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NICMOS STScI Analysis Newsletter 9

NICMOS STScI Analysis Newsletter 9

March 1998


        - NICMOS NEWS
          + Proceedings of the 1997 HST Calibration Workshop now available
          + New NICMOS Information on the WWW
          + Making Associations From Individual NICMOS Observations
          + New HST field-of-view plotting task available in STSDAS

Proceedings of the 1997 HST Calibration Workshop now available

The printed Proceedings of the 1997 HST Calibration Workshop are now
available, and hardcopies have been mailed to all participants.  The
electronic Proceedings are available at the URL

in both PDF and PostScript formats.  A limited number of printed copies
are also available and can be requested by e-mail to
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:

   * LIST of NICMOS Reference Files (update 13-mar-98)
Since last month, the following Instrument Science Reports have been made
available on the NICMOS documentation Web Page:

   * NICMOS Camera 3 Pointed Flats   
   * First NICMOS On-Orbit Flat-Fields: Results from SMOV Data
   * NICMOS Focus Monitoring
   * NICMOS Focus Field Variations and Focus Centering

Making Associations From Individual NICMOS Observations

Many observers have asked if it's possible to build an association from
originally unassociated NICMOS images, so that they may be processed as
a group with the CALNICB task to create mosaics. Yes, this is in fact
possible. Note, however, that CALNICB does *not* currently handle rotations
amongst images in an association - only simple x/y (RA/Dec) translational
shifts. Therefore the images must not have field rotations, or if they do,
you must first remove the rotations using, for example, the IRAF task 

The following description of what you need to do in order to create an
association does not give a lot of detailed explanation of how each piece
of information that you supply will be used by CALNICB. Please read through
the STSDAS help page for the CALNICB task to discover these details.

There are two basic things that you need to do: 1) create an association
table that will be used to drive the CALNICB processing; and 2) edit a few
keywords in the headers of the images that will be in your association.

Making an Association Table
The first thing you need to do is to make an appropriate association (ASN)
table. ASN tables are stored in a FITS file, in a FITS binary table extension.
You can either copy and edit an existing ASN table that you have for some
other (associated) NICMOS data, or you can copy and edit a template ASN table
that is available from STScI. To get a copy of the template ASN table, ftp to
node, login as "anonymous", and go to directory /pub/bushouse.
There you will find a file named "template_asn.fits". Copy this file (using
binary transfer mode) to your computer.

To edit the ASN table, use the IRAF/TABLES task "tedit". Change the root names
of the datasets in column 1 ("MEMNAME") to match those of the _cal files that
you want to have in your association. Add or remove rows as needed to
accomodate the number of images you have. You also need a row at the bottom
of the table that contains the root name of the output mosaic (_mos) image
that you want CALNICB to produce. Column 2 ("MEMTYPE") of the table should
contain the string "EXP-TARG" for your input images, which indicates to
CALNICB that these are (input) target exposures. Column 2 should contain the
string "PROD-TARG" for the output image (last row), to indicate to CALNICB
that this is the output ("product") target image. Column 3 ("MEMPRSNT")
should contain a boolean "Y" or "yes" in all rows.

Finally, make sure your association table has a file name of the form
"_asn.fits", where "" is any name you like, such as
"cooldata_asn.fits" or "mosaic1_asn.fits". CALNICB will create an updated
association table as output, with a file name of the form 

Here's an example of the contents of an association table for a simple
dither pattern of images:

 nitprint n3af15010_asn.fits
#  Table n3af15010_asn.fits[1]  Tue 19:08:52 24-Feb-98

# row MEMNAME        MEMTYPE        MEMPRSNT

    1 N3AF15ITQ      EXP-TARG            yes
    2 N3AF15IWQ      EXP-TARG            yes
    3 N3AF15IZQ      EXP-TARG            yes
    4 N3AF15J2Q      EXP-TARG            yes
    5 N3AF15010      PROD-TARG           yes

Modifying Image Header Keywords
Now you need to modify a few keywords in the headers of your images that
make up your association. These keywords are divided into 2 groups: the first
group are keywords that contain values that apply to the whole association,
so you only need to modify them in the first image listed in your association
table, since CALNICB will only read them once, from this image; the second set
of keywords contain values that are unique to each image, so you need to
modify them in every image.

The first group of keywords are:

 NUMITER      The number of exposure iterations at each pattern position
 PATTERN      The pattern sequence name
 NUMPOS       The total number of pattern positions observed

These 3 keywords are contained in the primary header (extension zero) of
your NICMOS images. If you use the IRAF task "hedit" to modify them, be sure
to specify that you want to edit extension zero of the FITS file by appending
the string "[0]" to the end of the FITS file name 
(e.g. clhedit n0000001q_cal.fits[0]).

If you've got just one image at each of the various sky positions in your
pseudo-association, set NUMITER to 1. The PATTERN keyword must contain the
name of a recognized NICMOS observing pattern (see the NICMOS Instrument
Handbook). If all of your observations contain the target of interest and
you want them all mosaiced together into a single final image, just use the
"SPIRAL-DITH" pattern name. If you've got images that are of background
regions adjacent to your target and you want them handled separately, use
one of the CHOP pattern names (e.g. "ONE-CHOP", "TWO-CHOP", etc.).
Now set NUMPOS to the total number of positions within your (fake) pattern.
If you've got all target images, just set NUMPOS to the total number of
images. If you've got one pair of chopped target - background images, set
NUMPOS to 2.

The second group of keywords, which must be set correctly in each image
header, is:

 PATT_POS     The position of this image within the pattern
 WCS keywords, giving the sky coordinates of each image

PATT_POS must be set to a monotonically increasing number, starting with 1
for the first image in the pattern, up through NUMPOS for the last image.
The WCS keywords are the "CRPIXn", "CRVALn", "CDn_n" keywords that are
already in the headers of each image and most likely already contain valid
values that you shouldn't have to modify. CALNICB uses these to make a first
guess at the registration of the images, but then refines the guess using
cross-correlation of the images.

The PATT_POS keyword is also contained in the primary header (extension zero)
of your NICMOS FITS image files, so again be sure to use the "[0]" notation
at the end of your file names when editing this keyword. The WCS keywords are
contained in the SCI image extension (extension one) of the NICMOS FITS files.
If you need to edit them, specify "[1]" on your file names.

Now you're finally ready to give CALNICB a try. Just execute the task using:

nicalnicb _asn.fits

and let it (try) do the rest.

New HST field-of-view plotting task available in STSDAS
        by J.-C. Hsu and S. Baggett
STFOV, a new STSDAS task to overplot the science instrument apertures
of the Hubble Space Telescope (HST) on a gray scale image, is now
available. STFOV is basically a "wrapper" script based upon the
existing tasks disconlab and siaper, but with a more user-friendly
interface. The input image can be any image with WCS (= World Coordinate
System) parameters in the header or a Digital Sky Survey (DSS) image;
the input aperture can be any one of the HST apertures (i.e., not
limited to WFPC2 apertures). The orient parameter (spacecraft roll
measured in degrees east from north) is the same as that specified in
the Phase II proposal template.
Other features include:
(1) Appending a new FOV, with user-specified color, to an existing
chart, thus allowing convenient multiple overlays on the same gray
scale image.
(2) Providing rudimentary estimates for magnitudes of stars in the
field, which is useful when planning exposures, e.g., to avoid bright
stars near WFPC2 which may result in scattered light.
STFOV is now available in the newest STSDAS release (2.0.1), under the
graphics.stplot package; please refer to the STSDAS help for more
details and examples.

                          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
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