay more questions from Joel. Question #4 may be a killer. Do you know the available clearance for question 4. He may be able to countersink the holes in the masks to minimize the protrusion of the screws holding the filter to a minimum A few final questions: 1. What thread size are the retention screws, and how long are they? 2. Is the area around the screw holes recessed at all, or flat with the plane of the disk? 3. How deep is the slot? If using 3mm thick filters, does the filter sit above the plane of the disk? 4. Typically these masks are 2mm thick, and usually filter wheels have plenty of room to accommodate this extra thickness above the plane of the disk. Let me know if you think this might interfere with operation. This shouldn't take long to design once I have this info.
Screws are 4-40. The disk is 0.158" = 4 mm thick. The screws must not protrude. The holes are flat with the plane of the disk. Note that the toothed "gear" part of the carousel is thicker, and the top of it is 0.107" = 2.7 mm higher than the plane of the disk. The slots are 0.108" deep = 2.73 mm. The top of 3 mm filters will sit a little above the disk. In the standard filter wheel, the distance from the top of the disk to the inside face of the filter wheel lid is 0.166" = 4.2 mm. There should be plenty of room.
Thanks for all your help, Doug. I have been able to resolve the issue with the help of you and Joel Short at buckeyestargazer.net. He was, in short order, able to fabricate masks for the wheel that solved the problem. See the enclosed for a before and after shots of the flat exposure with an Ha filter. Also, shots of the masks installed. These are now available on his the buckeyestargazer website. These are specifically for 3 mm thick filters, but Joel would very likely be able to make masks for thinner filters.
