The design of the filter is pretty common I'm assuming. Totally basing it on the 2 examples of mounted filters I've seen here .
The mounting is a couple of pieces of aluminum. I'll call the bottom part the seat and the top part the fastener. The glass rests on the seat and is held in place by the fastener. On the glass, there is a ring of black on the very outside edge. I don't know what this is, but my guess is some kind of anti-reflective coating. Anyway, it is THAT circle around the glass of the filter that is not completely covered by the aluminum mounting. That's what seems to cause the "not quite circular" effect. When I spin the glass in the filter to different positions in the mounting, I get the results shown in the latest screenshots. These 3 screenshots are a small sample of the number of actual test images I took (and the only ones I actually remembered to screenshot). I must have assembled and disassembled that EFW and that L filter 30 times trying to spin the glass and find a "sweet" spot where the "not quite circular" part would be the least intrusive. The third screenshot above is about the best I could get things to line up.
Yes, the known good filter shows a tiny bit of cutoff in the corners. However, as you point out, it is evenly distributed. As to why it's happening at all? I don't know. LYAstro wrote in post #50
...The another reason is that using the 1.25 inch filter is at the very limits of providing proper coverage of the large ASI294mm sensor. Due to the combination of a focal reducer, the Antlia 1.25" filter cannot be completely covered. 1.25 inch filter's effective aperture > 26mm, filter thickness is 2mm.
This could be true, I suppose. My purchase and my knowledge was/is based on the calculator at astronomy.tools telling me the 1.25" was plenty big enough, by people here on these very forums using 1.25" filters with this camera/wheel/scope, by ZWO providing a kit including the 1600MM pro and 1.25" filters, etc. According to the calculator at astronomy.tools, for my camera, scope, reducer combo, the minimum filter size is:
24.72mm if the filter is 10mm from the sensor
25.48mm if the filter is 15mm from the sensor
26.23mm if the filter is 20mm from the sensor
Last I checked, 1.25" is 31.75mm. This _should_ give me more than enough coverage. Maybe the mounting holding the glass is more than 5.5mm? I really don't know. What I do know is that the filter absolutely cannot be more than 20mm from the sensor, otherwise it would be flush up against the back side of the EFW because the total distance of sensor + EFW = 26.5mm.
Edited by jonnybravo0311, 11 March 2021 - 07:57 AM.