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NICMOS STScI Analysis Newsletter 13

NICMOS STScI Analysis Newsletter 13

July 1998

CONTENTS:

       
        - NICMOS NEWS
          + New NICMOS Information on the WWW
	  + Updated versions of CALNICA and CALNICB
	  + NICMOS Coronagraphic Program Summary

        - RECENT PREPRINTS

        - APPENDIX:  NICMOS CONTACTS
 
============================================================================
  
New NICMOS Information on the WWW
---------------------------------
	by Doris Daou

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
(http://www.stsci.edu/) and following Observing links to the NICMOS
Web pages.
 
Since last month, the following items have been newly posted:

     * May 26-27, 1998 Sardinia Workshop Proceedings, 
       Posted from the Documentation PAGE (July 15, 1998)

     * The NICMOS Frequently Asked Questions PAGE, Updated: 16 July 98

     * The NICMOS Reference Files List, Updated: 21 July 98

Since last month, the following ISRs have been published:

     * Characterization and subtraction of well-exposed HST/NICMOS
       Camera 2 point spread functions for a survey of very-low-mass
       companions to nearby stars (June 8, 1998) 
     
     * Second NIC3 Campaign Results (July 13, 1998)

     * NIC1 Narrow-band Earth Flats (July 22, 1998)


Updated versions of CALNICA and CALNICB
---------------------------------------
	by Howard Bushouse

CALNICA v3.2 and CALNICB v2.2 were installed in the OPUS production
pipeline on July 16, 1998. The changes to CALNICA mostly involve
enhancements to the MultiAccum-mode zero read signal correction step,
and the implementation of two new data quality (DQ) flags. The new
version of CALNICB also includes the implementation of the new DQ flags,
as well as the addition of several new task parameters which allow users
greater flexibility in controlling the software when reprocessing
associated data sets.

CALNICA v3.2 includes the following specific modifications.

 - The ZSIGCORR calibration step (which detects and corrects for non-zero
   signal in the zeroth read of MultiAccum exposures) now uses the new
   ZSIGCORR, ZSIGDONE, and SAMPZERO header keywords, which have been
   included in NICMOS science images since April 1998. Setting the
   ZSIGCORR keyword to "OMIT" will turn off this step when reprocessing
   data. When processing older NICMOS images that do not contain these
   keywords, suitable defaults are used.

 - The ZSIGCORR algorithm has been enhanced to use any available good data
   in the zeroth read for pixels that are already saturated in the first
   readout. These data are passed to the CRIDCALC step for populating the
   output _cal file for these pixels. The ZSIGCORR step also now sets the
   new ZEROSIG DQ flag (value 2048) for all pixels with detected signal
   in the zeroth read. Hence users may expect to see these DQ values in
   their output _ima and _cal files if the images contain very bright sources.

 - All routines within CALNICA have been modified to handle the new ZEROSIG
   DQ flag, as well as another new flag (value = 16) which will be used in
   the bad pixel mask reference file (MASKFILE) to flag pixels affected by
   "grot" within the instrument. Note that both of these new flag values
   are treated by CALNICA as warnings only. Thus pixels containing only these
   DQ values are not rejected from any processing step, but the flags are
   passed on to the output files so that users may inspect their images
   for pixels affected by these conditions.

 - New image statistics keywords - GOODMEDN, QAMEDN, QBMEDN, QCMEDN, QDMEDN -
   have been added to NICMOS image headers. CALNICA v3.2 populates these
   keywords in the output _ima and _cal files with the median of good 
   (i.e. unflagged) pixel values from the whole image and from each image
   quadrant.

CALNICB v2.2 includes the following specific modifications.

 - All processing steps have been updated to handle the new ZEROSIG and GROT
   data quality flags. As with CALNICA, the ZEROSIG flag is treated as a
   warning only and pixel containing this flag will not be rejected. The GROT
   flag, however, is treated as an error condition by the image combining
   steps, so that pixels with this flag will be rejected and replaced by
   good pixel values (if available).

 - CALNICB also populates the new median image statistics keywords in the
   output _mos file headers.

