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NICMOS STScI Analysis Newsletter 1

NICMOS STScI Analysis Newsletter 1

6 May 1997

CONTENTS:

       - NICMOS NEWS
         + Update
         + Status Summary
         + Policy Summary
         + Recommendation Summary

       - APPENDIX:  NICMOS CONTACTS

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NICMOS Update
-------------

Due to the changes to NICMOS performance on-orbit, it has been
necessary to consider changes to the way in which NICMOS science will
be carried out on HST.  As part of this response, GOs and GTOs have
been notified of the possible need to rework some of their observing
proposals to take advantage of the changed capabilities of NICMOS.  To
assist observers in deciding whether and how to revise their NICMOS
observing programs, we have produced a document detailing the latest
information on the status of NICMOS and the first on-orbit measurements
of its characteristics.  The summary sections of this document are
reproduced below.  The full document can be found under the NICMOS home
page on the world wide web which can be found by visiting the STScI WWW
page (http://www.stsci.edu/) and following Observing links to the
NICMOS web pages.

Paper copies of the document will be mailed to the PIs of proposals
that have already been submitted.  We desire to quickly implement the
NICMOS science program, so the deadline for a response to this document
is short:  

*** Observers must resubmit revised Phase 2 proposals, or notify STScI
to proceed with their existing proposal, by MAY 16, 1997.  ***

The document we have produced updates or corrects the information
provided in the Instrument Handbook and the Call for Proposals. We have
made every attempt to include the most accurate information and advice
possible in this document, but users should be aware that in many cases
we are presenting our current best conclusions before completing all
the planned tests and calibrations included in the Science Mission
Orbital Verification (SMOV). Users should bear in mind that at the time
of writing, the instrument has not reached an equilibrium state but is
still evolving. Thus, the final performance cannot be fully predicted.
New and more complete information will be described in future monthly
electronic Space Telescope Analysis Newsletters (STANs) and posted to
the WWW as it becomes available.

*** NICMOS users should continue to consult these resources regularly.  ***

Please be especially careful to check the Advisories Web page before
re-submitting your programs.  If you have questions related to your
Cycle 7 proposal, you should consult your Contact Scientist or Program
Coordinator.


NICMOS Status Summary
---------------------

Soon after launch, early tests of NICMOS revealed dynamic changes to
its characteristics and performance. It is thought that thermal cycling
of the NICMOS dewar during ground operations cryopumped nitrogen to one
end of the dewar. On-orbit expansion of the column of solid nitrogen
has deformed the dewar leading to a thermal contact between the
instrument cold well and the inner vapor-cooled shield and also causing
changes in focus for all three NICMOS cameras. The expected lifetime of
the NICMOS cryogen has been reduced as a result. Best estimates are
that, if the current rate of cryogen loss continues, the useful
lifetime of NICMOS will be approximately eighteen months. If the
thermal short breaks and the cryogen loss rate is decreased, that time
could be longer. As described in this document, NICMOS camera 3 (NIC3)
has suffered the largest loss in performance. Cameras 1 and 2 (NIC1,
NIC2) are performing well and are capable of meeting most observational
requirements. Details on the performance of all three cameras in NICMOS
are covered in the full document, available on the NICMOS WWW site.
Based on our best estimates of the current situation, we will work from
the following set of assumptions:

  --  The useful lifetime of NICMOS will be reduced; we are planning
      for a termination of NICMOS observations by November 1998.

  --  Thermal backgrounds are much lower than estimated in the
      Instrument Handbook.

  --  Image quality in NIC1 and NIC2 is excellent. Both cameras are,
      however, slightly vignetted.

  --  The best focus positions for NIC1 and NIC2 are different, but the
      image quality of NIC1 at NIC2's focus or NIC2 at NIC1's focus is
      only slightly degraded.

  --  The coronograph performance in NIC2 is degraded.

  --  NIC3 cannot currently be focussed. The image quality is very poor
      when either NIC1 or NIC2 are in focus as the prime camera. NIC3
      is vignetted over approximately 25% of the detector.

  --  NIC3 may return to focus as cryogen is depleted, but when this
      may occur cannot be predicted.

  --  The spectral resolution and sensitivity of the grisms are
      seriously degraded for point sources. For extended sources, the
      loss of performance is less severe.

