Re: [ATM] Gimbaled Secondary mirror

[Aplanatic@aol.com]

Hi Jack,
 
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.  In light of this I wonder if it's a  good idea to 
use the secondary position as a means of focusing the  telescope.  Probably 
not.
 
How fast do you want the corrections to be performed?  A while  back I 
analyzed the problem of moving a large, heavy secondary fast  enough to outperform 
an AO-7.  No chance.  Well, sorry, anything is  possible, but it takes a huge 
stack of piezo material and a massive,  high-voltage audio amplifier to move a 
secondary of this size with a  closed-loop bandwidth of, say, 50 Hz.  I 
realized then that the AO-7  is a bargain.  There is no advantage to moving the 
secondary that I can  think of, except the small light gain due to the absence of 
the tertiary mirror  in the AO-7.
 
Anyway, if you're thinking of gimbals then I assume that the correction  rate 
will be on the order of a few Hz or less, which is achievable, just not the  
best way of doing business IMO.  See above.  What I don't understand  is the 
"12 minute tip/tilt without translation."  Is that 12  arcminutes?  Perhaps you 
have some interesting application that I can't  envision.
 
Did you know that the isoplanatic region of a 20" telescope is less than a  
couple of arcminutes in diameter?  In other words, even an  ideal tip/tilt 
system will only produce significantly better images over a  small region, and 
only on nights where de-focus is not the dominant atmospheric  seeing 
degradation.  For a graphic example of the limited size of the  isoplanatic region, see 
 
_http://brucegary.net/ASD/_ (http://brucegary.net/ASD/) 
 
While you're there check out some of Bruce's other work.  I don't  think 
you'll be disappointed.  Well, I guess that depends...
 
-- Dave
 
In a message dated 6/29/2005 7:54:28 P.M. Pacific Daylight Time,  
atm@hudler.org writes:

Hi  all,

I'm working on a design to gimbal our secondary mirror for active  optics.
Plus the design includes of a movable ram to allow for a wide range  of back
focus settings (8 to 25 inches) with a precision of 5 microns over  a 5 cm
travel. 



I'm looking for ideas on a gimbal that will  allow a 12 minute tip/tilt
without translation. I would like this to work  without any projections
larger than the secondary mirror however; there is  10.8 inch diameter baffle
that surrounds all this. 



Any  pivot joints will be using CFlex bearings and I hope to be able to  drive
the tip/tilt with a couple of PI LV Piezo Actuators.  



Optical specs on the system are here if you're  interested.



http://www.lonestarobservatory.org/Website/WebPage.aspx?Page=Optics



Trying  to keep the cost below  $8000.





Thanks,

Jack



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