Subject: DIS2 status

From: Ed Turner

Submitted: Fri, 15 Mar 2002 18:39:40 -0500 (EST)

Message number: 89 (previous: 88, next: 90 up: Index)

 After many trials and tribulations, we are pleased to announce
 that a significant milestone in our
 project to upgrade the DIS detectors has been achieved. DIS is now ready
 for on-sky science observations with the new detectors, although there
 are still some problems.

 Although there is still work to be done to realize the full
 performance potential of the DIS detectors upgrade, the present
 performance of DIS now appears to be better than it was with the old
 chips.  We intend to let scheduled science users try to use DIS in
 its current state on a shared-risk basis over the next few weeks.
 Also during this time, scheduled engineering and open time will be
 used to both finish fixing the remaining known problems, and
 systematically characterize the new performance parameters.

 A preliminary technical description of the new DIS detectors is given
 This page describes the basics, and will soon be linked to new data
 characterizing actual performance.

 The major changes in the detector upgrade are smaller pixels, lower
 read noise, and a larger detector format than we had with the old DIS
 CCDs.  The new electronics also allow for simultaneous readout, which
 saves about 40 seconds of observing overhead per exposure.

 As we understand it, the current status is:

 o The DIS red camera is well-focused and aligned, has good
 throughput, and has low read noise when the camera is read serially
 (i.e., not in parallel with the blue camera).

 o The DIS blue camera is almost focused, but needs a dewar
 modification to get the last ~10% tweak in focus and focal plane
 tilt.  Due to an inadvertent specification error, the AR coating on
 the new blue camera field flattener lens is defective, which reduces
 the throughput of the camera by about 30 to 40% below about 4600
 Angstroms.  This lens will be replaced in April to recover the
 throughput.  The blue camera has low read noise when the camera is
 read serially (i.e., not in parallel with the red camera).

 o In parallel read mode, there is electronic noise in the data for
 both cameras of about 20 ADUs, similar to the read noise levels in
 the old DIS cameras.  This seems to be largely eliminated when the
 cameras are read out serially.

 o The DIS controls in the Remark remote interface appear to work well.

 For tonight, the software cannot be changed to enable serial reads of
 the cameras, so the noise level in tonight's data will be similar to
 that of the old chips unless the users choose to just read one camera
 in an exposure.  By tomorrow night, we hope to be able to implement
 the software changes to make serial reads of the camera the default.
 Over the next weeks, tests will be run to determine a way to read the
 two cameras simultaneously without the noise penalty.

 We are attempting to contact the PIs of the DIS programs for tonight
 and this weekend's observing, and more information will be posted as
 it develops.  We encourage the PIs of the DIS programs over the next
 few weeks to use the instrument in its current and evolving
 condition; this will help us debug and characterize the new
 instrument, and hopefully obtain useful science data.  If you feel
 that the current DIS performance limitations seriously impact your
 science programs, you should consider using an alternative instrument
 for a different approved science program, or contact me for other

 You should also contact your Observing Specialist well in advance to
 determine which gratings are to be mounted for your observations.  Due
 to the new chip format, the entire spectral range is available with the
 medium dispersion gratings and a much greater coverage is possible with
 high dispersion.  On-chip rebinning makes it possible to reduce
 resolution in order to gain sensitivity without going to low resolution

 Kudos to the DIS upgrade installation team, including Jeff Morgan,
 Peter Doherty, and Russ Owen (UWashington) plus Mark Klaene, Craig
 Loomis, Dave Woods, Russet McMillan, Camron Hastings, Jack Dembicky,
 Jon Davis and John Barentine at APO.

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