Re: [ATM] Gimbaled Secondary mirror

[Aplanatic@aol.com]

Regarding the secondary spacing issue, I'm afraid I disagree.  The  primary 
to secondary spacing is very important in any Cassegrain.  For  example, I ray 
traced an optical system similar to the one that you have.   I find that 
incorrectly spacing the secondary mirror by a half an inch causes  the on-axis 
Strehl ratio to drop from 1.0 to about 0.5.  
 
With your long focal ratio you may not notice a degradation of  this 
magnitude because of atmospheric seeing, but it is there nonetheless.
 
Yes, there is nothing new about movable secondary mirrors, but the pros  
carefully keep the focal plane in the correct location so that when the  secondary 
is used to focus the telescope, it focuses at its aplanatic  position.
 
-- Dave
 
 
In a message dated 6/30/2005 2:18:00 A.M. Pacific Daylight Time,  
atm@hudler.org writes:

Don't  have any experience with Dall-Kirkham's but, perhaps they are
sensitive to  LSA/TSA due the intrinsic nature of spherical mirrors.

I've run this  through OSLO and CODEV and I don't see any appreciable change
in aberration  for the various back foci. There's nothing new about a movable
secondary,  most large telescope use this method of focus. 

PI has already spec'd  the actuators for this mass and frequency and has
eaten most of my  budget.

I'm just more interested in the gimbal; it may not be worth it  due the small
angles involved and yes that 12 arc minutes to move half the  focal plane.

>Did you know that the spherical aberration of a  Cassegrain telescope
>changes substantially with the primary to  secondary spacing?  With an  f/3 
>primary and a secondary  magnification of ~2.5, the primary to secondary  
>spacing in our  20" corrected Dall-Kirkham needs to be set within about +/-
>1.5  mm  to achieve best possible performance.   


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