Re: Hobby-Eberle Was Re: [ATM] ATM edge support

["Scott Ewart" <scott.e@quicksurfer.com>]

    Correct.  It measures 11m corner-to-corner, 10m side-to-side.  It has a 
glass corrector for spherical aberration at prime focus which "sees" 9.2m at 
a time.  One thing I found interesting is that the mirror(s) and OTA do not 
move AT ALL during observations.  Only the prime focus instrument moves 
above, and only +/- 6 degrees in two axes (three, if you include its curved 
path through the sphere's focus).  Its altitude is fixed at 55 degrees and 
moves in azimuth by inflating 8 perforated air bags, air hockey style. 
These aspects do limit its access of the sky, but it was done that way to 
reduce cost.  A LOT!
    While the segments of the Keck must be kept aligned and in phase to a 
fraction of a wave (twice per second), the HET's segments need only be 
aligned once in a while by using the tester that sits in the ROC tower.  The 
tolerance for phase or "step" between segments is a whopping 25 microns.  It 
does not take images, only spectra.  Even the spot size is large at around 1 
arc-second.  They're trying to get it down under 0.8.  But without AO, 
that's about as good as the seeing ever gets anyway.
    I too took the tour during this year's TSP.  Our guide deferred to the 
HET operator, who led that part of our tour.
    Any other questions?  I'll see what I can remember.

Scott Ewart


----- Original Message ----- 
From: "Jerry" <wa4guu@verizon.net>
To: "'Richard'" <cnc@cncservo.co.uk>; "'The Alarm Co.'" <atm@atmlist.net>
Sent: Tuesday, May 10, 2005 5:55 PM
Subject: RE: [ATM] ATM edge support


Probably corner to corner.

Jerry

-----Original Message-----
From: Richard

Hi,

Sorry, if you'd said HET I would have know the one you are
talking about. This is the mirror made of 91 hexagonal segments
with an effective diameter of 9.2m. I guess the 11m is the
extreme edge to edge dimension?

-- 

Best regards,
 Richard




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