NICMOS STScI Analysis Newsletter 17
March 1999
CONTENTS:
- NICMOS NEWS
+ New NICMOS Information on the WWW
+ Dealing With NICMOS Image Anomalies:
Current Situation and Future Goals
+ Data header errors
- DATA ANALYST POSITION AVAILABLE AT STScI
- RECENT PREPRINTS
- APPENDIX: NICMOS CONTACTS
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New NICMOS Information on the WWW
---------------------------------
by Sherie Holfeltz
As with all Space Telescope instruments, the NICMOS home page is updated
regularly. This page can be found from the STSCI home page
(http://www.stsci.edu) by following the "Instruments" link.
Since the last STAN, the following ISRs have been published:
ISR-99-001:
Analysis, results and assessment of the NICMOS warm-up
monitoring program (T. Boker, J. Bacinski, E. Bergeron, D.
Gilmore, S. Holfeltz, B. Monroe, M. Sosey, February 22, 1999)
ISR-99-002:
Color Dependence of NICMOS Flatfields (A. D. Storrs, L. E.
Bergeron, S. T. Holfeltz, February 26, 1999)
Since the last STAN, the following items have been updated:
Interpolated flat field software and calib. files, off of the
software link on the NICMOS page
Dealing With NICMOS Image Anomalies: Current Situation and Future Goals
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by Howard Bushouse
As most users of NICMOS data are already aware, certain unpredictable
behaviors of the instrument often lead to image anomalies that are not
handled or removed by the routine pipeline calibration processing system.
So while the original calibrated data received by a PI or an archival
researcher may be adequate for an initial "quick look" analysis of the
data, it is generally necessary to reprocess the data, employing
additional techniques in order to produce scientifically accurate results.
There are several different types of image anomalies that can appear in
NICMOS data, depending on the specific spacecraft and instrumental
environment at the time of an observation. All currently known anomalies
are described and illustrated in the "Image Data Anomalies" section of the
NICMOS instrument pages at the STScI web site:
http://www.stsci.edu/ftp/instrument_news/NICMOS/nicmos_anomalies.html
Some of the more common anomalies are bias offsets (commonly known as
"pedestal"), residual shading signal, bias jumps, bars, and cosmic-ray
persistence. In the near future we will be adding information to these
pages describing known correction procedures to treat the anomalies. In
the meantime, here are a few tips to recognize and alleviate some of the
most frequent problems, together with an indication of the work in
progress at STScI on the subject.
*** Pedestal ***
The "pedestal" effect is a DC offset or bias that is leftover in an image
after it has had the dark reference file subtracted from it. Normally the
dark reference data would be a perfect representation of the actual
instrumental bias and dark current, but the NICMOS detector bias is
temperature dependent and the temperature of the detectors varies over
both long and short timescales. Typical pedestal amplitudes are a few DN
up to about 15 DN. Each quadrant of an image can have a different pedestal
value. The effect of leaving a residual bias in the image during the
normal calibration processing is that multiplication of this constant by
the flat field image then leaves an imprint of the flat field response in
the calibrated data.
The simplest forms of pedestal removal involve just measuring and
subtracting the median value of each quadrant (excluding sources) before
flatfielding the image. However, care must be taken to also account for
the detected background signal, which will not be constant across a
quadrant because it will be modulated by the QE response of the detector.
Thus more sophisticated methods use iterative algorithms to measure the
pedestal by minimizing the imprinted flat field pattern on both large and
small spatial scales. These methods tend to work better than the simpler
ones for images containing extended low surface brightness objects. These
techniques are ususally applied by processing the raw data with the
CALNICA program up to, but not including, the flat field correction step,
removing the pedestal, and then using CALNICA again to apply the flat
field and other remaining normal calibration steps.
*** Residual Shading ***
Another effect that appears similar to pedestal is a spatial variation in
the residual bias after dark subtraction has been performed. This is due
to the fact that the exact shape of the NICMOS detector shading signal is
also temperature dependent. Because the detector temperatures can vary on
short timescales (due to the changing spacecraft environment and
instrument mode switching), as well as long timescales (due to the gradual
warming of the instrument as the cryogen was depleted), the change in the
shape of the shading will result in a residual signal after the dark
reference data have been subtracted. Unlike the pedestal, however, this
residual shading signal varies spatially across each image quadrant. The
most common method for removing this effect is to again process the raw
data with CALNICA up to the point of the flat field correction, then use a
standalone task to compute and subtract the median value of each column or
row in the image (depending on which camera is in use), and then complete
the CALNICA processing on the corrected image.
*** Bias Jumps ***
Bias jumps appear as vertical or horizontal (depending on the camera in
use) bands across an image. It is currently believed that these bands are
caused by a pull-down of the detector amplifier bias voltage when the
amplifiers of another camera are being used at the same time. This effect
can easily be removed using the same technique employed to remove residual
shading. Simply measure and subtract the median value of the offending
band in each quadrant of the CALNICA intermediate product before applying
the flat field correction.
