L. Colina1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, Email: colina@stsci.edu
(1)Affiliated with the Astrophysics Division, Space Science Department, ESA
This document summarizes the goals and status of the various calibration programs that form the basis of the NICMOS Cycle 7 calibration plan. Pointers to technical reports and/or WEB pages showing the results of the analysis and providing detailed information are also given.
HST, NICMOS, calibrations, near-infrared, detectors, imaging, spectroscopy
Astronomers working with ground-based telescopes are used to get simultaneously their calibrations (i.e. darks, flats, standards, etc) and scientific data. The extension of this practice to satellites in general, and HST in particular, would make a very inefficient use of telescopes in orbit as the fraction of available observing time devoted to calibrations would increase very dramatically.
The approach taken by STScI regarding the calibrations of HST instruments is that STScI is responsible for (a) the definition of the goals and objectives of the calibrations, (b) the implementation and execution of the calibration programs, (c) the analysis of the calibration data, and (d) the delivery of the calibration products in the form of reference files and/or technical reports. This approach has two benefits, first, it saves a considerable amount of telescope time (about 10% of the total number of orbits allocated to a given instrument go into its calibration) and second, it generates an homogeneous database of calibrations that would be difficult to create otherwise.
This document summarizes the content, goals and status of the Cycle 7 calibration plan for NICMOS and also points to the WEB pages where detailed information on specific topics of interest can be found.
The NICMOS calibrations available during Cycle 7 are based on three distinct calibration activities. First, NICMOS was extensively tested on the ground. These tests included a limited amount of calibration, particularly during the System Level Thermal Vacuum (SLTV) testing. Second, a period following the installation of NICMOS into HST for testing and initial calibration was completed during the first three months on orbit. This activity is known as the Servicing Mission Observatory Verification (SMOV). Finally, the routine Cycle 7 calibration plan is now underway.
It is important to distinguish between the various goals of these calibration activities. SLTV was intended to demonstrate the proper functioning of NICMOS and to obtain an initial calibration of a subset of its capabilities. SMOV was intended to demonstrate that the instrument is functioning as expected, based on the SLTV experience, to characterize those parameters not measurable during SLTV (e.g. the thermal background generated by the HST optics), to establish necessary operation parameters (e.g. plate scale), and to begin the calibration of NICMOS. A complete calibration of NICMOS various operating modes is conducted during Cycle 7 with SMOV being used to demonstrate that the planned calibrations are in fact feasible.
As a consequence of the camera 3 (NIC3) defocus, the Cycle 7 calibration plan that went into operation in June 1997, did put the emphasis on the calibration of NIC1 and NIC2 cameras executing only a limited number of calibration programs for NIC3. Revisions to the original cycle 7 calibration plan took place after experience with the detectors was gained during the first 5 months of Cycle 7 (i.e. in November 1997), and most recently in March 1998. The first revision supplemented the original version with programs designed to cover the calibration of NIC3 during the first campaign. The second revision included programs aimed at the calibration of modes of operation not covered in the previous versions. In this version, few additional programs were also added to monitor the behaviour of the detectors during the warming-up phase of NICMOS.
In its present form, the Cycle 7 calibration plan consists of several calibration programs that are grouped in one of four different categories: special calibration programs, routine monitoring programs, NIC3 specific programs and Cycle 9 oriented programs (see Table 1):
The NICMOS calibration programs may each be examined by using their ID number and the HST Program and Schedule Information page. The reference calibration files and tables created as a result of the various calibration programs are available on the NICMOS Reference Files List page. Instrument science reports (ISR) and/or published conference papers presenting the latest calibration results can be found on the NICMOS Documentation WEB page.
All three detectors are being characterized to the same extent. High quality darks were obtained for all available MULTIACCUM sequences during June and early July 1997 (proposals ID 7703 to 7710). The mode of operation of the detectors was modified in August 1997 (amplifiers are now kept on all the times instead of keeping them off, and switching them on only when exposures start) and as a consequence a new set of darks were obtained (proposals ID 7789, 7793 to 7799). As a result, there are now two separate set of dark references files that should be used for pre-fix data (i.e. data obtained before August 22, 1997) and post-fix data (i.e. data obtained after August 21, 1997), respectively.
