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NICMOS STScI Analysis Newsletter 2

NICMOS STScI Analysis Newsletter 2

May 1997

CONTENTS:

        - NICMOS NEWS
          + New NICMOS Information on the WWW
          + NICMOS Focus Update
          + Description of the ORIENTATION update
          + Mr. Staypuft and a Redefinition of Zero

        - PARALLELS -- COMING SOON TO AN ARCHIVE NEAR YOU

        - APPENDIX:  NICMOS CONTACTS

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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
(http://www.stsci.edu/) 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 posting were made available:

	* CORRECTIONS to the SMOV NICMOS UPDATE.
	* ORIENTATION update


NICMOS Focus Update
-------------------

As reported in last month's STAN, an expansion of the dewar led to a
large motion of the cold-well which changed the focus of all three
cameras from their nominal values and pushed NIC3 beyond the usable
range of the internal (PAM) focus mechanism (NIC1 and NIC2 are still
well within the PAM range). In early April the expansion stopped and
the cold-well began moving back slowly towards its nominal position.
As of this writing in late May, the bench has come back almost half of
the distance it was off the end of the PAM range, bringing the NIC3
focus 3.5 mm nearer. To bring NIC3 fully back into focus will require
another 4 mm of motion in this same direction.  However, the motion
appears to be slowing at this time, and the measurements over the last
week indicated very little change in the cold-well position.  The
nominal "best focus" positions for each camera have been updated to
reflect the changes since the last update, and the NIC3 position is
still set to the closest possible PAM setting.


Description of the ORIENTATION update
-------------------------------------

Use of ORIENT, PATTERN-ORIENT, and POS TARG
May 20, 1997

An extensive advisory on the use of the ORIENT visit-level special
requirement, the PATTERN-ORIENT exposure-level optional parameter, and
the POSition TARGet exposure-level special requirement has been posted
to the STScI NICMOS ADVISORY page. Observers who are mapping well
defined sections of the sky should review this to ensure that they will
get what they want. It is always a good idea to add a comment
describing in words what you intend to do with a complicated exposure.


Mr. Staypuft and a Redefinition of Zero
---------------------------------------

Two effects which NICMOS GOs should be aware of have come to light
during the SMOV and ERO observations to date.

The first is an effect which may have been seen by some ground-based
users of NICMOS arrays, but which we had not encountered until
recently.  NICMOS arrays are divided into four quadrants of 128x128
pixels each.  When a very large number of counts are recorded in a
source at pixel i,j of any one quadrant, a faint `ghost' of this image
appears at pixel i,j of each of the other three quadrants.  A faint
band is seen running along detector rows which pass through these
ghosts, which could be mistaken for a diffraction spike.  The amplitude
of the ghost images is of order a tenth of a percent of the `real'
image count rate.  It appears to be an electronic phenomenon and is not
an optical ghost.  Inside the NICMOS group, for convoluted reasons, we
have been referring to this as the `Mr. Staypuft anomaly'.

The second is a more serious problem, and will affect any observations
of bright sources taken during the early part of Cycle 7.  The total
counts accumulated during a NICMOS observation are determined by
differencing the `zeroth' read and the final read.  For the case of
ACCUM mode observations the differencing is done on-board the
spacecraft, while for MULTIACCUM it is done in the calibration
pipeline.  The accumulated counts are then used to carry out a
linearity correction.  The linearity correction is now known to be
larger than was expected prior to the Thermal Vacuum testing last
summer, and it becomes significant at much lower total counts.

There is a problem inherent in using the difference between final and
zeroth reads:  rather than resetting and reading each pixel at the
beginning of an exposure, the entire array is reset and then the entire
array is read.  The elapsed time between reset and read is 0.203
seconds.  For a bright source, this is sufficient time for significant
counts to accumulate.  The counts which have accumulated during this
0.203 second period are discarded by the zeroth read subtraction, which
is the first thing that is done by the calibration pipeline developed
by STScI in collaboration with the NICMOS IDT; the same is done
on-board for ACCUM mode.  For a bright source, this may amount to many
hundreds or thousands of ADUs.  When there are large total accumulated
counts, the linearity correction can be several percent, but the
recorded counts in the processed data are now offset by the number of
counts that were thrown away with the zeroth read -- so the linearity
correction is incorrect.

Secondly, because of the offset of the data, the total counts recorded
in the processed data may now never reach the value defined as
`saturated' in the calibration reference files!  In this case, the
observed count rate, which is used by CALNIC for detecting Cosmic Ray
hits, will suddenly drop at the point where the pixel has in fact
saturated. However, because the accumulated counts never reach the
expected saturation threshold, CALNIC never flags the pixel as
saturated.  Now, since the count rate changes drastically during the
course of the exposure, CALNIC may flag many, or in some cases all, of
the reads during the MULTIACCUM exposure as Cosmic Ray hits!

