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Old 29-12-2018, 04:41 PM
Wavytone
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Wavytone is offline
 
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
OK Martin, nice pic which tells all. Two problems evident - and its not your spider vanes:

1. You have 3 mirror clips protruding over the edge of the primary. These cause 6 weak diffraction spikes.

Lose the clips and find another way to restrain the mirror without impinging on its circular perimeter. Google is your friend.

2. Large lump at the bottom protruding into the light path, this will create a big pair of diffraction spikes. At a guess this is the bottom of the drawtube of your focusser. From the photo you can estimate how far it sticks in, moving the mirror up by the same amount closer to the secondary mirror will move the focal plane out and ideally enough to ensure the draw tube does not stick into the optical path.

FWIW the intensity of diffraction spikes is related to the area of the optical path obstructed by each object that obstructs the path - the bigger the obstruction the more intense the spikes.

There are three ways to eliminate diffraction spikes:

a) find the cause and remove it, out of the optical path (schiefspieglers, maksutov-newtonians);
b) ensure an obstruction has a curved shape of the same radius as the perimeter of the primary mirror (curved spider vanes);
c) ensure an obstruction is precisely circular and concentric with the primary mirror (secondary mirror cells).
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