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NICMOS STScI Analysis Newsletter 17

NICMOS STScI Analysis Newsletter 17

March 1999


        - NICMOS NEWS
          + New NICMOS Information on the WWW
	  + Dealing With NICMOS Image Anomalies: 
		Current Situation and Future Goals
	  + Data header errors





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 
( by following the "Instruments" link.

Since the last STAN, the following ISRs have been published:
        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) 

        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
       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: 

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 "".

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 

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 

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.


                         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 
by the STScI Library not yet published in the journals.  Please remember to
include our Library in your preprint distribution list.

R.; O'DOWD, M.; TREVES, A.  "The HST Survey of BL Lac
Objects: Gravitational Lens Candidates and Other Unusual
Sources"  ApJ accepted


                     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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