Cycle 1 Calibration - NIRCam

The activities listed below are those parts of the Cycle 1 Calibration plan for the Near-Infrared Camera (NIRCam) 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 1453 - CAL-NRC-001 - Dark Current and Read-Noise Monitor

Program 1475 - CAL-NRC-002 - Sky Flat Field Monitor

Program 1476 - CAL-NRC-003 - L-Flats, Photometric, and Distortion Monitor

Program 1477 - CAL-NRC-005 - Total-count and Count-rate Linearity Characterization

Program 1478 - CAL-NRC-006 - Persistence Characterization

Program 1479 - CAL-NRC-010 - LW Grism Spectral Calibration

Program 1480 - CAL-NRC-011 - LW Grism L-Flat Correction

Program 1481 - CAL-NRC-012 - Coronagraphic Distortion Monitor

Program 1482 - CAL-NRC-014 - Coronagraphic PSF Characterization & TA Verification

Program 1483 - CAL-NRC-016 - Subarray Dark Current and Read-Noise Monitor

Program 1453 - CAL-NRC-001 - Dark Current and Read-Noise Monitor

This program monitors the FULL frame noise properties by taking dark observations throughout Cycle 1 using the equivalent of every readout pattern. The Darks are set up to characterize the dark current, 1/f noise, IPC, cross-talk, and superbias. We note that the dark current itself is so low (1.9/27 e-/ks for the shortwave/longwave channel) that achieving a good S/N per pixel is prohibitive for the shortwave channel. However, hot pixels are still characterized, and the overall temporal evolution of the dark current can be monitored with the median of the full frame. For this activity, the pupil wheels will be set to the "Dark" position. Subarray darks are obtained in NIRCam-016.

Program 1475 - CAL-NRC-002 - Sky Flat Field Monitor

Instead of taking dedicated sky flats during Cycle 1, we will use the deep field GTO data to reconstruct P-flats in a subset of filters. Analysis of the CV3 flats suggest wavelength dependence in the P-flat is generally <1% in all detectors (though it appears to reach as high as 2% in some pixels on the tail-end of the distribution), so there is no need to take additional flats in the remaining filters unless on-sky commissioning data present different results. Since we cannot control when the GTO data are obtained, we will also monitor the stability of the sky flats with the F150W and F444W filter pair. We will obtain 6 epochs, which are scheduled in parallel with NIRISS sky flats. Large scale frequency variations in the NIRCam illumination pattern (L-flats) will be measured in CAL-NIRCAM-003, in combination with the data from this program.

Program 1476 - CAL-NRC-003 - L-Flats, Photometric, and Distortion Monitor

This activity will serve 4 calibration activities, all of which can be obtained by observing the LMC calibration field throughout Cycle 1: stellar flats, photometric monitor, astrometry/distortion monitor, and flux array dependence. Stellar Flats: The stellar flats (L-flats) are meant to supplement the LP-flats. These observations step well-measured stars across the detector to map changes in response across the frame. HST/WFC3 and HST/ACS stellar flats improved photometric accuracy by 0.6%-6%, depending on the filter. A dense stellar field with minimal crowding is required. Since relatively short exposures are required, stellar flats can be used to monitor changes in the wavelength dependence of the flat field since Commissioning by observing in all (wide, medium, and narrow filters). Stars will be stepped across the detectors at 9 positions. Photometric Monitor: Repeat observations of the LMC field will monitor the stability of the photometric zeropoint calibration in all filters. Astrometry/Distortion Monitor: Determine the plate scale, orientation, and geometric distortion for each SCA in each NIRCam module over the full wavelength range. This requires dithering in the LMC calibration field with a representative subset of filters. The LMC calibration field has been carefully chosen and mapped with HST’s ACS to facilitate such a calibration. Characterize absolute flux array-dependence: Since absolute flux calibrators are bright, they can only be observed on small subarrays. Thus, to check the array dependence, we will include subarray observations of the LMC along with the FULL observations.

Program 1477 - CAL-NRC-005 - Total-count and Count-rate Linearity Characterization

The purpose of this activity is to verify that the linearity behavior has not changed. The program requires observations of a rich stellar field that is bright and dense down to the confusion limit. To spread the flux more evenly, we may use a weak lens to introduce defocusing. The goal is to probe the linearity of a large sample of pixels across the detector. This program will likely target the LMC calibration field, which is in the CVZ. Alternatively, we could target Omega Cen, which was used for the WFC3 linearity study.

Program 1478 - CAL-NRC-006 - Persistence Characterization

This program characterizes the NIRCam persistence (latent images) and checks for changes since the commissioning persistence check. The plan is to observe a rich stellar field, e.g. Omega Cen or 47 Tuc, with a long enough exposure time to probe a wide range of over-saturation levels, followed by a series of darks and a final short dithered sequence in a narrow-band filter to recover the source photometry. The illumination exposure is preceded by a series of dark integrations providing a baseline measure of the dark and noise floor and verify that no persistence from previous observations has been imprinted on the detector. The length of this preliminary dark can be shortened if scheduling can guarantee that the detector has not been exposed in the previous ~10,000s. The dark has to be taken "on site", i.e. after the target has been acquired. This is needed to prevent spurious exposure to bright sources during the telescope slew and target acquisition maneuver.