*** Development Plan ***
The solutions outlined above require dedicated software. The STScI NICMOS
and Science Software Groups have begun development work on a package of
IRAF/STSDAS tasks that can be used to perform these specialized analysis
and correction steps for NICMOS data, outside the current pipeline. This
package will include a combination of display, analysis, and correction
tools. The display and analysis tools will make it easier for a NICMOS
observer to obtain the necessary information from their data sets to allow
them to identify and evaluate the existence and severity of various
anomalies. The correction tools will be tailored to then remove or correct
for specific problem conditions.
The residual shading problem is being addressed by producing a set of
temperature-dependent dark reference files which will provide a better
match to the shape of the shading signal in observations of various
epochs. The new dark reference files are being produced now and will be
made available for observers to reprocess their data in the late spring
(May). The bars problem can be identified and corrected in a
non-interactive fashion. We have developed and will include a bars
correction step in the next release of the CALNICA program, also due out
this spring (May). The new algorithm searches for and flags pixels
affected by bars in each readout of a MultiAccum exposure. When the
readouts are subsequently combined in the "CRIDCALC" step of CALNICA, the
pixels flagged as affected by a bar are rejected from the fitting process,
thus removing their effects from the final combined image. Revised
nonlinearity correction data are also being produced now and will soon
(May) be made publically available.
Many of the techniques and algorithms for performing the most common
corrections, such as pedestal removal, have already been developed in the
form of IRAF CL scripts. We plan to make a public release of these scripts
by late April 1999. We will also develop these and other new algorithms in
the form of IRAF native tasks, which will offer much greater efficiency
and flexibility than the CL scripts. We plan to have an initial set of
these tools available by June 1999 and will demonstrate them for
interested observers at the Chicago AAS meeting. An updated version of the
HST Data Handbook will be released in the summer of 1999 and will contain
information on the NICMOS image anomalies, the available correction tools,
and "cookbook" descriptions of how to use the tools.
In addition, the STScI NICMOS group is considering hosting a special
NICMOS data reduction workshop at STScI in September or October 1999. The
workshop would allow STScI NICMOS instrument scientists to demonstrate for
observers the best known techniques for handling the various problem
conditions described above. We will only hold this workshop if there is
demonstrated user interest, so if you will attend such a workshop, please
send e-mail to "bushouse@stsci.edu".
In the meantime, we encourage observers who are having difficulties with
their NICMOS data reduction to visit STScI so that we may assist with your
data reduction needs. While the public versions of the correction tools
won't be available for a few months yet, development versions of them
already exist at STScI and we can help you apply them to your data. To
arrange a visit, contact your CS or send e-mail to the helpdesk at
"help@stsci.edu".
Data header errors
------------------
by Alex Storrs and Al Schultz
It has become apparent that recent data headers have had significant
errors in them. The value of ORIENTAT for moving targets (at least) has
not been properly set, and the value of the start time of the exposure
(TIME-OBS, EXPSTART) can be off by a minute or more. Observers may want
to contact their CS if these parameters are vital to their data
reduction.
Since July 1998, the position angle for moving targets has not always
been properly set by the SMSG/Scheduler. The Support Schedule
Extractor obtains the position angle for observations from the Science
Mission Specification (SMS), the weekly calendar of events for HST.
This information is stored in the Science Operations Ground System
(SOGS) database and is written into the science header keywords during
OPUS pipeline processing.
The ORIENTAT error was first identified in the header keywords for
WFPC2 observations. However, several NICMOS moving target observations
have incorrect values for the keyword as well, indicating the problem
probably affected all moving target observations regardless of the
instrument used. Operations Problem Report (OPR) #37390 was filed to
fix the problem. The fix will be delivered to the Science Planning and
Scheduling Team (SPST) in March 1999.
Many of the moving target observations have correct values for the
header keyword ORIENTAT. This problem shows up in the first observation
following a change in the pointing of HST. There will be no
reprocessing of the moving target observations due to the problem of
regenerating the database entries from the original SMSs and the need
to set up a separate pipeline to process the observations. Future
observations should not be affected by this problem.
Several sets of observations of moving targets in October of 1997
showed more than a minute of error in the exposure start time in the
header (keywords EXPSTART and TIME-OBS). More recent observations have
much less error. It is not known whether this problem effects fixed
target headers as well. Most observers rarely need this kind of
precision in this parameter.
It is possible to determine the proper value for the start time of an
exposure using the time of the table load from the mission schedule for
the observation, and the knowledge of the pre-exposure overhead. Your
contact scientist can determine these values if you need them.
=============================================================================
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.
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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.
SCARPA, R.; URRY, C.M.; FALOMO, R.; PESCE, J.E.; WEBSTER,
R.; O'DOWD, M.; TREVES, A. "The HST Survey of BL Lac
Objects: Gravitational Lens Candidates and Other Unusual
Sources" ApJ accepted
http://www.stsci.edu/science/preprints/prep1332/prep1332.html
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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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