Re: [ATM] Cassegrain, primary mirror perforation size

[Michael Peck <mlpeck@hughes.net>]

>I'm on my way to finish rough grinding on my recent project, a 12"
>RC-Cassegrain. The next step will be drilling the central hole into the
>primary mirror, but I'm still a bit concerned about the diameter of that
>hole. So I would like to present my data for discussion. I used Michael
>Peck's spreadsheet (
>http://home.earthlink.net/%7Emlpeck54/astro/cassbaff/ ) to calculate the
>baffles and got 89.66 mm at the primary baffle front. So I decided for a
>hole diameter of 97 mm (3.425") to be on the save side and get the
>baffle-tube through the hole. Are there any big mistakes in my
>calculations?


At 12:27 PM 2/28/2007, Richard wrote:
>Hi Heiner,
>
>passed. This has been verified against other programs. It's up to you
>what you add on for baffling, etc, but 97mm for a 300mm primary sounds
>huge.

My posts to the list have been disappearing lately, but with Simeon's 
help we may have figured out why, so I'll try again. Sorry if this is 
a duplicate.

My spreadsheet calculates baffle dimensions to give a fully shielded 
(from direct sky flooding) and unvignetted field of any specified 
linear diameter as seen from the focal plane. For some reason Heiner 
wants over a 58mm diameter shielded field. That results in nearly a 
50% obstruction (146 mm) from the secondary baffle, and a 90mm 
diameter primary baffle. A slightly larger perforation than that 
should be fine provided of course he really wants those baffle dimensions.

The spreadsheet calculation gives an exact algebraic solution 
(assuming I actually did the algebra correctly). Of course in 
practice one might be willing to trade off some vignetting and/or sky 
flooding for a smaller obstruction or easier construction.

Without knowing what he plans to use the telescope for it's hard to 
say if a 50% obstruction is reasonable. If he's doing imaging with a 
really large format CCD it might be.

Mike Peck



------
Michael Peck
mpeck1@ix.netcom.com

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