Program 1479 - CAL-NRC-010 - LW Grism Spectral Calibration

This program will calibrate and characterize the long wavelength (LW) grism, including spectral calibration and the line-spread function (LSF) characterization. The wavelength solution will be determined by observing 1 JWST spectral calibration source. This will likely be a planetary nebula, since PNe have a weak continuum and a rich set of strong spectral lines. The PN should also be sufficiently compact (e.g., sources in M31) to enable the LSF to be measured. Wavelength calibrator SMP LMC 58 (K = 14.5 mag) can be observed with MEDIUM8 readout, 7 groups, and 3 exposures/dithers to achieve S/N up to 200 per wavelength in grism mode with both F322W2 and F444W.

Program 1480 - CAL-NRC-011 - LW Grism L-Flat Correction

This program will determine the L-flat correction for longwave (LW) grism observations by observing a rich stellar field (the LMC calibration field or a globular cluster), which will provide a large number of stars across the field for mapping spatial variations in the grism throughput. If the LMC is observed, the LW detector will contain about ~115 stars brighter than kmag=16. To detect these stars with S/N>5 per wavelength, observations should use the SHALLOW2 readout with 5 groups and 3 exposures/dithers. This data can also be used to characterize the spectral trace across the detector. Science data will be used to supplement this program.

Program 1481 - CAL-NRC-012 - Coronagraphic Distortion Monitor

This program monitors the NIRCam coronagraphic distortion/astrometry. Commissioning will determine the absolute distortion solution to within 3 mas. In cycle 1, we will revisit the LMC calibration field twice to monitor changes. This will involve observing stars behind the ND squares as well as in the vicinity of the coronagraphic masks to quantify the distortion behind the coronagraphic substrate. We will use the NIRCam Engineering Imaging template to observe with the FULL array, including dithers and a mosaic to overlap the shortwave (SW) detectors (an important cross-check). Since only module A is enabled for science observations, we will restrict detector overlap to the 2 SW detectors used for coronagraphy on module A.

Program 1482 - CAL-NRC-014 - Coronagraphic PSF Characterization & TA Verification

This program characterizes the NIRCam coronagraphic PSF with different observing configurations (all masks and 1 or 2 filters) using a relatively bright photometric standard star and the small-grid dither patterns. Data from this program can also be used to verify target acquisition (TA) and flux calibration (CAL-NIRCam-013). PSF Characterization: PSF characterization will use a JWST standard star and all mask/filter combinations that span the coronagraphic imaging wavelength range. These observations will check our ability to bootstrap accurate (to ~1-2%) absolute photometry with coronagraphs (including the coronagraphic wedge, substrate, Lyot stop) with respect to the standard imaging using the whole optical system (phased telescope + instrument). We will characterize the behavior of the PSF with wavelength and position along the bars. The same star will be observed in imaging mode through the same filters to make a direct comparison. We will also observe the same star with the coronagraph wedge in place, but placing the star a few arcsec outside the mask. Since we use a photometric standard (likely SNAP-2, a G2V star with K=14.2 mag), these data can supplement the absolute flux calibration. Target Acquisition: The PSF observations can also be used to as an additional check of the TA offsets, which will be verified during Commissioning (COM-NRC-30). This check requires no additional dedicated observations. After the TA exposure and subsequent slew to the occulter position, the fine steering mirror is used to perform a 5- or 3-point dither sequence in steps of ~10 mas. This will allow a verification of and, if necessary, an update to the ideal position of the bright star behind each occulter. This program will test the faint target acquisition (with SNAP-2); bright targets observed in the absolute flux calibration program (CAL-NIRCam-013) will test TA using the neutral density squares.

Program 1483 - CAL-NRC-016 - Subarray Dark Current and Read-Noise Monitor

This program takes Dark frames for the NIRCam subarrays using the equivalent of every readout pattern. For this activity, the pupil wheels will be set to the "Dark" position. The data will be used to further characterize readnoise, 1/f noise, IPC, cross-talk, and superbias. Dark frames are particularly important for subarrays because of "glow" from the amplifiers, which accumulates each time a pixel is read out. Since subarrays have short frametimes, the glow accumulates much faster than it does in the full frame images. The glow may produce significant structure across subarrays, and is also expected to change if the ASICs are re-tuned. Note that at the time of submission, APT does not include subarrays other than SUB640, SUB320, and SUB160 in the Dark template (Module B only). A few other subarrays are available in Engineering mode, but those time estimates are inflated (they include slews). This APT file therefore does NOT include an accurate time estimate---it is only a placeholder used to help estimate time and data volume. Only 1 epoch is included in this APT file. All together, we will have 5 epochs, for a total of about 50 hrs. The required subarrays are expected to be included in future versions of APT.



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