Re: [ATM] Cassegrain, primary mirror perforation size

[Otterstedt@t-online.de (Heiner Otterstedt)]

Hi Richard,
> As far as I can see, you have not given any of your design parameters
> anywhere, so I cannot comment.
Ups, sorry, the parameters were only the spreadsheet. Here is the output 
of my cassegrain program:

Primary mirror diameter ..................    300.0000 mm
Primary mirror focal length ..............    750.0000 mm (f/2.5)
Primary mirror conic constant ............     -1.0914
Primary mirror sagitta ...................      7.5000 mm
Secondary minimal diameter ...............     95.6190 mm
Secondary diameter .......................    101.8615 mm
Secondary Radius of curvature ............    695.4113 mm
Secondary Magnification ..................      3.2000
Secondary mirror conic constant ..........     -4.5269
Secondary mirror sagitta .................      0.9325 mm
System focal length ......................   2400.0000 mm (f/8.0)
Back focal length ........................    254.0000 mm
Linear Obstruction .......................     33.9538 %
Obstructed Area  .........................     11.5286 %
Distance secondary from system focus .....    764.9524 mm
Distance secondary from prime focus ......    239.0476 mm
Distance secondary from primary ..........    510.9524 mm

My desired fully illuminated field of view is 0.7° (29.3 mm).

Provided this data your program calculates a hole diameter of 57.681 mm 
which I think is a bit too tight. At a focal ratio of f/8 I need at 
least (254 mm backfocus / 8 + 29.3 mm desired field) 61.05 mm to let all 
paraxial rays pass. Michael Peck's spreadsheet suggests a perforation 
diameter of about 75 mm, which I think is meant as the minimum diameter. 
Because his spreadsheet calculates a baffle front diameter of about 90mm 
I decided for my 97 mm perforation diameter.

Best regards, Heiner

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