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Дата изменения: Wed Feb 14 17:44:53 1996 Дата индексирования: Sat Dec 22 16:58:49 2007 Кодировка: Поисковые слова: dwarf spheroidal |
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.
STEP-TIME Constraints
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: Target acquisition constraints
