PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT NOTE = "Procedure for HSP Calibration." PUBLICATION_DATE = 2004-09-20 END_OBJECT = TEXT END Procedure for HSP Calibration Josh Colwell September 20, 2004 Background: The UVIS HSP was designed to measure variations in intensity of light due to partial or complete occultation of the source (usually a star) by intervening material, usually Saturns rings or the atmosphere of Saturn or one of its satellites. It was not intended to be an absolute radiometric instrument. In a typical observation a background level is measured of the unocculted source to get a data level, I0, to which the occulted signal, I, is compared. No absolute calibration of I0 in physical units is required for this observation, which is the primary mode of operation of the instrument. Nevertheless, it is possible to make an approximate estimate of the brightness of an object in the FUV from the HSP data rate, provided that there is knowledge of the sources spectral shape in the FUV. The procedure for doing that is outlined below. HSP Wavelength Response: The estimated wavelength response for the UVIS HSP is shown in Figure 1 and Table 1. 0.20 0.15 xx xx x x x x x x x x x x x x xxxx x x x 0.10 x x x x x x x x x x 0.05 x x x x x x 0.00 x | | | | | | 1000 1200 1400 1600 1800 2000 Figure 1:Relative Sensitivity of the HSP detector as a function of wavelength. Table 1: Relative Sensitivity of the HSP detector as a function of wavelength. Wavelength (Angstroms) Relative Sensitivity 1130.00 0.00000 1140.00 0.0270549 1150.00 0.0390556 1160.00 0.0511593 1170.00 0.0560159 1180.00 0.0645439 1190.00 0.0705074 1200.00 0.0778497 1210.00 0.0810836 1220.00 0.0915620 1230.00 0.0993767 1240.00 0.102783 1250.00 0.110385 1260.00 0.114058 1270.00 0.118549 1280.00 0.123220 1290.00 0.123048 1300.00 0.121375 1310.00 0.129776 1320.00 0.133781 1330.00 0.140251 1340.00 0.145286 1350.00 0.143884 1360.00 0.147813 1370.00 0.146146 1380.00 0.144396 1390.00 0.150510 1400.00 0.151467 1410.00 0.149956 1420.00 0.145171 1430.00 0.143898 1440.00 0.136842 1450.00 0.144043 1460.00 0.145993 1470.00 0.149858 1480.00 0.149426 1490.00 0.148534 1500.00 0.148939 1510.00 0.149116 1520.00 0.147436 1530.00 0.146430 1540.00 0.152721 1550.00 0.147942 1560.00 0.133399 1570.00 0.132017 1580.00 0.127328 1590.00 0.112406 1600.00 0.118397 1610.00 0.108828 1620.00 0.130915 1630.00 0.122729 1640.00 0.136309 1650.00 0.130725 1660.00 0.131107 1670.00 0.126259 1680.00 0.119278 1690.00 0.109329 1700.00 0.103477 1710.00 0.0957879 1720.00 0.0880108 1730.00 0.0806759 1740.00 0.0761473 1750.00 0.0680325 1760.00 0.0620168 1770.00 0.0560484 1780.00 0.0500040 1790.00 0.0450547 1800.00 0.0403837 Radiometric calibration of the HSP is accomplished by comparing the observed HSP count rate with the convolution of the HSP relative sensitivity (Table 1) and the spectrum of the source object. The ratio of these two numbers (the count rate, I, and the scalar product of relative sensitivity, S, and the source spectrum, F, gives the HSP scalar calibration factor, f=I/(FS). Measurements of stars with known spectra provide independent measures of f. With an HSP measurement, I, and the calibration factor f, one can then derive the product FS for the source object. Similarly, if one assumes a spectral shape for F then one can calculate the absolute magnitude of F from the HSP measurement, I, the calibration factor, f, and the relative sensitivity, S. Conversely, the predicted count rate, I, is given by I=f*F*S*dlambda, where the source spectrum and brightness are known. With F in units of erg/cm2/sec/A, the HSP scalar calibration factor is f = 1.7 x 10^11. This factor is based on observations of a limited set of stars during Cassini cruise to Saturn. Objects with different spectra may have different values of f due to uncertainties in the HSP relative sensitivity, S. Also, the HSP may exhibit temperature-dependent and time-dependent changes in sensitivity.