[ATM] FW: Secondary Cage Calculations

["Geogre Nikolidakis" <geoniko@otenet.gr>]

I know I am new to ATM forum but applying some Geometry
I found this formula.
Ring ID = Aperture + 2 * Tan (AFOV/2)* A 
Where A= distance from top ring to primary mirror.

George Nikolidakis

 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Robert Houdart
Sent: Monday, October 08, 2007 2:27 PM
To: atm@atmlist.net
Subject: Re: [ATM] Secondary Cage Calculations

Arjan,

> For a given aperture you'd expect the ring ID to grow with decreasing
> focal length...
> 
> > DW> ring ID = aperture + 1% of focal length
> >
> > Without knowing the focal ratio and the design field of view that
> > statement is meaningless (like a lot of "rules of thumb")

No, you'd expect the difference between ring ID and aperture to remain
more or less constant.

The largest Field of View is limited by the focuser size. For example,
the largest field stops in commercial 2" eyepieces are about 45 mm
(1.8") wide. The upper cage rings should not block any ray for this
largest field of view.

The top of the upper cage will be at a distance of the primary mirror
approximately equal to the focal length of the mirror.

Simple geometry then shows that the ring ID should be equal to aperture
+ the size of the field stop for the largest Field of View.

In other words, with a 2" focuser the correct ring ID equals aperture +
45 mm (1.8").
If you use a 1.25" focuser, this becomes aperture + 30 mm (1.2").

Robert Houdart
www.cruxis.com/scope/scope1100.htm

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