Date: Thu, 19 Aug 1999 13:41:29 -0700
From: Patrick Waddell
Subject: focus and vane adjustments
I am not sure what Russell is recommending by changing the "primary to
common corrector distance", but a focus value for the EC of -3.4 mm is
pretty surprising.  Is it possible that work on M2 has put the nominal
position of the optic closer to the primary by ~2.7 mm?
Recall from the following discussion on collimation:
http://www.astro.washington.edu/waddell/collim/2_5coll.html
the vanes for M2 must be adjusted such that when the M2 is at mid range of
travel in piston (focus = 0.0 mm), the M2 to C1 spacing should be at the
nominal spacing for the spectroscopic design, or 3620.62 mm.  This will
allow for the range of piston control needed for "zoom" or focal surface
scale adjusment.  During the initial setup of the telescope, in April98 we
had a special metering rod made to make this spacing measurement as the
vanes were tuned.  I am sure that French knows where these parts are.
Some older EC focus values:
As recommended by Jim Gunn the height of the ec was adjusted to be such
that the difference in the M2-M1 despace between the mosaic camera and the
ec (for on-axis objects) is 1.9 mm in the sense that the m1-m2 despace is
greater for the ec.  As a check on this I noticed that on very different
nights in March99 we had focus values for the ec ranging from -650 mm to
-0.700 mm, and around +1.503 for the mosaic camera; a delta of about 2.2 mm.
-pw
206.795.5469
>
>The focus for the engineering camera last night was thousands of
>microns from zero. This was the one value I was told about; I'm not
>sure how it varied over the evening. In any case, it pushed the
>actuators to fairly near their reverse limit and suggests that the
>primary to common corrector distance could be improved.
>
>Unfortunately I don't have the list of focus values seen, so I can't
>suggest a better value. I hope the values were recorded.
>
>What is the situation on open-loop focus correction? Is the TCC
>receiving current weather data yet? Does anybody have a reasonable
>estimate for the temperature coefficient based on the temperature of
>the secondary truss? Given this information we could immediately
>enable open loop focus correction, which I think would be a very good
>thing to do.
>
>-- Russell
>