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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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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. |