Changes in the detectors darks are being monitored with a monthly periodicity (proposal ID 7596). These data are currently being analysed and show indications that darks are changing with time. A strategy to create incremental darks is under investigation and will be provided in the future to users. Dark frames have also been obtained for a limited subset of ACCUM exposure times (proposal ID 7688) but the analysis has not yet been completed. Observers using ACCUMs with exposure times different than those provided by the calibration program will require to interpolate, producing therefore a less satisfactory calibration of their data.
The pixel-to-pixel and the large scale responses of NICMOS filters are provided by high signal-to-noise flat-fields generated using internal lamp exposures. Flat-field exposures for all NIC1 and NIC2 polarizers, broad- and medium-band filters were obtained during July and August 1997 (proposal ID 7690). Flat-fields for all narrow-band NIC1 and NIC2 filters were taken in May 1998 (proposal ID 7956) and February 1998 (proposal ID 7689), respectively. Lamp flats for all NIC3 filters were taken separately in December 1997, just before the first NIC3 campaign (proposal ID 7814).
The temporal evolution of the pixel-to-pixel response as a function of camera and wavelength is being monitored on a monthly basis using a subset of filters in the three cameras (proposal ID 7690).
The operating temperature of NICMOS has been rising steadily during Cycle 7. As a consequence, variations in the pixel-to-pixel and large scale sensitivity are expected. A second set of internal lamp flat-field exposures (proposal ID 7957) will be taken in August/September 1998 for those filters for which the reference flats are based on data taken in July/August 1997, i.e. all medium- and broad-band NIC1 and NIC2 filters.
Several image anomalies have been found in NICMOS exposures. While in general the pipeline calibration appears to work well, on occasion one or more anomalies will occur. Complete descriptions of these anomalies including work-arounds, if available, can be found on the NICMOS Image Anomalies WEB page.
The focus of all three cameras is being monitored throughout the entire Cycle 7. During the first few months focus measurements were obtained every other week with the three cameras (proposal ID 7608). The frequency of this monitoring program decreased to once a month after the first NIC3 campaign. In its present form, measurements of NIC3 focus are obtained every month while the other two cameras are monitored on a bimonthly basis (proposal ID 7901). The results of the focus monitoring program are posted regularly on the NICMOS focus WEB page.
No calibration program aimed at measuring the point spread function of the three cameras as a function of wavelength and location was included in the Cycle 7 calibration plan. Observers requiring PSFs in specific filters and/or locations within the field of view, were advised to include these in their own program. NICMOS PSFs can also be modeled using Tiny Tim V4.3. Tiny Tim software can be retrieved from the Tiny Tim WEB site.
Plate scale measurements for all three cameras were performed early during the SMOV phase as part of the NICMOS aperture location (proposal ID 7039). Additional detailed measurements of plate scale and camera distortions were performed taking a series of images of the star cluster NGC 1850. In each camera 5 images were taken, displaced from each other without rotation (proposal ID 7040).
The results of the program 7040 are already available and have been presented in a technical report by Cox and collaborators (OSG-CAL-97-07). Additional information regarding plate scale measurements can also be found on the NICMOS Plate Scale WEB page.
The absolute level and stability of the thermal background as seen by the NICMOS cameras was already measured as part of the SMOV program. The Cycle 7 calibration program extended the SMOV program with images obtained in the F237M (NIC2) and F222M (NIC3) filters (proposal ID 7611). Images were taken during June and July 1997 as pointed parallel observations to map possible changes in the thermal background as a result of temperature changes in HST optics.
The analysis of these data is now complete and the results can be found in a recent instrument science report (NICMOS-98-010) written by Daou & Calzetti.
The photometric characterization of NICMOS comprises various aspects covered by different calibration programs. The absolute photometry for all available filter plus camera combinations has been obtained by observing the two primary NICMOS absolute spectrophotometric standards, the white dwarf G191-B2B and the solar analog P330E (proposal ID 7691 for NIC1 and NIC2 and proposal ID 7816 for NIC3). Images of additional calibration standards (white dwarfs, solar analogs and red stars) have also been taken to help defining the transformations between HST and ground-based JHK photometric systems (proposal ID 7904 for NIC1 and NIC2 and proposal ID 7816 for NIC3).
