Quote:
Originally Posted by Bombardon
Despite poor seeing last night, using another John Bambury mask, comprising a disc of thin plywood with a 4" offset hole, on my Dob 10" F5 last night
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Hi Eugene,
As you point out there can also be benefits in using an off axis aperture mask on a Newtonian, this can become very beneficial as the aperture gets larger and the seeing gets worse.
The off axis mask essentially turns your 10"/F5 obstructed Newtonian into a 4"/F12.5 Unobstructed Newtonian. This helps by reducing diffraction as the spider vanes and central obstruction are cut out and the smaller aperture can cope better under turbulent seeing conditions. If the scope has a decent mirror it essentially becomes a high quality 4"/F12 APO refractor. I have at different times in the past used a 7" off axis mask on my 18"/F4.5 Obsession which turn it into an unobstructed 7"/F11.5 scope, or in other words a 7"/F11.5 APO. We also have a 10" Off Axis mask for one of the 25" 3RF Obsessions but we haven't used it in a while as the bigger scopes generally only get pointed at DSO type targets.
Nice work on splitting the Pup under less than ideal conditions.
Cheers,
John B