Other
Scientific paper
Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10383b&link_type=abstract
HST Proposal ID #10383
Other
Hst Proposal Id #10383
Scientific paper
The NICMOS grism is an underutilized resource for science, in particular for precision IR spectrophotometry. This cycle 13 program is a minimal follow up and closeout for the 10 orbit cycle 12 program 9998 and for 14 orbits in 10014. If GO grism usage remains light in cycle 14, this two orbit sensitivity monitoring program could also suffice as a cycle 13 closeout calibration for this mode. The goals of the Cycle 12-13 grism calibration campaign are: 1. Provide a robust characterization of the absolute flux calibration for the two shorter wavelength NICMOS grisms, including quantitative measurement of repeatability and estimates of the uncertainty. 2. Verification to better than 1-percent accuracy of the relative fluxes of the three primary WD standard stars and of the relative fluxes of the three secondary solar analog standard stars to 1.9 microns. The same verification has already been done with STIS {Bohlin, Dickenson, & Calzetti 2001} below 1micron. 3. Characterization of the differences between the WD and the solar analog flux scales in the IR; and establishment of the best basis for all absolute IR flux calibrations, including the other NICMOS filters, SNAP, JWST, and WFPC3. 4. Establishment of one fainter IR standard stars to prove the ability of the NICMOS grism to do precision spectrophotometry at 3-5 magnitudes fainter than the other six standard stars. The fainter standard will provide the basis for relative fluxes on SNAP and may be faint enough for the higher dispersion spectral modes of NIRSpec and MIRI on JWST. SNAP requires standards with 1-percent relative flux from 0.35-1.7microns to accurately determine the parameters of the equation of state for the dark energy that causes the accelerating expansion of the universe. 5. In summary, these data will establish for the first time an absolute flux basis in the IR that will be of comparable fidelity to the optical flux standards. 6. This cycle 13 proposal will quantify the repeatability of grism observations with a star that has already been observed in cycle 12. CYCLE 12 RESULTS AND CYCLE 13 FOLLOW UP: All cycle 12 data was successfully acquired. One problem is that the repeatability as measured by the comparison of the cycle 11 to the cycle 12 observations of P330E is only +/- 2%, as shown in Fig. 1 {attached to the Phase 1 distribution}. We have extracted and corrected the cycle 12 data for intra-pixel and pixel gap sensitivity variations at each of the 15 dither positions. The rms of these sets of 15 dithered spectra, ie the scatter about the mean of an individual spectrum, is typically 1-2-percent, making the error in the mean <0.5-percent for all the pixels in the central 90-percent of the wavelength coverage.The goal and expectation is for repeatability to better than 1%. The somewhat poorer results may be attributed to flat field differences between the Thompson dither strategy and that used in 9998; or perhaps, there is some synoptic change in the system throughput. In order to distinguish these possibilities and hopefully recover a repeatability of 1%, an observation of one of the cycle 12 standards must be repeated early in cycle 13, using the same dither strategy as 9998. To minimize the measured amount of any synoptic sensitivity change, the observations should be made before the end of 2004. The bulk of the cycle 12 observations were made in the 2004 January through 2004 July time frame, so that the ideal repeatability target was observed early in 2004 and can be observed again this fall. The bright Sloan standard BD+17d4708 is the ideal selection, having been previously observed on 04Jan10. P330E would be a good choice, except that it was previously observed too recently on 04Jun19 to measure any sensitivity change back to 04 Jan; and the scheduling would be rushed to get it before going into solar avoidance in October.
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