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Thanks for the comment Mark. On closer inspection I think I can see traces of glue between the OTA and the rear cell so looks like you were right!
I have been spending a lot of time working on the collimation of this scope. On a night of good seeing and after a long cool down time, I can see some pretty well-defined symmetrical diffraction rings just outside focus.
However, if I rack the focuser further in the same direction into focus and out the other side, I get a blurry, flared ,yellowish doughnut with little defined ring structure. (I have checked my eyepieces and the colouring is not due to them). From other blogs that I have read, this seems to be due to a turned down edge.I don't think this is caused by mirror slop as I am always pushing the mirror in the one direction.
Any advice or comments much appreciated.
John
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