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NICMOS STScI Analysis Newsletter 3

NICMOS STScI Analysis Newsletter 3

June 1997


        - NICMOS NEWS
          + New NICMOS Information on the WWW
          + NICMOS Exposure Time Calculator has been Updated
          + NICMOS Update



New NICMOS Information on the WWW

Similar to all Space Telescope Instruments, the NICMOS home page on
the World Wide Web is updated with all instrument developments. The
NICMOS home page can be found by visiting the STScI WWW page
( and following Observing links to the NICMOS
Web pages.

The NICMOS ADVISORY page is a very important source of information,
updates, news and status reports on the instrument. The observer is
encouraged to visit this page periodically. Since last month's STAN
issue the following postings were made available:

   * A NICMOS Focus Update is available and updated on a biweekly basis
   * A complete list of NICMOS reference files for DATA CALIBRATION
   * The Exposure Time Calculator has been updated using a new grid of PSFs
Since last month, the following Instrument Science Reports have been made
available on the NICMOS documentation Web Page:
	* NICMOS Cycle 7 Calibration Plan
	* Cosmic Rays on NICMOS: Results from On-Orbit Data
	* Persistence in NICMOS: Results from Thermal Vacuum Data

NICMOS Exposure Time Calculator UPDATE!

Now that tests of the absolute photometry capabilities of NICMOS and of
its thermal background have been carried out and analyzed, we have made
further adjustments to the ETC.  First, we have now been able to
compare observed PSFs of bright targets with those predicted by the
revised tinytim (with NICMOS included), and find that they agree to a
remarkable extent.  Therefore, the ETC has been revised to use a grid
of monochromatic tinytim NICMOS PSFs, and to synthesize a PSF for a
given filter by combining PSFs across the filter passband, with
appropriate weighting for the input source spectrum.  This has led to
some changes in ETC sensitivity predictions.  Secondly, a small error
has been identified in the treatment of the secondary mirror
obscuration by the ETC, and this too has been corrected.  In the light
of these corrections, the ETC agrees very closely with the predictions
of the Synphot package available in the STSDAS IRAF package.  The
differences between the newly revised ETC and the former version are
significant, at the few tens of percent level, and GOs may wish to
check the new version.

Camera 3 and the grisms are now available once again in the ETC.  We
are currently using Camera 3 PSFs calculated for a PAM position of
-9.5mm and no secondary mirror adjustment, which means the images are
slightly out of focus.  It is possible that campaigns may be run during
which the secondary is moved to allow Camera 3 to be refocused.  We may
make a further revision to the ETC in the near future to accommodate
this possibility.  If this occurs, the resulting Camera 3 sensitivities
will be slightly improved over those now expected.

Finally, we note that while the ETC and synphot agree very closely, the
two do not accurately reflect the observed on-orbit throughput.  We
find that the observed throughput is lower than predicted by a few tens
of percent at the shortest wavelengths, with reasonable agreement at
the longer wavelengths.  The reason for this is unclear, but it appears
that the instrumental throughput is simply lower than one can predict
based on the measured mirror reflectivities, filter transmissions,
detector quantum efficiencies, and known aperture efficiencies.  STScI
and the NICMOS IDT have not to date been able to determine the reason.
We are working on an empirical, wavelength dependent correction to the
DQE files which will bring our ETC and Synphot predictions into
agreement with observations and we hope to put this into the online ETC
soon.  Watch this space for details!


The NICMOS science program began in June with early execution of 
a number of observations using the in-focus NIC1 and NIC2 cameras.  
The instrument is returning excellent quality data with 
diffraction-limited performance in both cameras.  Results of one of 
the first science observations, measurement of the gamma-ray-burst 
source GRB 970508 (described elsewhere in this Newsletter by A. 
Fruchter), are publicly available from the HST archive.