  --  A small percentage of pixels in each camera have reduced
      throughput, possibly from debris on the detectors. The pattern of
      affected pixels is time variable.

  --  Except as noted here, NICMOS performance is as previously
      described.



Policy Summary
--------------

In order to make the most efficient use of the shortened lifetime of
NICMOS, the following policies have been adopted. Questions about the
applicability of policies to specific proposals should be directed to
your Contact Scientist or Program Coordinator.

  --  Observations that require prime use of NIC3 will not be
      supported until that camera reaches a stable configuration;
      these visits will be placed on hold. If not reactivated sooner,
      this hold will be reviewed in November 1997. Proposers who elect
      to have their NIC3 observations executed before the hold is
      lifted must provide their own calibration data from their TAC
      allocation. All such observations will be at the observer's
      risk.

  --  Visits that require the use of the coronograph in NIC2 will be
      put on hold until the coronograph can be characterized in a
      stable configuration.

  --  Coordinated parallel observations will be made with NIC1, NIC2,
      and NIC3 even though optimal focus cannot be achieved. Parallels
      will be executed unless explicitly directed otherwise by the
      observer.

  --  Observations will not be eligible for repeats based on degraded
      performance of NICMOS.

  --  Due to changes in performance, proposers are invited to
      consider modifying their programs to make the best use of cameras
      and observing modes that are available now. However, observers
      must be able to complete their observations within their original
      TAC allocation consistent with their original science objectives.
      No additional orbits will be made available for these changes.

  --  Revised Phase 2 proposals or directions to proceed with existing
      Phase 2 programs must be received at STScI no later than 16 May
      1997 in order to be scheduled. Major revisions made after that
      date will risk long delays in the scheduling of observations,
      possibly resulting in the loss of the program. An opportunity for
      small modifications that do not affect scheduling will be
      available approximately 10 weeks before each visit is executed.



Recommendation Summary
----------------------

We give here a summary of general recommendations. However, observers
are strongly advised to read the technical sections in the full Update
document and decide an optimal observation strategy based on the
demands of their individual scientific goals.

  --  Proposers that currently use NIC3 have the option of redesigning
      their proposal to use NIC2 or NIC1 if their scientific objectives
      can be met. If proposers decide to redesign their program for
      NIC2 or NIC1, they should do so by the 16 May deadline.

  --  Proposers who cannot meet their scientific objectives with NIC1
      or NIC2 should make no changes. Their proposal will be placed on
      hold pending a resolution of the situation.

  --  Where possible, proposers should separate NIC3 exposures into
      separate visits from NIC1 and NIC2 exposures so that the latter
      may be scheduled without delay.

  --  Proposers who make use of the grisms in NIC3 may consider using
      narrow band filters in NIC2 or NIC1 to meet their science goals.
      Otherwise, their program will be placed on hold with other NIC3
      proposals.

  --  Proposers that use the coronograph should decide whether their
      program can be effectively done without the coronograph. If so,
      the program should be redesigned by the deadline. If not, the
      proposal will be placed on hold pending a complete
      characterization of the coronograph performance.

  --  Proposers that use both NIC1 and NIC2 as prime cameras should
      minimize the number of changes between cameras.

  --  Because of problems associated with darks, proposers should avoid
      programs that rely on single long exposures with MULTIACCUM.
      Exposures should be divided into smaller individual MULTIACCUMs.

  --  Proposers should dither observations that could be adversely
      affected by bad pixels. For most observations in NIC1 and NIC2,
      however, dithering will not result in improved resolution.

  --  Proposers with coordinated parallel observations should continue
      to use all of the NICMOS cameras where possible.

  --  Proposers should eliminate restrictive scheduling requirements
      (e.g., ORIENT) wherever possible to decrease the chance that
      their program will experience long delays.

============================================================================

                          APPENDIX: NICMOS Contacts
                          -------------------------
 
Any questions about the scheduling of your observations should be
addressed to your Program Coordinator. Post-Observation questions can
be addressed to your Contact Scientist. If you do not know who these
persons are, you can find the information on the WWW at
http://www.stsci.edu/public/propinfo.html.

Analysis, STSDAS or any other HST-related questions can also be
addressed to help@stsci.edu.
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