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Hubble Space Telescope
NICMOS STScI Analysis Newsletter 39

NICMOS STAN

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| NICMOS 
| STScI Analysis Newsletter 
| 19 September 2008 
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Visit the NICMOS web site http://www.stsci.edu/hst/nicmos "New in the 
Last 45 Days"for all new information about NICMOS. 

CONTENTS: 
-NICMOS / NCS Safing and Status 
-New routine rnlincor to correct for NICMOS count-rate non-linearity
-Instrument Science Report 2008-004: "Summary of NICMOS SMOV4 Plans"
-Instrument Science Report 2008-003: "Verifying the NICMOS Count 
 Dependent Non-Linearity Correction"
-Instrument Science Report 2008-002: "Improvements to Calnica"
-Instrument Science Report 2008-001: "Bright Earth Persistence in NICMOS"
-Technical Instrument Report 2008-001: "Light leaks from NICMOS flatfield 
 lamps into WFPC2"
-Electronic Distribution of NICMOS Instrument Science Reports 
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NICMOS / NCS Safing and Status

As part of the preparations for Servicing Mission 4 (SM4), the flight
software for the spacecraft computer (NSSC-I) had to be updated in order
to accommodate the new instruments that will be installed. Since this
computer manages the health and safety of all the on-board instruments,
including NICMOS and the NICMOS Cooling System / NICMOS Cryo-cooler
(NICMOS NCS/NCC), this involved shutting down the NCS/NCC as part of
the procedure, on Sep 10, 2008. After the NSSC-I update, the NCS was
restarted but went into safemode soon afterwards, before the cooldown
had completed. A project tiger-team was formed, including staff at
STScI and GSFC as well as the engineers who had designed and built the
NCS, and three more NCS restart attempts were carried out. In each
case, the NCS safed with telemetry that was suggestive of the possible
presence of small amounts of icy contaminants interfering with the
circulator pump. The leading theory suggests that small amounts of
these contaminants, likely water ice, became dislodged as part of
the shutdown and restart process, and were transported from benign
areas to the location of the turbo circulator mechanism and sensors,
where they are interfering with the nominal operation of the system.

There is no indication of any damage to the mechanical systems in
the NCS/NCC or to NICMOS itself, for the brief amounts of time that
the system was running during each restart attempt. However, continued
operation in this state may pose a significant health and safety threat,
hence the decision has been made across the Project to defer further
restart attempts until the system has warmed up sufficiently to enable
the contaminants to thaw and reduce their level of interference. With
SM4 imminent, this means that the cooler will remain off through SM4
(it would have had to be powered down during SM4 in any case). The
current plan is to restart it soon after SM4, as was also originally
planned, with the main difference being that the expected cooldown may
take a few weeks longer than originally planned, due to the warmer
initial state of the system. The net result is that the remainder of
the Cycle 15 and 16 NICMOS observations will be deferred for execution
after SM4, and will execute along with the rest of the observations
currently planned for Cycle 17.

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New routine rnlincor to correct for NICMOS count-rate non-linearity

A new software routine "rnlincor" has been released to correct NICMOS
data for the count-rate dependent non-linearity effect. The routine 
is being released within the STSDAS package in the hst_calib.nicmos 
subpackage.The routine is written in python and requires PYRAF (iraf 
is not sufficient),or can be called from the command line or from a 
python script. This routine supersedes an earlier python script that 
was released as a Beta test from our web site.

Unfortunately, the test "rnlincor" version that was silently released 
with STSDAS 3.7 contained a small error in the sky value correction. 
This correction is only applied when the SKYVAL header item is 
detected, normally populated after running "pedsky" in default mode. 
The error affects mainly the photometry of objects close to sky value. 
We recommend that users who used the STSDAS 3.7 version of rnlincor 
reduce their data again with the new STSDAS 3.8 version released 
Feb 26, 2008.

