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Instrument Science Report NICMOS 2003-004

Measuring the Cold Mask Offset
E. Roye, J. Krist, A.B. Schultz, T. Wiklind April 23, 2003

ABSTRACT An unexpected increase in measured thermal background during the Cycle 11 early calibration program caused speculation that the cold mask position could have shifted since Cycle 7. To address this concern, a single orbit NICMOS program was executed (Program ID: 9704) to obtain deep PSF images of the star LHS1846 in all three cameras. Analysis of this data using the Phase Retrieval software package revealed a minimal amount of cold mask shift since Cycle 7 and provided new, more accurate cold mask values for the Tiny Tim PSF modeling software. It was concluded that the cold mask position was not the cause of increased thermal background observed during the Cycle 11 early calibration program. Increased thermal background has since been determined to be the result of increased thermal load on the HST aft shroud due to the addition of ACS and NCS during SM3b.

Introduction
During the Cycle 11 early calibration program, NICMOS data analysis revealed increased levels of thermal background by approximately 20% in Camera 2 (NIC2) and Camera 3 (NIC3). (Thermal background effects are observed at wavelenghts longward of about 1.9 microns, thus NIC1 is not appreciably impacted.) The increase was unexpected, and the cause for the excess thermal background was unknown. A possible culprit for such an observation was a shift in the positions of the cold masks. The NICMOS instrument houses three cold masks within its dewar, one for each camera, in order to eliminate stray IR emission from peripheral warm surfaces. If the cold masks were to become shifted relative to the optical telescope assembly (OTA) pupil, it is

Copyright© 2003 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.


Instrument Science Report NICMOS 2003-004 possible that excess thermal background would be observed in NICMOS images at longer wavelengths. Hence, in order to test this hypothesis, a NICMOS calibration program intended to measure the cold mask offset was executed (Program ID: 9704). The program was designed to obtain deep exposures of a single bright star in each camera. These single deep exposures would provide data out to the furthest wings of the PSF. The deeper an exposure is, the more accurately the PSF-fitting analysis software can model the optical characteristics of the detectors. The analysis of the 9704 data would not only reveal whether or not the cold mask had shifted appreciably since Cycle 7, but would also provide up-to-date and accurate values for the cold mask positions to be used in the Tiny Tim PSF modeling software (Krist & Hook, 2001).

Data
The M4V star, LHS1846 (J=8.65, J-K=0.75) was observed on December 4, 2002 (Program ID: 9704). MULTIACCUM mode exposures were taken in each camera in separate visits of the single-orbit program. The 192 second Camera 1 (NIC1) exposure was taken with filter F170M, using SAMP-SEQ=STEP64 and NSAMP=11. The 192 second NIC2 exposure was taken with the F180M filter, also using SAMP-SEQ=STEP64 and NSAMP=11. Finally, the 48 second NIC3 exposure was taken with the F222M filter, using SAMP-SEQ=STEP16 and NSAMP=9. A NIC-ONE-CHOP pattern was utilized for the NIC3 observation in order to provide an image for thermal background subtraction from the long wavelength NIC3 exposure. Additionally, the NIC3 observation had to be shorter, with smaller MULTIACCUM read durations than in NIC1 and NIC2 to accommodate for the larger pixel size and resulting shorter saturation time.

Analysis
The data were calibrated using calnica. Only the early MULTIACCUM reads for each visit were used in the analysis, so as to provide deep images of the PSF in each camera prior to core saturation. The data were then analyzed by J. Krist using the Phase Retrieval software package (Krist & Burrows 1997, Roye & Schultz 2002). The phase retrieval software fits the PSF to an anylitical model and determines important optical characteristics of the data such as focus, coma, astigmatism and cold mask offset. There were no calibration programs specifically tailored to measure the cold mask offset in Cycle 7. Therefore, the current values are more precise and accurate than those obtained in Cycle 7.

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Instrument Science Report NICMOS 2003-004

Results
Analysis using phase retrieval showed no appreciable change in cold mask positions since January 1999 (see Table 1). Although the results of the phase retrieval revealed a slight deviation for cold mask offset values from those obtained in Cycle 7, it is probable that the differences are simply a reflection of the greater accuracy of the current measurements. Likewise, we attribute the small rotations to a better fit to the data than that achieved during Cycle 7.

Table 1. Current Cold Mask Positions From Phase Retrieval Compared to Cycle 7 Values
NIC1 Current Mask X offset Mask Y offset Mask Rotation -0.046 0.051 0.213 Cycle 7 -0.09 -0.04 0.00 NIC2 Current -0.086 0.053 0.017 Cycle 7 -0.13 0.025 0.00 NIC3 Current 0.019 -0.043 -0.856 Cycle 7 0.00 0.00 0.00

The mask offsets are given as a fraction of pupil radius in the phase retrival software's coordinate system (not Tiny Tim's coordinate system). The rotations are given in degrees from the default aperture angle ( 45 ° ). Cycle 7 values=0.00 indicate that no careful measurement for those parameters were obtained.

Conclusions
The analysis of the deep-exposure PSF data obtained from program 9704 revealed that the cold mask positions have not changed appreciably since Cycle 7. Furthermore, the analysis provided the most accurate cold mask offset values to date, which have been inserted into the Tiny Tim PSF Modeling software. Increased thermal background in post-NCS NIC2 and NIC3 data has since been determined to be the result of an increased thermal load on the HST aft shroud caused by the addtion of the ACS and NCS during SM3b. The increased aft shroud temperature warms the NICMOS fore-optics causing increased thermal background in longer wavelength filters (Sosey et al., 2003).

References
Krist, J. E. & Burrows, C. J. 1997, "STScI Phase Retrieval Software (FITPSF) User's Guide, " (Baltimore: STScI). Krist, J. & Hook, R. 2001, "The Tiny Tim User's Guide Version 6.0," http:www.stsci.edu/ software.tinytim, (Baltimore: STScI). Roye E. & Schultz, A. 2002. "NICMOS Focus Data Analysis: Using the Updated Phase Retrieval Software," NICMOS Technical Instrument Report, NICMOS 2002-004, (Baltimore: STScI)

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Instrument Science Report NICMOS 2003-004 Sosey, M., Wheeler, T., and Sivaramakrishnan, A. 2003. "Analysis of the HST Thermal Background as Seen By NICMOS + NCS." NICMOS Instrument Science Report 2003-006, (Baltimore: STScI)

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