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NICMOS STScI Analysis Newsletter 8

NICMOS STScI Analysis Newsletter 8

February 1998


        - NICMOS NEWS
          + New NICMOS Information on the WWW
	  + NICMOS Camera 3 Pointed Flats


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.
Since last month, the following items have been newly posted:

   * NICMOS Parallel Program *PAGE* has been created
   * NICMOS Frequently Asked Questions *PAGE* has been upated
   * NICMOS Photometry Update *PAGE* has been created
   * NICMOS Instrument Status *PAGE* has been upated
   * NCIMOS Focus Update *PAGE* has been upated
   * NICMOS PlateScale Update *PAGE* has been upated
   * NICMOS Coronographic Hole Update *PAGE* has been upated
   * The NICMOS reference files for DATA CALIBRATION list has been updated
     (24 February 98)

Since last month, the following Instrument Science Reports have been made
available on the NICMOS documentation Web Page:
   * Cosmic Ray Persistence in NICMOS Data


A new Instrument Science Report (NICMOS-98-001) describes the impact of
cosmic ray persistence on NICMOS data quality, the tradeoff between
data quality and timing of observations relative to SAA passages, and
strategies to minimize the problem.  These topics are discussed briefly
below. Observers of faint targets may wish to contact their PCs and CSs
about implementing some of these suggestions.

The illumination of NICMOS arrays by a bright source of infrared
photons often results in image persistence, i.e., the continued
accumulation of charge after illumination has ceased.  A similar effect
can arise from exposure to cosmic rays.  Recent examination of NICMOS
exposures of faint galaxy fields reveals that a fraction are affected
by image persistence from cosmic rays, which results in reduced data
quality and sensitivity.  In these data, the count rate from persistent
cosmic rays can approach 0.1 DN/s, comparable to peak pixel counts of
faint galaxies with integrated K magnitudes of 17 in a 4" aperture.

The number of persistent cosmic ray images is found to be strongly
correlated with the timing of the observation relative to HST passages
through the South Atlantic Anomaly (SAA).  Observations taken soon
after emergence from the SAA (< 25 min out of Model Contour 5) and
following long SAA passages ( 10 min spent in Model Contour 2) are
more severely affected.  This correlation was confirmed by the
inspection of darks taken for the NICMOS calibration program.

When the effect of cosmic ray persistence is measured as the additional
spatial noise that is produced (e.g., using the image RMS), 1/3 of the
faint galaxy data are found to be affected (i.e., they have RMS values
larger than data taken in orbits unaffected by cosmic ray
persistence).  This is probably representative of the fraction of
NICMOS data that is affected by CR persistence at faint galaxy flux
levels.  The frequency with which HST crosses the SAA (~1/2 of all
orbits) is roughly consistent with the fraction of darks and faint
galaxy data that are affected by cosmic ray persistence.

Since February 9, 1998, NICMOS has been operating in a new operational
mode that may reduce the impact of cosmic ray persistence on NICMOS
data.  In the current mode of operation, the detectors are turned on
and are in autoflush immediately upon exiting SAA Model Contour 5.  The
longer periods spent in autoflush before science observations are begun
increases the likelihood that cosmic ray persistence is reduced.  The
impact of the new operational mode on cosmic ray persistence is
currently under investigation.

Persistent cosmic ray images are difficult to remove using standard
data processing techniques.  Because they are produced by a finite
count rate, they typically survive cosmic ray rejection in CALNICA.
Cosmic ray persistence will affect programs that require deep imaging,
low spatial noise, and/or a specific level of photometric accuracy.
There are several options that observers may wish to consider in order
to minimize the problem. It is not possible with the HST
scheduling system to simply specify SAA-free orbits:

1. Observing strategy:

(i) In order to reject pixels that are affected by cosmic ray
persistence, frequent dithering between a fairly large number of dither
positions (4) is advantageous. (ii) Because data quality is
preferentially reduced at the beginning of a sequence of observations
compared to observations taken further downstream from the SAA,
observers with programs that, for example, involve imaging in multiple
filters may wish to alternate the filter that is observed first in the