We are working on a technique to fix this problem in CALNIC by using
reads early in the exposure, prior to any possible pixel saturation, to
determine the count rate and working backwards to determine how many
counts should have been accumulated during the initial 0.203 seconds.
This result can then be used to update the counts in subsequent reads,
and so obtain the correct linearity correction and saturation
threshold.  This is a major revision to CALNICA, which will not be
ready in time for GO observations which may be scheduled during June,
July or August.  The problem is more insidious for ACCUM mode:  in this
mode, the zeroth read is subtracted on-board, and never returned to the
ground.  We thus have no way to establish in CALNIC whether a pixel was
really saturated in ACCUM mode, or whether the linearity correction
applied was correct.  Observers planning to use ACCUM mode for
observations of bright sources, or at wavelengths where the background
count rate is itself very large, should be aware of this problem.

Finally, these CALNICA problems also lead to problems with CALNICB,
which is invoked for producing mosaicked images of associated data.  If
the observations are dithered, a particular part of a source may
saturate in one image but not be flagged as saturated because of the
`non-zero zero read' problem; however, that same pixel may not be
saturated in a subsequent observation because the dither moved it onto
a much less sensitive part of the detector.  If CALNICA fails to flag
problem pixels, then CALNICB can fail when it combines the multiple
images.  This problem should go away when CALNICA is repaired to deal
with the zeroth read correctly -- but not for ACCUM mode, where the
problem is irreparable.

Observers whose data is affected by this problem should consider
re-calibrating their data once the revised algorithms become
available.  We will send notification of availability via the STAN and
the WWW.


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PARALLELS -- COMING SOON TO AN ARCHIVE NEAR YOU
-----------------------------------------------

The capability of HST to obtain parallel observations has been vastly
expanded with the addition of NICMOS, STIS and the Solid State
Recorder.  Likewise with three imaging instruments all of which can
operate in parallel with each other, and each of which provides unique
capabilities for parallel science studies the value of pure parallels
is very high.  The Cycle 7 Telescope Allocation Committee recognized
the expanded capability of HST for pure parallel science, and
recommended that STScI take on the task of managing large, archival
pure-parallel programs as a community service.  Data to be obtained in
such programs will be made available immediately to the full community
via the HST archive.

Pure-parallel, archival observations with STIS and NICMOS have
commenced as of 2 June 1997.  These initial parallels are implemented
with very simple single orbit exposure sets that are simply replicated
onto as many opportunities as possible in a given week.  As the ground
system is developed further this summer, we anticipate that a new set
of pure parallels will be implemented that more fully utilize the
existing capability by using information on sky position and
characteristics of the primary observing window to craft the individual
parallel observations.  A committee including TAC members and
astronomers with interests in parallel science opportunities will
provide recommendations on the appropriate science to address and the
parallel observations needed to carry this out.

The current parallel observation units are: for STIS a short image with
the CCD followed by a long G750L slitless spectroscopy exposure at the
longest wavelength setting (coverage from 6460-11490A), for NICMOS the
F110W, F160W and F222M filters are cycled through with NIC2 in focus
using the SPARS256 MULTI-ACCUM sequence, F110W and F160W on NIC1 and
F160W plus a grism, G141 exposure on NIC3 using the same sequence.
Given the lack of parfocality of NIC3 with the other two cameras
operated simultaneously, the grism observations are not expected to be
scientifically useful.  They will give useful technical data on the
NICMOS background characteristics, however.

The proposal numbers for these community support programs are 7675 and
7676 for STIS and NICMOS respectively, and 7700 and 7701 for shorter
visit versions of the same that can fit in smaller opportunities.  In
the first two weeks the ~40 minute parallel units for both STIS and
NICMOS schedule about 15 times for one orbit pointings of the prime, 6
times for 2 orbits, 6 for 3-4 orbits, and 6 for 5-10 orbits.  The
shorter ~20 minute parallel units receive a comparable number of
"hits".  As an illustration, a 6 orbit prime window results in total
exposure times of 1800s STIS/CCD clear image, 10,800s for the G750L
slitless spectroscopy, and 4608s (F110W), 6144s (F160W), and 4608s
(F222M) on NIC2.  The STIS and NICMOS exposure time calculators may be
used to determine how deep such exposures will go for your favorite
objects -- the limiting magnitudes are impressive for multi-orbit
cases.  For NICMOS the high quality diffraction limited infra-red
imaging, coupled with a lower than anticipated thermal background allow
exceedingly faint limiting magnitudes particularly in the H band.

A web page, http://archive.stsci.edu/pure_parallels.html, has been set
up to quickly show users what observations have been done in these
programs, with the most recently archived datasets listed first.
Extensions of these programs, which may entail additional proposal IDs
will be incorporated in this web page in a transparent fashion.
Registered archive users will then be able to retrieve the data through
this page.

The priorities for WFPC2 are to finish the approved Cycle 6 GO
parallels before phasing in an appropriate archival, pure-parallel
program.  The Cycle 7 approved GO parallel programs will have high
priority for execution beginning this fall when the more robust ground
system implementation for parallels becomes available.

There is no proprietary period for the community service pure
parallels, so these data will be the first STIS and NICMOS observations
broadly available to HST users and may be used not only for science,
but also to obtain a measure of how well the new instruments perform.


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                          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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