Constraints on Acquisitions and the STEP-TIME Parameter
GHRS Instrument Handbook
Target Acquisition Reference Information
Very Bright and Blue Stars
There are limits to the brightest and bluest stars that can be acquired with mirrors N1 and N2 because of the very high fluxes of these objects. Figure 7.3 on page 91 shows the limits for these two mirrors as set in the Constraints and Restrictions Document (CARD). Objects that are too blue and too bright relative to these limits should be acquired with mirror A1 or A2.
Once you have predicted an acquisition count rate, you then determine the integration time per dwell point during the target acquisition: STEP-TIME. The goal is to have GHRS see 103 to 104 counts at the peak dwell point of the spiral search for either an acquisition or a peakup. A minimum of about 100 counts is acceptable for fainter objects. The STEP-TIME then is just the number of counts desired (103 to 105, but at least 100) divided by the predicted count rate. Remember that valid values of STEP-TIME range from 0.2 seconds to 12.75 seconds. If you have questions about acquiring very bright and faint objects you may wish to consult a GHRS Instrument Scientist.
"Wrapping" of the Counters
To avoid the possibility of a failed ACQ/PEAKUP, you should ensure that no single diode of the eight used for target acquisition exceeds 65,000 total counts (the number accommodated in a 16 bit register) in a given STEP-TIME. While this is usually a problem only for very bright targets, it can always be avoided by using a STEP-TIME of 0.6 seconds or less. This problem is avoided if BRIGHT=RETURN is specified because a 32-bit on-board register is used. If instead a BRIGHT limit is explicitly given, the register that holds it is limited to 16 bits.
Figure 7.3 is provided to allow for a visual check of potential problems arising from the choice of STEP-TIME. This Figure shows the constraints placed on using a particular mirror and STEP-TIME.