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Hubble Space Telescope
NICMOS STScI Analysis Newsletter 32

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| NICMOS
| STScI Analysis Newsletter
| NICMOS STAN 32
| August 2004
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Visit the NICMOS web site http://www.stsci.edu/hst/nicmosw
"New in the Last 45 Days" for all new information about NICMOS.

CONTENTS (Optional):

-HST/STIS Failure

-New NICMOS Photometric Calibration Constants

-NICMOS Coronagraphic Polarimetry

-NICMOS Calibration Lamps

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http://www.stsci.edu/

HST/STIS Failure

K. Noll

On Tuesday, August 3, HST/STIS entered a "suspend" state in
response to the loss of 5-volt power in the Side 2
electronics.  The HST/STIS was commanded to Safe Mode on
August 6, 2004.  The input power and temperatures have
reached their nominal safe mode ranges and stabilized.


     The HST/STIS Side 1 electronics suffered a short
     circuit in May 2001 and are currently not working.
     Failures in the two redundant Sides make HST/STIS
     unusable. While it is possible further investigation
     will point out a way to restore the HST/STIS to a
     useful state, we believe it unlikely that HST/STIS
     can be revived without physical servicing. Fortunately,
     all other science instruments and the observatory
     itself continue to function normally.
					STScI Director
					Steve Beckwith


Due to the reduced heat load in the aft shroud following
the safing of the HST/STIS, the temperature of the aft
shroud has been decreasing.  This additional cooling has
also decreased the NICMOS detector temperature.  The
NICMOS Cooling System (NCS) set point will be raised to
maintain the NICMOS temperature at 77.1 K.

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http://www.stsci.edu/hst/nicmos

New NICMOS Photometric Calibration Constants

R. de Jong

The NICMOS group has derived new calibration zero-points for
the NICMOS cameras, both pre- and post-NCS installation.
Due to the change in the nominal operating temperature of
the detectors from ~62K before the NCS-installation to the
current 77.1K, the photometric keyword values have changed.
The QE has increased by 20-90% (depending on wavelength) and
different sets of calibration constants have to be used for
pre- and post-NCS data. 

In the effort to calibrate NICMOS after NCS installation,
all pre-NCS Cycle 7 calibration data were also analyzed
again with the latest data reduction techniques. This has
resulted in new calibration values for these data as well.
These photometric values have changed for Cycle 7 data by
typically 5-10% due to a change in the way the aperture
corrections (ACs) are applied in regards to the photometric
keywords for each filter.  Photometric aperture radii of
0.5" for NIC1 (11.5 pixels) and NIC2 (6.5 pixels) and 1.0"
for NIC3 (5.5 pixels) were used to measure the flux from 
the standard stars (as before), but now instead of a fixed
AC for all filters of a given camera, a different AC was
used for each filter to correct the small aperture flux to
an infinite aperture flux.  The new photometric keywords
should be applied to infinite-aperture measured fluxes.
This is a change from photometric keywords released
previously, which required a specific AC be applied to flux
measured in a specific aperture radius.

The new PHOTTAB tables with calibration constants for
Cycle 7 and Cycle 11 and beyond were installed in the OPUS
pipeline on July 29, 2004. Any observations retrieved from
the archive before July 29, 2004 will have the old
photometric keyword values. Retrieving this data again
after July 29, 2004 will fix the problem, as the on-the-fly-
reduction (OTFR) will automatically populate the headers
with the correct calibration values.  Alternatively,
headers can be updated by hand using the tables listed on
our web pages:
http://www.stsci.edu/hst/nicmos/performance/photometry/

No new photometric calibration of the polarimetry filters of
NIC1 and NIC2 and the grisms in NIC3 has been performed. The
calibration values for these optical elements in the
calibration tables reflect the initial calibration of NICMOS
performed in Cycle 7 and have only been corrected for the
observed detector sensitivity gain for the post-NCS era.
THESE VALUES WERE DERIVED WITH A DIFFERENT METHODOLOGY AND
SYSTEMATIC OFFSETS WITH THE ZERO-POINTS OF THE OTHER FILTERS
OF ORDER 5-10% CAN BE EXPECTED.

All updates to the calibration standards will be announced
through STAN announcements.  A description of the new
photometric calibration and the most up-to-date values
can be found on the NICMOS photometry web site:
http://www.stsci.edu/hst/nicmos/performance/photometry/ .