The photometric stability of all three NICMOS cameras as a function of time and wavelength is being monitored on a monthly basis with observations of an absolute spectrophotometric standard (P330E) in a subset of filters covering the entire NICMOS wavelength range (proposal IDs 7607 and 7902).
Relative photometry as a function of the position in the detectors has been measured for each camera using images of the solar analog P330E in a few filters. The grid used for this program consisted of 144 positions covering the entire field-of-view in each camera (proposal ID 7693).
Results regarding NICMOS photometric performance can be found on the NICMOS Photometry Update page.
The instrumental polarization and zero position angle of NIC1 and NIC2 polarizers have been measured taking images of two bright near-infrared polarized standards, HDE283812 and CHA-DC-F7. Changes in the polarization as a function of position within the detectors will also be measured by moving one of the polarized targets in a spiral pattern across the detector. Additional images of the HST unpolarized standards HD64299 and BD+32d3739 have also be taken (proposal IDs 7692 and 7958).
The analysis of these data is in progress but preliminary results can be found in the contribution of Dines, Schmidt & Lytle to the 1997 HST Calibration Workshop.
NICMOS coronograph consists of a laser-ablated hole in the NIC2 mirror on the field divider assembly (FDA). One of the effects of the dewar expansion and subsequent contraction has been a change in the relative geometry of the NIC2 detector relative to the FDA. For the coronograph, this has caused the apparent location of the coronographic hole to drift relative to the fixed acquisition aperture on NIC2. Because it is necessary to center a star in the coronographic hole to within 1/4 pixel or better, a new addition to the NICMOS onboard software has been implemented to locate the coronographic hole at the beginning of each observation.
The long term drift of the NIC2 coronographic hole has been continuously monitored during Cycle 7 using internal lamp flat exposures obtained as part of the focus monitoring program (proposal IDs 7608 & 7901). The latest information on the status of the coronograph can be found on the NICMOS Coronographic Hole page.
The wavelength dispersion solutions for all three NIC3 grisms have been computed using the emission lines of the compact planetary nebulae HB12 during the first NIC3 campaign (proposal ID 7695). The absolute sensitivities of the three grisms were derived using observations of the two primary NICMOS absolute spectrophotometric standards ( G191-B2B and P330E; proposal ID 7696). A check on the stability of the wavelength and absolute sensitivity will be performed during the second NIC3 campaign in June 1998 (proposal ID 7959). Finally, the stability of the wavelength calibration for grism G141 is measured on a by-monthly basis (proposal ID 7903).
The analysis of the calibration programs executed during the first NIC3 campaign has been completed and the results can be found on the NICMOS ST-ECF WEB page.
This paper has summarized the status of NICMOS Cycle 7 calibration plan as of May 1998. The calibration of NICMOS is an ongoing effort that will continue long after the nitrogen is exhausted and Cycle 7 observations are finished. Calibration proposals will be executed during the warming-up phase of NICMOS and will stop soon before the shut-down of the instrument.
The calibration of NICMOS detectors is not yet in a stable situation. Improvements in the calibration pipeline continue, new reference files are being delivered as the analysis of the calibration data is completed and new technical reports presenting the results of the calibration programs are posted continuously on the NICMOS WEB page.
The definition, implementation and analysis of all the calibration programs that form part of the NICMOS Cycle 7 calibration plan has been a project that involving a large number of persons including members of the STScI NICMOS team (D. Axon, E. Bergeron, D. Calzetti, D. Daou, D. Gilmore, S. Holfeltz, J. Mackenty, J. Najita, K. Noll, C. Ritchie, A. Schultz, C. Skinner, W. Sparks, A. Storrs, A. Suchkov), the instrument definition team (R. Thompson, M. Rieke, G. Schneider and D. Hines), and the ST-ECF (W. Freudling and N. Pirzkal).
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