[ATM] Secondary Offset

["Mitchell R" <funnybone101@embarqmail.com>]

     I know this question comes up often but I want to understand the
workings of Secondary mirror offset better. 

 

Why exactly is it optically correct to offset it? 

How much does it affect performance?

Is it much more crucial on a larger mirror where it is usually a greater
offset?

Does a faster focal ratio need more or less offset usually? Is it more
important in fast focal ratios?

How do you calculate offset?

Is there a "rule of thumb" when it reaches a certain amount it is necessary
to offset?

 

Specifically I am making the structure for my 10" F/4.8 Mirror. It was
concluded on my first scope (6" F/6.5) the offset was not necessary but I
don't want to just draw the same conclusion here. If secondary size matters
I am planning on a 2.6". My calculations for 70% illumination say a 2.8"
diagonal; Mel Bartels says a 2.5". So 2.6" (stock size by Antares) is a good
enough in-between as this will be a deep sky scope. 

 

I know these are a lot of questions; short and sweet answers are fine. Just
want to get this settled so I can order my spider so I can build the
secondary cage, assemble the OTA, etc and get things rolling.

 

Mitchell 

 

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