STScI Logo

Other Acquisition Information

GHRS Instrument Handbook


Target Acquisition Reference Information

Effective Areas of the Acquisition Mirrors

Table 7.3 on page 98 lists the effective areas of the four acquisition mirrors (in cm2) as a function of wavelength, to use to predict acquisition count rates. These values are those from the Science Verification Report for the GHRS, adjusted by the observed ratio of post- to pre-COSTAR sensitivities. Note that A1 and N1 may only be used with detector D1, and A2 and N2 with detector D2.

Also note that the proper use of this table requires compensation for the different energies of photons of different wavelengths, hence the last column, which is in picoergs per photon.

Figure 7.4: Relative sensitivities of the GHRS acquisition mirrors. The effective areas shown are from Table 7.3 on page 98.



Geocoronal Lyman-alpha Background

The acquisition mirror sensitivity curves illustrated above show that the mirrors still reflect well at the Lyman-a line at 1216 Å. Note that Ly-a is suppressed by mirror A1. Tests have shown that the count rate from geocoronal Lyman-a can be as high as 12 counts per second per diode. If you wish to acquire a faint target with the least Ly-a contamination, we suggest that you specify SHADOW as a Special Requirement in Phase II. Doing so, however, will limit the schedulability of your program.