Although the detectors behaved well from
September 2015 until June 2016, we were
not out of the woods yet. Following a ther-
mal cycling event of the cryostat in June
2016, a new vertical structure appeared in
the bias of CCDs 2 and 3, as well as repetitive
sharp horizontal lines. A few months later
these structures became even stronger, now
seriously affecting the science quality of the
data; in particular, due to the increased noise
over significant parts of the CCD array. Once
again we looked for measures to minimize
the effect and carried out many tests follow-
ing advice from the chip manufacturer… but
nothing seemed to improve the situation; it
seemed desperate.
LSST: Gemini South´s Neighbor
Comes Closer to Reality
New large telescope facilities always attract
attention, and the Large Synoptic Survey
Telescope (LSST), currently under construc-
tion on Cerro Pachón, is no exception. In
December 2016, for instance, two eminent
astronomers — Ewine van Dishoeck (Pro-
fessor of Molecular Astrophysics at Leiden
Observatory and President-elect of the In-
ternational Astronomical Union), and Eu-
ropean Southern Observatory Director Tim
de Zeeuw — visited both LSST and Gemini
South. The images in Figures 2, 3, and 4,
Figure 2.
Staff and visitors inspect
the LSST pier and
enclosure.
Then one day in February 2017 a fault on a
compressor used for the cooling of GMOS-
S caused the cryostat to warm up. We all
feared the worst and were anxious to see
the first bias frame come out of the in-
strument. To everyone’s surprise, the bias
looked normal, without any of the vertical
banding or horizontal stripes!
A Happy Ending?
Well… yes and no. At this moment the
detectors are performing well, and we
are monitoring their behavior; but clearly
something’s not right as more than once
serious problems have come and gone
without any clear indication why. We need
to take stock of the situation and determine
the best way forward.
Figure 3.
View of the LSST
telescope enclosure from
where the LSST coating
plant will be located.
At the time of writing, similar
detectors are being installed in
GMOS-N with great care to avoid
the problems we identified in
GMOS-S; all tests carried out to
date show no indication of prob-
lems with detector bias.
April 2017
GeminiFocus
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