HELP · SEE_ALSO
tacount -- Calculate the observed count rate given the predicted count rate for GHRS target acquisitions.
tacount modifies a predicted count rate for a target acquisition using the Goddard High Resolution Spectrograph (GHRS), accounting for paired-pulse effects and the incoming light's point spread function (PSF). The observed count rate and count rate for each of the eight target acquisition diodes are placed in the task's parameters and optionally output to standard out.
The GHRS uses an array of diode detectors to count photons of light entering one of the apertures. Once a photon strikes a diode, it is counted in the electronics which takes some time, albeit a short amount of time. Within this interval, the diode+electronics are unable to count photons. If a photon should happen to strike this diode within the "dead time", it will not be counted. This is called the paired-pulse effect. For low count rates, i.e. less than 2000 counts/diode/second, the error rate is less than 1%. The error is less than 10% for rates up to about 20,000. The detectors saturate at a count rate of about 100,000.
The equation relating observed count rate to true count rate is as follows:
y = (1.0 - e**(-t*x)) / t where x = True count rate t = Dead time constant ~ 10.2x10e-6 seconds. y = Observed count rate
The input is simply the predicted, or true, count rate of the target to be acquired. This rate can come from a number of sources, including the "synphot" package. Since GHRS target acquisitions use eight of the 500 science diodes, a PSF is needed to distribute the incoming flux over the eight diodes. These are specified in the parameters psfX where X is the target acquisition diode number.
The observed count rate, i.e. what the GHRS would see, is stored in the parameter obs_rate. The count rates for the individual diodes are stored in the parameters diodeX, where "X" is the diode number. If the verbose parameter is "yes", this information is also sent to standard output.
The reason one would be interested in the count rate for each diode is that the memory location which stores the counts can only hold a maximum of 65536 counts per diode. If the target acquisition exposure time is long enough at a specified count rate, the counter will overflow and wrap back down to zero. Knowing the count rate will help an observer avoid this situation.
- input [real]
- The predicted, or true, count rate of the target to be acquired.
- (obs_rate = 0.) [real]
- Output: On task completion, this will have the value of the observed count rate.
- (diode1, diode2,...,diode= 0.) [real]
- Output: On task completion, these parameters will contain the count rates for each of the target acquisition diodes.
- (verbose = yes) [boolean]
- If "yes", the observed count rate and individual diode count rates will be printed on standard output.
- (psf1, psf2,..., psf8) [real]
- These parameters contain the Point Spread Function (PSF) as seen by the target acquisition diodes.
- (dead_time = 10.2) [real]
- The "dead time" constant, expressed in microseconds (10e-6 seconds) for the diodes.
1. Determine the count rate that GHRS will observe from an object whose predicted count rate is 4000 counts/diode/second.
cl> tacount 4000 # For predicted count rate 4000., observed count rate is 3738. # Count rate for each diode is: # Diode Rate 1 35.9339848272905 2 83.64116955106521 3 252.6865667694634 4 1496.623069154898 5 1496.623069154898 6 252.6865667694634 7 83.64116955106521 8 35.9339848272905
The count rate that GHRS would observe is 3738 counts/diode/second. The maximum count rate any one diode will see is 1497 counts/diode/second.
Steve Hulbert, Science Instrument Branch, STScI.
Jonathan Eisenhamer, STSDAS, STScI.
Ball Aerospace, "GHRS Science Verification Program for the Hubble Space Telescope", Dennis Ebbets, ed., 24Feb1992, under NASA contracts NAS5-26000 and NAS5-30433.
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