What is the peak count rate using the PR110L prism in the SBC for the HST standard star HS2027+0651 (V = 16.9) that was used for the STIS prism calibration (this spectrum is not in the ETC
list, therefore we quote below the flux which could be found by dearchiving the STIS spectrum)?
The sensitivity peaks in the 1500 Å to 1600 Å region. To find the count rate at 1537 Å, inspection of Figure 6.22 gives the sensitivity of 9.9x1014 counts/second per erg/cm2/s/Å. Multiplying by the stellar flux of 5.3587x10-14 gives 53.0 counts/second, summed in the cross dispersion direction. For the fraction of light in the central pixel = 0.31, the brightest pixel at 1437.6 Å is 17.6 counts/second/pixel, well below the bright object limit.
The SBC has no readout noise, and the dark current rate is negligible, while the main sky contribution for PR110L is from Lyman-. For daytime Ly-
intensity of 20kR = 6.1x10-13 ergs/cm2/second/arcseconds2, S'= 1.7x1014, and d, the dispersion in Å/pixel, is 2.58. Therefore, the background count rate is 6.1x10-13*1.7x1014*0.0322/2.58 = 0.041 counts/second/pixel. This value varies somewhat over the field, as the plate scale varies from the nominal 0.032 arcseconds/pixel. For faint source spectroscopy, it is better to use PR130L, which is on a CaF2 substrate to block Ly-
.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |