Cycle 1 Calibration - NIRSpec
The activities listed below are those parts of the Cycle 1 Calibration plan for the Near-Infrared Spectrograph (NIRSpec) that have a dedicated observational component. It is important to remember that the Cycle 1 Calibration plan is not final. The plan as laid out here is based on our current understanding of the telescope, instruments, and all other planned observations in Cycle 1. This program should be considered provisional and may change in response to system developments and the final science program.
The text for each calibration activity below includes a title, program ID number, and the abstract also included with the APT file. The program ID number links directly to the STScI webpage for that program. Users interested in obtaining APT files can either follow that link or retrieve the file by running the APT and retrieving it by its program ID.
Program 1484 - CAL-NRS-001 - Dark Monitor Full Frame
This program will obtain a set of dark exposures taken with full frame readout. The data will be used to construct dark current, super bias, and read noise reference files. Master bias observations are only to be taken during the first visit of the program. Dark exposures using NRSIRS2RAPID use expoure durations of approximately 3000 sec (200 groups). This duration is the longest being recommended to users, and it may be changed in the future. Dark exposures with NRSRAPID are approximately 1000 sec (88 groups) in duration. The observations are split into individual visits - 100 for the NIRSIRS2RAPID darks and 50 for the NRSRAPID darks. Two bias frames, one in each of the readout patterns, will also be acquired. These visits are executed in parallel with no constraints on pointing.
Program 1485 - CAL-NRS-002 - Spectroscopic Flats
This program provides additional data for the MOS S-flat (spectroscopic flat field) reference files. The MOS S-flat is a cube with complete but sparse sampling in wavelength space per pixel. This allows the calibration of MOS spectra, regardless of the location of the MSA shutters used. Data will be obtained through 20 MSA long slit configurations using the CAA internal calibration FLAT lamps to expand the wavelength interpolation of the S-flat. The S-flat is one part of the 3 component NIRSpec flat field: the F-flat traces the field-dependent throughput of the OTE and instrument FORE optics, the S-flat traces the light path from the micro-shutter array up to but not including the FPA and the D-flat consists of the pixel-to-pixel variations of the detector.
Program 1486 - CAL-NRS-003 - Spectroscopic Subarray Flats
This program will acquire the spectroscopic (lamp) flats for the FS mode using the ALLSLITS subarray. Observations will be acquired for all of the NIRSpec disperser/lamp combinations. These observations are essential for minimizing flat field calibration noise for FS and/or BOTS observations that need improved signal-to-noise. The commissioning program will acquire a minimum set of flats, these observations for Cycle 1 calibration will greatly decrease the noise from flat fielding process for science observation processing. The S-flat is one part of the 3 component NIRSpec flat field: the F-flat traces the field-dependent throughput of the OTE and instrument FORE optics, the S-flat traces the light path from the micro-shutter array up to but not including the FPA and the D-flat consists of the pixel-to-pixel variations of the detector.
Program 1487 - CAL-NRS-006 - Slitloss Extension
This program will obtain observations of a spectrophotometric standard star in order to further characterize wavelength dependent throughput variations as a function of position in the slit for FS and IFU modes. Data will be taken for the fixed slit 2-point and 5-point dither patterns and the 21st through 40th points in the IFU cycling pattern and will build on the data already obtained in commissioning (the fixed slit 3-point dither pattern and the first 20 points of the IFU cycling pattern).
Program 1488 - CAL-NRS-007 - MSA Operability Monitor
Internal lamp observations during slews will be used to monitor the status of NIRSpec MSA failed shutters to update MSA failed shutter masks. Undispersed MSA images will be taken through the ALLOPEN, ALLCLOSED, CHKBD1x1-1, and CHKBD1x1-2 shutter patterns, using NRSIRS2RAPID readout, with 7 groups x 1 integration. These exposures will be acquired every two weeks to monitor the failed open and closed shutters on the NIRSpec MSA. The CHKBD3x3-1 shutter pattern will be observed to check shutter image location on the detector against the instrument model, and is also useful for grating wheel sensor calibration. An additional long exposure (65 groups x 1 integration,949 s, 1506 s with overhead) through the ALLCLOSED configuration is obtained once per year, to yield a high dynamic-range contrast map.
