Absolute Flux Calibration of STIS MAMA Imaging Modes

The absolute flux calibration of STIS MAMA imaging modes is tested by comparison of observed and predicted count rates. As part of this work, we have derived new, wavelength-dependent aperture corrections for MAMA imaging, and have revised the NUVMAMA imaging throughput at short wavelengths. FUV-MAMA imaging modes clearly show a time dependent sensitivity loss that is consistent with the time dependent sensitivity changes seen in G140L spectra. Once this time dependence is taken into account, count rates measured using 1” apertures for FUV 25MAMA, F25LYA, F25ND3, and F25SRF2 observations are within 5% of predictions made using the prelaunch throughput determination. However, FUV F25QTZ observations show a much larger scatter than expected from Poisson statistics. NUV-MAMA imaging modes also appear to be consistent with the time dependence observed for G230L spectra, although this effect is not as large as for the FUV-MAMA. While most NUV-MAMA imaging modes show adequate agreement with the tabulated sensitivity curve, results for the F25CN182 mode are discrepant. We suggest a revision of the short wavelength throughput of the NUV-MAMA imaging modes to fix this problem. This revision brings the results for all NUV imaging modes to within 5% of predictions. Introduction The primary white dwarf stars used as flux calibration targets for HST are too bright to be directly imaged with most of the filters used with the STIS/MAMA detectors. Calibration of these modes therefore requires the use of fainter secondary standards. Our approach in this report will be to convolve the measured spectral energy distributions for a Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report STIS 2003-01 number of faint standard stars with the adopted wavelength-dependent instrument throughputs to predict the count rates for MAMA imaging observations of these stars. These predicted count rates will be compared to the actual imaging observations. We will then infer what, if any, changes to the adopted throughputs are needed to bring the predicted and observed count rates into agreement. We will assume that the wavelength-dependent throughput of the HST Optical Telescope Assembly (OTA) is known. The throughputs of the STIS filters have been directly measured by comparing filtered and unfiltered slitless spectra of hot stars. There remain, however, uncertainties in the combined throughputs of the remaining optical surfaces and detectors in STIS. For our initial comparisons between observed and predicted MAMA imaging count rates, we will adopt the prelaunch FUV-MAMA imaging throughput curve given in the reference file h2315581o_pht. For the NUV-MAMA, we will use the throughput curve given in the reference file j7c1059ko_pht, which was delivered in July 1999 based on preliminary on-orbit measurements. The calibration targets for our check of the MAMA imaging absolute sensitivities are summarized in Table 1. Four of these imaging targets are isolated field stars; the remainder are in globular clusters. In the cluster NGC 2808 we only used one UV-bright star, NGC 2808-UIT1, for which STIS spectra are available from the GO program 7436. A number of MAMA images are available for this cluster from the STIS calibration program 8511. In the cluster NGC 6681, the brightest blue horizontal branch stars give count rates in the unfiltered MAMA detectors of 100 to 300 counts/second/star, making it an excellent target for the calibration of the broad band MAMA imaging modes. Numerous exposures have been taken to provide tests of geometric distortion, of flat-fielding, and as a monitor of the full field sensitivity. As part of programs 8422 and 8849, FUV/G140L, NUV/G230L, and CCD/G430L spectra were taken through the 52X2” slit, to determine the spectral energy distribution of a number of these HB stars. The slit position chosen included a dozen hot HB stars of various effective temperatures for which copious imaging data from the full field sensitivity monitor are available. The MAMA photometry of the full field imaging data is described in a separate report by Davies & Mobasher (2003).