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http://www.stsci.edu/hst/nicmos

NICMOS Coronagraphic Polarimetry

D.C. Hines (SSI) & G. Schneider (Steward Observatory)
 
A recent commissioning program has shown that the HST/NICMOS
Camera 2 polarizing filters can be used successfully in
combination with the coronagraph. This significantly enhances
the imaging polarimetry mode by enabling polarization
measurements of regions near a bright object, such as a star
or active galactic nucleus.

Image artifacts within about 2 arcseconds of the coronagraph,
caused by scattering off the edge of the hole, are repeatable
and induce only a small instrumental polarization (IP ~2%).
This IP component and the current calibration data limit
calibratable polarization measurements to about 8-10%
(4 sigma in percentage polarization) per 2x2 pixels
(approximately resolution element) near the hole. However,
the observed stability of the IP component implies that
future refinement to the calibration and further
characterization of the scattering about the coronagraphic
hole may improve this limit.  A more complete analysis
(now underway by us) will place stronger constraints on
the range of polarizations available to this mode, both as
a function of polarization of the source and as a function
of radial distance from the coronagraphic hole.

Observers considering the use of the coronagraph combined
with the polarizing filters should follow the standard
recommendations for two-roll coronagraphic imaging, and
remember that images through all three polarizers must be
obtained at each roll.  We also recommend that observers
include observations of an unpolarized standard star in
addition to their primary target object, and that the
standard star be observed at sufficient depth to obtain
similar S/N to the primary target in each polarizer. Thus,
a minimum of two orbits per target is typically needed; i.e.,
target star and unpolarized standard star.  A single, well
exposed unpolarized standard star should be sufficient for
a multi-target science program.

Cycle 14 proposers are encouraged to contact the HST Help
Desk (help@stsci.edu) for assistance when proposing for
NICMOS coronagraphic polarimetric observations.  Details
about the coronagraphic polarimetric observing mode will be
presented in a future NICMOS Instrument Science Report (ISR).

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http://www.stsci.edu/hst/nicmos

NICMOS Calibration Lamps

A. Schultz and E. Roye

A review of the NICMOS flat field monitor data shows a
continuing decrease in the flat field lamp intensity.  The
Lamp=1 filter count rates have decreased by ~0.5% and 
LAMP=2 by ~1.5% over 22 months. LAMP=1 is used for NIC1 and
NIC2 flat fields and NIC3 narrow band filter flats, while
LAMP=2 is used for NIC3 broad band filter flats. The
small decrease in the flat field lamp intensity will not
affect the photometric calibration as the photometric
calibration is not dependent upon the flux from the lamps.
The photometric calibration has remained relatively constant.

The decrease in lamp output is consistent with the expected
decrease in lamp output due to the number of lamp-on hours.
Details can be found in NICMOS Instrument Science Report (ISR),
ISR-1999-012, title "NICMOS Calibration Lamp Stability," L.
Bergeron and J. Bacinski 01 Mar 1999.
http://www.stsci.edu/hst/nicmos/documents/isrs/isr_99_012.pdf

The life time of the lamps will not be a problem as the
lamps are expected to be fully operational until the HST end
of life (EOF). A summary of the flat field monitor data
review will be presented in a NICMOS ISR.

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| Recent NICMOS Publications
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There are currently 33 NICMOS refereed publications for
2004.  Many NICMOS observations were retrieved from the
HST Archive in support of ground-based and HST observing
programs. Some publications reported new NICMOS observations
while others reported follow up observations to existing
programs.  A few publications of note are listed below:

Bloemhof, E.E. 2004, "Remnant Speckles in a Highly Corrected
Coronagraph," ApJ, 610, L69-L72.

Kneib, J.-P. et al. 2004, "A Probable z = 7 Galaxy Strongly
Lensed by the Rich Cluster A2218: Exploring the Dark Ages,"
ApJ, 607, 697-703.

Riess, A. et al. 2004, "Type Ia Supernova Discoveries at
z > 1 from the Hubble Space Telescope: Evidence for Past
Deceleration and Constraints on Dark Energy Evolution,"
ApJ, 607, 665-687.

Ueta, T., Murakawa, K., & Meixner, M. 2004, "HST/NICMOS
Imaging Polarimetry of Proto-Planetary Nebulae: Probing of
the Dust Shell Structure via Polarized Light," preprint.

Yan, H. & Windhorst, R.A. 2004, "Candidates of z =
5.5-7 Galaxies in the Hubble Space Telescope Ultra Deep
Field," ApJ, 612, L93-L96.

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