NICMOS STAN +----------------------------------------------------------+ | NICMOS | STScI Analysis Newsletter | 19 September 2008 +----------------------------------------------------------+ 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 +----------------------------------------------------------+ 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. +----------------------------------------------------------+ 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 +----------------------------------------------------------+ 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 +----------------------------------------------------------+ 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 +----------------------------------------------------------+ 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 +----------------------------------------------------------+ 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 +----------------------------------------------------------+ 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 +----------------------------------------------------------+ 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" +----------------------------------------------------------+ For more detailed information and updates, please consult the NICMOS webpages: http://www.stsci.edu/hst/nicmos +----------------------------------------------------------+ | Help: help@stsci.edu +----------------------------------------------------------+ | To subscribe or unsubscribe to the STAN, send a message | to majordomo@stsci.edu with the subject line blank and | the following in the body: [un]subscribe nicmos_news +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