2. Use of preceding ACCUMs:

In principle, by analogy with photon induced persistence, it may be
possible to hasten the reduction of cosmic ray persistence by flushing
the array by inserting ACCUMs with a large number of reads before a
science exposure.  In practice this approach has not been validated
on-orbit.  Preliminary analysis of the NICMOS calibration darks
indicates that single ACCUMs with NREAD=10 (corresponding to 20
detector readouts) produce no measureable reduction in cosmic ray
persistence.  Thus, the number of reads required for this approach to
succeed must be quite large, and the procedure is not guaranteed 
to work at all.

A single NREAD=25 ACCUM DARK with the minimum exposure time (14.279s)
will typically require ~1 minute, including overhead.  Since DARKs are
an available mode for NICMOS, GO's wishing to use ACCUM DARKs for this
purpose must contact their Contact Scientist to obtain approval for the
use of this available mode.  Similarly, the use of NREADs other than 1
or 9 is also an available mode and Contact Scientist approval is
required.  In order to ensure that the preceding ACCUMs are executed
immediately before the science exposure, observers adopting this
strategy should use the SEQ NON-INT special timing requirement to link
the two exposures together. 

NICMOS Camera 3 Pointed Flats

On December 22, 1997, NICMOS Camera 3 flat field observations
were obtained with the calibration lamp.  Observations were obtained
with all filters, broad, medium, and narrow band filters.
The resulting Camera 3 calibration reference files were delivered
and can be retrieved from the HST Archive.  These reference files
were installed before the start of the January 1998 NIC3 Campaign
on January 10, 1998.  All Camera 3 observations obtained before this
date were calibrated with either a ground-based or preliminary SMOV flat
field and should be recalibrated.

A one orbit monitor program was executed during the January 1998 NIC3
Campaign on January 22, 1998.  Flat field (lamp-off and lamp-on) observations
were obtained using filters F110W, F150W, F160W, and F222M.  Delta-flats
have been created using this data, and have been delievered as well.

A comparison between the delta-flats and the recommended flats created
from data obtained before the NIC3 campaign indicate no significant
differences.  However, the bottom ~15-20 rows are variable due to different
amounts of vignetting over time.  The vignetting edge - the edge of the Field
Divider Assembly (FDA) - moves over time.  This appears to cause about a +/-
5% uncertainty in the throughput in this region.  The amplitude of the
difference in throughput for this region depends upon the amount of motion of
the FDA relative to the flatfield reference file epoch.

Data Analyst Position Available at STScI
The Space Telescope Science Institute currently has openings for
Data Analysts. Data Analysts in the Science Support Division help General
Observers and Archive Researchers analyze HST data, work with Instrument
Scientists in calibrating the HST instruments, and work with STScI staff
on grant-supported research projects. These research projects span a range
of size scales from comets and planets to the large scale structure of
the universe and a range of wavelengths from radio to X-ray astronomy.
Applicants should possess a B.S. degree (M.S. degree a plus) in astronomy
or physics, or equivalent; experience with astronomical research; familiarity
with scientific computing; expertise in data analysis; knowledge of IRAF, IDL
or other software packages for astronomical data analysis; and programming
ability. Additional mathematical, statistical, and computer skills are
desirable. Candidates should have the ability to work with a minimum of
direction, enjoy research, and possess skills to develop excellent working
relationships. Candidates should send a cover letter with current curriculum
vitae and the names of three references to:
Human Resources Manager
Space Telescope Science Institute
3700 San Martin Dr.
Baltimore, MD 21218
Women and minorities are strongly urged to apply. AAE/EOE.

                          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
Analysis, STSDAS or any other HST-related questions can also be
addressed to
To subscribe or unsubscribe send a message to with
the Subject: line blank and the following in the body:
                    [un]subscribe nicmos_news YOUR NAME
Comments, questions, suggestions, etc. can be e-mailed to
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