MDM Trouble Report for 2010 Nov 11

Telescope: 2.4m
Observer(s): Haojing Yan (OSU)
Instrument: OSMOS

Problem(s) Encountered:
  I really wish I would have nothing to report but unfortunately I
do. The same focusing problem struck again. But the good news is
that the source of the problem might have been located. And also
my earlier believe that the SDSS filter set and the BVRI set are
not parfocal might not be valid -- it might just be an illusion
from this focusing problem.

  It again started with the deep imaging. While I have carefully
focused using the same filter (SDSS-u') before the exposure began,
and nothing was wrong about the guide star, the 20-min exposure
still came out elongated. The next exposure was the same. I had
to stop, and spent the next two hours looking into this. First I
found that the  best focus  values changed from time to time, and
it was not just for the SDSS-u'. I tested SDSS-r' and broad-band V,
and they all came out the same. I tried to reduce the exposure
time to 10 min, and the images were still not focused. This really
suggested that something bad was happening on a very short time
scale. 

   Bob told me yesterday that the focus does change as the
temperature changes. I do believe this, but I just had a hard
time believing that it could ever change so rapidly. Bob also
showed me how to monitor this change at the telescope 
airconditioning system. While I didn't believe that temperature
would change that much within minutes (it would have to be at 
least 1-2 degree Celsius of change to have the impact like what
I saw), I decided to look at it. And this completely blew me off.
When I first looked at it, the  Truss Arm  temperature was 
fluctuating every second and the range was like 2 degrees. It 
soon went wilder and fluctuated by many degrees with minutes.
During a short period of time it would stay within a degree. It
must have been through many cycles of this already in the past
few hours. I then decided to see if this change (and hence the
suggested change of focusing values) was real. I put in a 10-min
exposure, stood in between the hand paddle and the computer room,
and tried to adjust the focus according to the suggested values.
While I (or more precisely, the hand paddle) could not respond
to the rapid change in real time, the exposure came out with
round, not-elongated star images. I tried this multiple times, 
and it actually worked.  

  This seems to suggest that there is some real temperature
change at the trust arms. However, the temperature sensors do
not seem to work properly, either -- at around 3:00am, the
temperature fluctuated in several hundreds of degrees, and the
champion was some  -3233 degrees ! I took a bunch of photos of
the computer screen with my iPhone while looking at these
changes. While I didn't catch that -3000 degrees thing, I did
have a shot showing the temperature was -7.1 degrees. 

  So in short: 1) the focusing problem seems to be related to
the trust arms temperature fluctuations shown in the air
airconditioning system ; 2) such fluctuations are really 
happening; 3) the temperature sensors might also have problems. 

  I hope we can do something about it soon. This is a great
telescope; and my impression is that this particular *site* that
this telescope resides might even be slightly better than elsewhere
on the mountain. For three days in a row, I recorded 0.8-1.0 
seeing at the beginning of the night right after just being done
with the telescope pointing. But each time it very quickly (within
minutes) turned to something like 1.3 -1.4  in the best case. I 
very much suspect that the whatever problem with the trust arms
does not strike until the telescope begins slewing from place to
place. If we can fix this problem, we might well be able to constantly
enjoy <1.0  seeing, which is nothing to snore at!

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Submitted on 2010 Nov 11 [8:26:29]