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Any reason why you’re using a flattener on a Newtonian Reflector ?? One would assume that flatteners were primarily designed to correct fields from refractors.
A good quality coma corrector is all you should need eg TS GPU on a standard Newt.
I’ve used 8” and 10” Newts for years and just required a coma corrector to eliminate coma from the corners.
I would check your tracking ( guiding ) if using a guide scope and PHD2 on the following -
Image Scale ratio between main scope and guide scope ( stick around 1:4 )
Differential Flexure between main scope and guide scope ( no shoe type guide scope brackets ) Use an OAG where possible
Check settings in PHD2 using the installation wizard
Get a good calibration at Dec = 0 ( utilise Calibration Assistant )
Utilise a Dark Library in PHD2
Check polar alignment and ensure your error is below say 2 or 3 arc min
Carry out Star tests on an area of the sky that has more stars ( brighter stars ) in the field to check all areas of the image.
Check collimation using both a Laser and Cheshire
Check focuser for any slop in the draw tube and also tilt when using grub screws
Martin
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