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| You need to flip the image to be able to make comparisons. Mine 6" flops too but that doesn't detract form achieving perfect flats. Your first image shows clearly that the image needs flipping before subtracting. |
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Okay I just flipped the light after the meridian flip and then substract it. pattern still appears.  andrea tasselli: |
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| You need to flip and match, obviously. Same as if stacking across the meridian flip. Then the residue will tell you nothing other then there is a residue from what is yet to be determined. On top of that you need to normalize the two frames otherwise there would always an offset due to different illuminations as seen by the primary. |
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This image is a substaction of two light, one before the flip the other after the flip, rotated and registrated.  Pattern still appears. there is something shifting in my imagetrain after moving my telescope. |
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| Yet you didn't normalized one against the other as can be seen for the residual stars present in the image. And yes, there is something shifting but then it is pretty normal in newtons, as far as primary movements are concerned. The issue is you really should not have a darker circle in the middle of the sensor at all, as illumination should followd a 4th power of the sine angle from the center of the optical axis law (which should coincide with the center of the sensor if properly collimated). |
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Your subtraction test clearly shows that things are not stable. However this test does not tell which part of the scope is responsible, so you have some detective work that needs doing. Focuser, primary cell, secondary spider or the tube itself are the key areas to check. You can sort of test the focuser by hanging a weight of some kind from a collimation laser and checking if the spot moves on the primary in different tube orientations. You could repeat the test without a weight on the laser, which may indicate that something else is the issue. Its fairly likely that you have a combination of many stability issues here, and it can be difficult to point out a single weak point. |
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Okay update guys. So I just tried yet another coma corrector. After trying the GPU/ Maxfield/ baader and starizona nexus, I tried the Tele vue paracorr. The problem is gone and the flats work perfectly. I got F4.6 now what is not what I first wanted to have, but at least it works now. I got 2 theroies. The GPU and Nexus as well the Maxfield got like a chambers inside which are highly reflective. See images. Maybe under heavy light pollution they cause are round shape pattern. No idea how to preveng it. Maybe by opening the corrector and flock it... not sure about that. Another reason might be that my focal plane is now 40mm further away, thats how the paracorr is calculated. The corners are pitch black because of it. (Do not bother me) Maybe by shifing the focuspoint further outside something happens... the only other cause i can imagin is something with the secoundary mirror. Causing a round pattern. Maybe the edge of the mirror is shiny and causes reflections. here a flat with the paracorr. Looks totaly different:  |
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GPU Cell Nexus cell  Tele Vue paracaorr  As you can see the paracorr got just one large chamber with no direct reflection. The other two got something going on in the inside. My secondary mirror is very large and can illuminate up to 50mm in diameter at 130mm distance from the tube. So maybe some light is hitting these sharp edges and causes some reflection. But just a theory |
