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Wonderful ist light and bodes well once its all sorted.
Yes you need a flattener. All RC scopes are corrected for coma only. They still suffer to some degree from spherical aberration meaning off axis stars will be to some degree out of focus. Only corrected Dall Kirkhams or corrected compund scopes like corrected Casasegrains adjust for this. In the RC a field flattener handles the bulk of this. In your image you can see the stars near the centre are sharply in focus yet the edge stars are clearly out of focus. It is also worse on the right side than the left. This may be some other issue like tilt in the camera or sag in the focuser under weight.
Basically the bottom line is to get ideal performance from any 2 mirrored scope it requires a corrector lens of some description usually a doublet sometimes a triplet to correct for the spherically induced aberrations.
Even your $50,000 RCOS ideally still needs a flattener (although not as badly needed).
Ask Paul his posts say he uses a Tak flattener.
Greg.
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