The routine uses reference files and header keyword items that have 
only been distributed from the MAST archive since Oct 2007. The easiest 
way to obtain the reference files and the correct header items for data 
retrieved before this date is to retrieve the data from the archive again. 
More details about the NICMOS count-rate non-linearity can be found at
http://www.stsci.edu/hst/nicmos/performance/anomalies/nonlinearity.html

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NICMOS ISR 2008-004: Summary of NICMOS SMOV4 Plans
This ISR outlines the NICMOS plan for Servicing Mission Observatory 
Verification 4 (SMOV4). It presents the NICMOS requirements, the planned 
activities to fulfill these requirements and a brief plan for the data 
reduction and analysis.
T. Wiklind et al.  
http://www.stsci.edu/hst/nicmos/documents/isrs/isr_2008_004.pdf

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NICMOS ISR 2008-003: Verifying the NICMOS Count Dependent Non-Linearity 
Correction
We re-investigate the NICMOS count dependent non-linearity, using the 
flat field lamps to reach high count regimes. The NICMOS count dependent 
non-linearity has been previously quantified and is currently corrected 
in NICMOS pipeline. In this document we show that the count dependent 
correction is still accurate to within 1.2% of true linearity for counts 
above 100 ADU independent of filter used. This is a response to the 
count-rate dependent non-linearity investigations, to eliminate questions 
about the current count dependent corrections.
B. Shaw, R. S. de Jong 
http://www.stsci.edu/hst/nicmos/documents/isrs/isr_2008_003.pdf

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NICMOS ISR 2008-002: Improvements to Calnica
This ISR describes a number of improvements to calnica, the STSDAS task 
that performs routine instrumental calibration of NICMOS raw images. We 
report two major and a number of minor updates to the code. The major 
updates consist of changes in the way the count rate is calculated from 
up-the-ramp fitting and changes in the cosmic rays rejection algorithm. 
All updates have been extensively tested and analysis shows improvement 
in the S/N of output images by 6-15% compared to the previous calnica 
version. A further improvement is that the error extension in the 
calibrated images now represents the true errors in a more consistent 
way compared to the old implementation of the software.
T. Dahlen et al.
http://www.stsci.edu/hst/nicmos/documents/isrs/isr_2008_002.pdf

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NICMOS ISR 2008-001: Bright Earth Persistence in NICMOS
We report the presence of image persistence in NICMOS data due to 
prior saturation of the array by the bright Earth as well as an 
algorithm to remove its impact. BEP (Bright Earth Persistence) may 
occur when ACS and NICMOS are used in parallel, and the data dump 
of ACS delays the insertion of the NICMOS filter blank as long as 
6 minutes beyond the Bright Earth avoidance angle. We have constructed 
a BEP frame which can be used to model and remove the presence of BEP 
in NICMOS data. The algorithm is seen to remove 99.5% of the variance 
in the background of BEP impacted Camera 2 data. Ongoing work involves 
broadening the applicability of the BEP correction to Camera 1 and 3 
and making it available as a pyraf script through STSDAS.
A. Riess, E. Bergeron 
http://www.stsci.edu/hst/nicmos/documents/isrs/isr_2008_001.pdf 

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NICMOS TIR 2008-001 Light leaks from NICMOS flatfield lamps into WFPC2 
We characterize the light leaks from the NICMOS internal flat-field 
lamps into WFPC2 by taking a set of images with WFPC2 using the F814W 
filter while having the NICMOS lamp in both on and off mode. We find 
that there is a light leak from the NICMOS lamp into WFPC2, increasing 
the overall background counts by ~25% in the F814W filter.Using the 
WFPC2 ETC, we estimate the effects of this light leak for different 
sky background levels and find an increase by almost a factor 2 in the 
case of a “low” sky background, while the effect is ~20% for a 
“high” background case. The effects on shorter wave-length filter 
observations are smaller, but may still increase the background level 
significantly. For example, in the F555W filter, the increase is 2% and 
16% for a “high” and “low” sky background, respectively. 
Besides the overall increase in background, there is also a particular 
feature, “dragon’s breath”, in WF4 with a significantly elevated 
signal (factor ~3) in ~400 pixels. We conclude that the NICMOS internal 
flat-field lamp should not be used while observing with WFPC2.
T. Dahlen, M. Mutchler

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Electronic Distribution of NICMOS Instrument Science Reports

We have implemented a new means of distributing NICMOS Instrument
Science Reports (ISRs), which will now be sent electronically to
this distribution list: nicmos_isr_dist@stsci.edu

If you would like to subcribe to this list then you can do
this by sending email to "Majordomo@stsci.edu" with the message:
"subscribe nicmos_isr_dist youremailaddress"

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For more detailed information and updates, please consult the
NICMOS webpages: http://www.stsci.edu/hst/nicmos

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