Calibration Goals and Plans

Transcription

1 CHAPTER 13 Calibration Goals and Plans In This Chapter... Expected Calibration Accuracies / 195 Calibration Plans / 197 This chapter describes the expected accuracies which should be reached in the calibration of NICMOS during Cycle 10. Since the NCS-operated NICMOS represents a new instrument, a full new suite of calibrations will be obtained during Cycle 10 to characterize the instrument performance under the new operating conditions. The list of activities which will be performed during the Servicing Mission Orbital Verification 3B (SMOV3B) to enable science with NICMOS is also detailed for reference. Expected Calibration Accuracies Remarks In Table 13.1 we list a provisional summary of our calibration goals for Cycle 10. These goals represent the calibrations and performance achieved by NICMOS during Cycle 7 and 7N. Calibration proposals which will be executed during the SMOV phase and during the Cycle 10 calibration program are aimed at reproducing (and possibly improving on) the same level of calibration obtained during Cycle 7 and 7N. Although observers may plan on NICMOS achieving these levels of performance, uncertainties 195

2 196 Chapter 13: Calibration Goals and Plans still exist, as explained in the next section. Science programs which require superior calibrations should request and justify additional observing time to reach the necessary calibration accuracy. Areas of Significant Uncertainty The figures reported in the Table below are drawn directly from the performance and calibrations achieved with NICMOS during its cryogenic life (Cycle 7 and Cycle 7N). Uncertainties in the actual performance during Cycle 10 are related to the temperature stability under NCS operations on both orbital and secular timescales. Current requirements set goals of 0.1 K temperature stability on orbital scale and 0.5 K over one year. Other areas of uncertainty include the quality of the transformations and (for a small portion of each Camera s field of view) the effects of vignetting.

3 Provisional Cycle 10 Calibration Goals Table 13.1: Summary of Cycle 10 Calibration Goals Attribute Accuracy Limiting Factor/Notes Calibration Plans 197 Detector dark current and shading <10 DN All MULTIACCUM sequences will have good calibrations. A small subset of the available ACCUM mode exposure times will have a direct calibration. Flat fields Photometry 1% broad-band 2-3% narrow-band 5% photometric zero point 2% relative over FOV 2% stability Color dependence may limit flats in some cases. The low spatial frequency may only be reliable to 3%. Photometric systems, Intrapixel effects, Filter leaks on red sources PSF and focus Maintained within 1 mm Breathing and OTA desorption are approximately equal effects. Coronagraphic PSF Polarization arcsec accuracy positioning in the hole ~1% relative intensity 3-5% polarization accuracy Photometric performance Ghosts/polarizing efficiency GRISM wavelength calibration.005 µm Limited by centroiding of target for wavelength zeropoint determination. GRISM photometric calibration 20 30% absolute and relative The figure is quoted without the NIC3 intrapixel sensitivity correction. With this correction, GRISM photometry will be better. Expected accuracy over central 80% of spectral range. Grism C flat field may not achieve this performance. Astrometry 0.2% plate scale 0.1" to FGS frame Calibration Plans The calibrations available during Cycle 10 will be based on two distinct calibration activities. First, the activity known as SMOV will provide science-enabling and initial calibrations for all NICMOS science modes. Second, the routine Cycle 10 calibration program will provide the necessary observations to achieve the goals set in Table It is important to distinguish between the various goals of these calibration activities. SMOV is intended to demonstrate that the instrument

4 198 Chapter 13: Calibration Goals and Plans is functioning as expected, based on both previous experience and expectations under NCS operations, to establish necessary operation parameters (e.g. plate scale), and to begin the calibration and scientific use of NICMOS. In many cases the complete calibration will be conducted during Cycle 10 with SMOV being used to demonstrate that the planned calibrations are in fact feasible. This approach is designed to enable the acquisition of science observations at the earliest possible date, even if the best possible calibrations do not become available until some time later. This contributes to the efficiency with which HST can be operated. As of this writing, only a preliminary draft of the SMOV activities is known and those activities relevant to NICMOS calibrations are listed in Table 13.2, for reference. The baseline of the SMOV plan for NICMOS consists of 30 activities, of which 24 are calibration-related. Some of these test specific components of the instrument while others characterize its performance within the HST environment. A number of proposals are aimed at testing the detector s temperature stability under NCS operations, and performance changes with temperature variations. Table 13.2: Planned (preliminary) NICMOS SMOV Activities Activity Title NICMOS Cool-down Dark Performance NICMOS Cool-down Flat-Field Performance NICMOS Temperature: Setpoint and Stability NICMOS Focus Characteristics with Temperature NICMOS Transfer Function Test NICMOS SAA Cosmic-Ray Persistence Test NICMOS Jitter Test NICMOS Target Acquisition Test (Mode 2) NICMOS to FGS Astrometric Calibration NICMOS Plate Scale and Astrometric Calibration NICMOS Fine Optical Alignment Demonstrates or Calibrates Dark performance as a function of temperature Flat-field performance as a function of temperature. Demonstrate detector s temperature stability within 0.1 K on orbital scales Demonstrate NICMOS optical alignment repeatability after temperature variations Set optimal detector DC offset voltages. Measurement of decay time of SAA cosmic ray events. Demonstrate that the NCS-induced jitter does not degrade NICMOS image quality Demonstrate Mode 2 (on-board) acquisitions. Establish the locations of the NICMOS detectors within the FGS coordinate system. Determine plate scales, relative field rotations, and field distortions for each camera. Optimal positioning of the PAM based on grid of small PAM motions. NICMOS NIC3 Fine Optical Alignment Optimal positioning of the PAM /FOM for NICMOS camera 3. NICMOS Focus Monitor Monitoring of NICMOS best focus position (stability).

5 Calibration Plans 199 Table 13.2: Planned (preliminary) NICMOS SMOV Activities Activity Title NICMOS Point Spread Function Characterization NICMOS Astronomical Persistence Test NICMOS Intflat Ability and Stability NICMOS HST Thermal Background Test NICMOS Absolute Photometry Test NICMOS Differential Photometry Test NICMOS Detector Noise and Dark Characterization, Cosmic Ray Test NICMOS Coronagraphic Performance Verification NICMOS Geometric Stability Test NICMOS Grisms Absolute Sensitivity NICMOS Grisms Wavelength Calibration Demonstrates or Calibrates Characterization of the imaging performance of NICMOS and initial set of PSF observations. Characterize the effect of severe overexposure of the NICMOS detectors.measurements of decay time for photon persistence. Demonstration of flat fielding capability and stability monitoring. Characterization of HST generated thermal background over a broad range of situations. Standard star observations across NICMOS wavelength range. Updates SLTV throughput calibration. Inter-camera photometric precision and stability test obtained from observations of a star at 25 positions. Characterization of detector noise and dark current. Comparison and bootstrap from Cycle 7N warm-up. Re-verification of CR rate. Optical characterization of coronagraphic stray light rejection and PSF of the obscured star. Stability of the coronagraphic spot, measurements of detector s lateral motions in the NIC2 focal plane. Determine Grisms sensitivity. Compare with Cycle 7 performance. Derive wavelength calibrations for Grisms. Compare with Cycle 7 performance.

6 200 Chapter 13: Calibration Goals and Plans