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Old 11-07-2019, 12:56 PM
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mental4astro (Alexander)
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mental4astro is offline
 
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,618
Cliff, this dark spot is seen in two circumstances in all scopes with a central obstruction:

1, when inside and outside focus it is seen in the centre of all out of focus stars.

How well concentric this dark spot is to the outside of the "doughnut" tells you how well collimated your optics are.

2, when using very low power under light polluted skies.

When I use my Newts (dobs) from home I see the shadow when I use a 30mm eyepiece. It is like a dark ghostly round spot that moves about as you move your head. In my SCT and Mak, I only see really see this shadow when I use a 50mm eyepiece. When you go bush with your scope this shadow isn't seen, only under light polluted skies.

Alex.
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