In addition to GO and GTO science data which are, in general,
proprietary, a large program of publicly available parallel
observations has been implemented.  In the first week of this program,
67 orbits of NIC1 and NIC2 imaging were completed and placed in the HST
archive.  NIC3 images were obtained at the same time but are far from
focus because the PAM mirror was set for the NIC2 parallels.  A
comparably high rate of scheduling is continuing into the following
weeks.  A special link has been established for fast access to the
public NICMOS data from the ARCHIVE page off the STScI WWW homepage.

Improved versions of the NICMOS pipeline were implemented in mid-June
with better handling of cosmic ray rejection.  A more sophisticated
mosaicking module (CALNICB build 2) is expected in late June.
Observers with observations taken early in June may benefit from
re-calibrating their data with the new versions of CALNICA and CALNICB.

Darks and flats continue to be measured on-orbit.  On-orbit flats 
have been measured for approximately half of the filters.  For the 
remaining filters, flats measured during thermal vacuum testing are 
being used.  However, the unexpected higher temperature of the 
detector on-orbit causes small flat-field residuals when the 
ground-based flats are used.  The measurement of on-orbit darks for 
all 16 available MULTIACCUM sequences is on-going.  The current 
status of pipeline calibration files is maintained on a page under 
the NICMOS WWW page.  

Still incompletely understood effects in the detector cause anomalous,
time-dependent dark currents that appear to be correlated with cycling
the amplifiers between idle and active states.  Several possible
solutions to this problem are being actively investigated at the time
of this writing.  Meanwhile, good progress has been made in developing
an empirical model of the various components contributing to NICMOS
dark current and it is now possible to produce acceptable synthetic
darks with this model.

Cosmic ray rates in NICMOS have been measured on-orbit and are close 
to predictions made before launch.  Approximately 2 pixels per 
second are affected by cosmic rays outside of the South Atlantic 
Anomaly (SAA).  An ISR describing these measurements is available on 
the NICMOS www page (D. Calzetti). 

During SMOV a population of pixels with low throughput was 
identified.  Contaminant inside NICMOS, possibly flecks of paint 
from a baffle that was compressed by the expansion of the dewar, has 
been identified as the likely source.  Approximately 100 pixels in each 
camera are affected, some single pixels, and a few larger groups of 
pixels.  Some of these bits of contaminant or "grot" have moved by 
small amounts while others have remained fixed.  This small 
population of bad pixels currently poses only a minor threat to 
science observations and will continue to be closely monitored.

The first measurements of throughput and quantum efficiency (QE) have
been made on orbit. Measurements of a G standard star revealed the
throughputs to be lower than expected by 10-30%.  Analysis of existing
flats made with the calibration lamp show no evidence of time
dependence in the throughput or QE.  Synphot and the NICMOS exposure
time calculator are in the process of being modified to reproduce
NICMOS's observed photometric performance.

We have continued to monitor changes in focus in all three cameras 
and are closely monitoring temperature inside NICMOS.  NIC3 is 
continuing to move back towards the focusable range of the Pupil 
Alignment Mechanism (PAM) mirror.  However, after an initially rapid 
motion, the movement of NIC3 has slowed in recent weeks. Updated
information on the latest focus measurements and focus history is
now posted on the NICMOS WWW pages.  An adjustment to the PAM
position for NIC1 and NIC2 was made in June to follow the evolving
focus of these two cameras.  NIC1 and NIC2 remain at separate foci,
although the performance of either camera at the other's focus is
good because of the relatively small difference in focus between
these two cameras.  

Also in June, the location of the NIC1, NIC2, and NIC3 apertures were
moved away from the geometric center of the detector arrays to nearby
locations.  The sites of the new aperture positions were chosen to be
relatively flat regions with good detector QE that lie close to the
center of the array and are well clear of contaminant.  Users are
reminded that the positions of these apertures may change with time as
changes in the detector arrays demand.  However, as always, the
NICn-FIX apertures will remain fixed at the geometric center of the

The NICMOS observer community is reminded that we are continuing to 
characterize NICMOS as rapidly as possible.  We will continue to 
post new results on our web site.  This should be checked regularly 
by those who need the latest information of the status of NICMOS and 
our understanding of optimum observing and data analysis strategies.


                          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

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addressed to
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