Narrowband Filters ASI2600MC Pro - Problem with Flats

MJoker

I'm having major problems with my Baader MPCC MKIII in connection with ZWO EFW, Optolong L-Ultimate and ASI2600MC Pro on my Skywatcher 200PDS.

Camera is connected to EFW with filter, then there is a spacer and MPCC that slides in the focuser.

Pics with all other filters calibrate just fine but with the L-Ultimate between the camera and MPCC, there is a horrible
ring that will not disappear using calibration frames. The flats have a bright very clear ring on them when using MPCC but the ring goes away immediately after removing the MPCC. The ring also disappears when I move the filter in front of the MPCC. Below are two pics on what it looks like with and without the ring (no flats but just 2s shot on a white wall to test).
Filter in front of MPCC:

Filter between MPCC and Camera:

This causes naturally all stacked pics to have the same ring visible but only on green and blue channels, not on red.

I found some discussions online saying that older MKII versions of the MPCC had problems with internal reflections when used with narrowband filters. They said that the problem either disappeared or significantly reduced when they moved to MKIII but since I have the MKIII already, there is no room for improvement there.

The strangest part in all of this is that I never had any problems when I was using ASI071MC Pro but now with the 2600MC Pro the problem has appeared. I don't have the ASI071MC Pro anymore so I can't test it unfortunately.

Have you heard of this kind of a problem earlier and would you have any solutions? Could it be that my MPCC is MKII and MKIII could solve the problem? If not, do you have any suggestions on a coma corrector I could perhaps use instead of MPCC?

MJoker

One more detail: I still have the tilter suoplied with camera on the camera. I’ve understood the filter should be as close to the sensor as possible so, I’ll try and remove it or push ot forward in the imaging train.

Are the any best practices to build this part of the imaging train?

Tech@ZWO

MJoker Does this ring changes if you adjust the EFW position?

MJoker

Tech@ZWO Not really. I've enabled the rotation in one direction and calibrated the EFW. The circle is always the middle and only appears with the combination of ASI2600MC Pro, L-Ultimate and Baader MPCC MKIII. If I remove any one of the three in the equation, the problem disappears.

If I change the EFW to filter drawer, the problem persists. Earlier when I owned ASI071MC Pro, there was no problem.

Could it be that the UV/IR cut glass in the 2600MC Pro cause the problem when connected with MPCC? Is it possible to order a glass that I could replace the current glass to something without filter built-in?

MJoker

This problem was probably solved by changing the Coma Corrector from Baader MPCC MKIII to TS Maxfield 0.95X. Based on some flat pics, the ring/circle is gone.

What still puzzles me a bit is that the same combo worked just fine with ASI071MC Pro but with ASI2600MC Pro it produces the circle. Probably has something to do with the UV/IR filter embedded in the sensor window as that is the only difference optically between the two cameras...