From: Jon Brinkmann
Date: Tue, 30 Dec 2003 10:31:07 -0700
Subject: Re: 2.5m obslog 20031229 (MJD 53003)
>
>                        APACHE POINT OBSERVATORY SDSS 2.5M
>
>                                  OBSERVING LOG
>                             Monday December 29, 2003
>
>                                   (MJD 53003)
...
>         Guide stars are streaks in 78 and gone by 80.  All axes are moving
>         and indicate tracking.  Maybe, but clearly not sidereal.
>         Slewed telescope so we crossed fiducials on all axes.  Found 3300
>         count error on azimuth.  Azimuth was around -34 at the time.
>         I can't believe we just ran over a moth after the temperatures
>         we've had the last few days.   I can't believe it, but that's
>         just what happened.  There was a fresh moth squish right next to
>         the encoder!  It was in the single digits F last night!!!
I believe moths have anti-freeze for blood!  Seriously, the lower
enclosure is a moth haven in cold weather, as there are many warm
surfaces, e.g., UPS units, motors, etc..
...
>         I don't recall ever having so much trouble getting the spectrographs
>         into spec.  I am wondering if we need to adjust the focus rings of
>         both the red and blue cameras so that the collimator can be moved
>         to "zoom out" a little and still be in focus?  It does however
>         bring up the uncomfortable question of what has changed so that
>         we would need to do this?
I hope not!  Proper focus is achieved when both red and blue cameras
are focused at infinitiy and the fibers are at the collimator's radius
of curvature (the collimator is presenting a parallel beam to the
gratings and cameras).  Focus is lost when either of these conditions
is not met.
Using the Hartmann doors alternately and noting the image shift is the
best way to adjust the collimator.  Once it is presenting a parallel
beam, the camera focus can be measured and adjusted.  Moving the
spectra around on the chip is done by tilting the collimator and thus
the parallel beam.
If the focus changes for no obvious reason, either the fibers,
collimator and/or cameras have moved relative to one another.  If the
position of the spectra changes, add the central optics to the list of
possible culprits.  This said, changes in telescope focus or cartridge
position in the focal plane of the telescope will affect spectrograph
focus, since the fibers are not entirely incoherent.  If the focus
changes similarly in both spectrographs for no obvious reason as it did
here, I would suspect the cartridge or telescope.
Jon