Program 1489 - CAL-NRS-009 - Instrument Model
From a set of internal CAA lamp exposures, this activity, along with the GWA tilt calibration monitor program, will serve to monitor the NIRSpec instrument model during cycle 1. The instrument model is a parametric model of NIRSpec optical geometry and is used to trace, extract and rectify the spectra and provides WCS information for each pixel in a 2D spectrum. Most model components are expected to remain stable, but a limited monitor will guard against changes not traceable via existing observations.
Program 1491 - CAL-NRS-013 - Grating Wheel Tilt Monitor
This program executes successive rotations of the GWA (grating wheel assembly), one position at a time, with an internal lamp exposure taken at each position (LINE4 for the PRISM, REF for the gratings). The data will enable monitoring of the calibration of the GWA tilt sensors, which is a critical part of the NIRSpec wavelength calibration. This calibration is part of the NIRSpec instrument model, but the tilt sensors are more likely to change with time than the other internal instrument model components and will be monitored more frequently. The monitor will run approximately once a month to provide data on the GWA tilt behavior and build up statistics for a full tilt calibration if necessary.
Program 1492 - CAL-NRS-015 - Characterization of LSF and Zero-Point Correction
This program will expand the NIRSpec LSF characterization and wavelength zero point calibration correction derived during commissioning. Observations will acquire spectra of spatially unresolved emission line sources used during commissioning in the FS and IFU apertures using nod/dither positions not yet covered in the commissioning program. The observed emission lines will be used to update the wavelength zero point for the instrument model. The unresolved emission lines will be used to characterize the LSF shape for each disperser, as both a function of wavelength and position within the aperture. MOS observations will also be acquire for zero point correction information for input into the instrument model update. This MOS observation includes a set of 10 exposures with the source stepped across the full pitch of a shutter at even intervals (roughly 25 mas per step). The MOS portion of the commissioning program will not provide the sampling necessary to measure these zero point offsets.
Program 1493 - CAL-NRS-017 - F-Flat Characterization
This program obtains observations needed to further calibrate the F-flat of the instrument flat field/throughput correction. The F-flat is one part of the 3 component NIRSpec flat field: the F- flat traces the field-dependent throughput of the OTE and instrument FORE optics, the S-flat traces the light path from the micro-shutter array up to but not including the FPA and the D-flat consists of the pixel-to-pixel variations of the detector, usually called the P-flat. Additional observations will be obtained at a wider range of positions than acquired during Commissioning to more accurately calibrate the field dependence, address the effects of polarization (at the level of 0.2% predicted by instrument models), and remove uncertainties from the S-flat due to imperfect knowledge of the instrument’s internal flat lamp profiles. This program also addresses the limited absolute flux calibration observations using only the S1600A1 fixed slit, and will be used to check against uncorrected effects that may affect observations using MOS mode.
Program 1494 - CAL-NRS-018 - MSA Anneal
The NIRSpec MSA Anneal process uses internal heaters to warm the MSA to ~+270K in an effort to un-stick failed open and closed MSA shutters that have evolved over the course of routine NIRSpec operations. The MSA is then cooled back to operating temperatures and a set of verification exposures is run to investigate the success of un-sticking the shutters. It will take approximately 24-48hrs before NIRSpec is back to operational temperature and available for science and close out of the Anneal verification exposures. This is anticipated to be done at least once in Cycle 1, but the frequency may depend on the evolving population of failed shutters in the MSA. This is a NIRSpec instrument engineering program.
Program 1495 - CAL-NRS-019 - Dark Monitor Subarray
Observations of a compact planetary nebula in the southern continuous viewing zone will be used to derive the grism wavelength solutions for the GR150R and GR150C grisms in the six blocking filters. A 3x3 mosaic of pointings is used to move the nebula around within the NIRISS field of view.
Program 1496 - CAL-NRS-020 - Wheel Characterization
This activity is needed to verify the basic functionality of the FWA and GWA. It will be done once during Cycle 1, unless normal telemetry monitoring indicates a possible problem that requires the more detailed information provided by this characterization activity. The procedure collects NIRSpec-focused telemetry data in the HC buffer at each commanded position and sends them to the ground for inspection after the procedure is completed. The FSW sends a series of mechanism move commands to step the FWA/GWA one position at a time through all 8 wheel positions in both the forward and reverse directions. At each position, the HC buffer is armed before a move and then dumped after the move. The procedure acquires prime internal calibrations and takes approximately 2 hours to execute.