 - There are six new task parameters which allow users to control various
   aspects of the processing. The new parameters are subbkg, meanbkg, readbkg,
   readoffsets, crthresh, and xcwin. The subbkg parameter switches the
   scalar background subtraction step on and off (default = on). The meanbkg
   parameter controls whether the scalar background is computed as the mean
   background level in all images, or computed and subtracted on an
   image-by-image basis (default = mean). The readbkg and readoffsets
   parameters instruct CALNICB to read the scalar background values and
   image offsets from the input association table, rather than computing
   them. This allows users to take an output ASC table from a previous
   execution of CALNICB, possibly modify the image background or offset
   values, and rerun CALNICB using the modified ASC table as input and
   overriding the values that would normally be computed. The crthresh
   parameter sets the cosmic-ray rejection threshold level used in all
   image combining steps. The xcwin parameter sets the size of the
   cross-correlation search window used when computing image offsets.

 - The routine that computes the scalar background level in each image now
   computes the median of the pixel values, rather than the mean. This helps
   to avoid biasing by sources that are present in the images.

 - The processing flow has been modified slightly, so that sources are
   identified in all images before computing any image offsets. This allows
   the cross-correlation routine used to compute the image offsets to use
   only pixels flagged as containing sources and helps to eliminate spurious
   correlations caused by hot pixels or residual cosmic-ray hits.


NICMOS Coronagraphic Program Summary
------------------------------------
	by Al Schultz

NICMOS flight software (FSW) version 4.0A was activated on 
3-Feb-1998.  NICMOS Camera 2 coronagraphic programs started
executing on 6-Feb-1998 following the successful execution of
an on-board system engineering test (program ID: ENG/NIC 7924). 
Monitoring of all the coronagraphic programs indicated an
anomalous jump in the position of the coronagraphic hole as
determined by the FSW, and a slight difference between the
FSW determined and "Ground-Truth" IRAF determined positions
of the coronagraphic hole. In total, 38 on-board ACQs were
evaluated. 

A review of FSW version 4.0A yielded no variable nor declaration
errors that could explain the quantization of the FSW determined
hole position that would result in the observed 1-pixel jump
in the inferred hole position. Instead, the hole centering
was finding a maximum instead of a minimum, which explains
the jump.

The review of FSW 4.0A did result in two suggestions to
improve the hole centering routine. The size of the checkbox used
to determine the hole centroid was increased from 7x7 to 15x15 pixels.
Second, the image of the hole would be inverted by subtracting the
image from a constant.

NICMOS FSW 4.0B was activated on 28-Jun-1998 with first on-board
acquisition following activation on 29-Jun-1998.  A total of 19
on-board acquisitions were reviewed (executed between 29-Jun-1998
and 13-Jul-1998).  Eleven independent on-board acquisitions were
evaluated. IRAF was used to reduce the data and the technique used
mimic the FSW. The FSW determined hole position is not saved,
but can be inferred from the target location and the resulting
slew which are saved. A comparison between the FSW inferred
hole positions and the IRAF determined hole positions show a mean
radial offset of ~0.08 pixels. Independent verification by the
NICMOS IDT through alternate methods of reduction/analysis yield
concurring results (radial offset of ~0.06 pixels).

These preliminary results strongly indicate that FSW 4.0B performance
is quite acceptable and meets or exceeds the stated requirements
(FSW tolerance goal within ~0.25 pixels) as stated in the Flight
Readiness Review (FRR) report "NICMOS Flight Software 4.0B".

In support of NICMOS coronagrahpic programs executing a second
on-board ACQ in the same orbit (visibility period), STScI has 
decided to use the updated hole solution from the first ACQ for
the second ACQ.  This is making possible 2-roll observations
reducible to the photon noise limit, except for those observations
for which noticeable focus change occurs due to breathing.
The FSW corrects the absolute pointing to better then the
requirements, and the suppression of a twice-per-visibility
position update enables precision re-targetting for PSF subtraction.

As of 21-Jul-1998, a total of 100 on-board acquisitions have
executed out of a total of 271, representing 36.9% completed.

============================================================================
 
                         RECENT PREPRINTS
                         ----------------
 
We draw your attention to these papers, based on NICMOS data, that
will appear in the next few months.  This list includes all preprints received
by the STScI Library not yet published in the journals.  Please remember to
include our Library in your preprint distribution list.
 
KRIST, J.E.; GOLIMOWSKI, D.A.; SCHROEDER, D.J.; HENRY T.J.
"Characterization and Subtraction of Well-Exposed
HST/NICMOS Camera 2 Point Spread Functions for a Survey of
Very-Low-Mass Companions to Nearby Stars"  PASP 9-98
www.stsci.edu/science/preprints/prep1263/prep1263.html

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

                     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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