Alternate Method for Predicting Target Acquisition Count Rates

Using the table, you would see that HR 8023, an O6 star with is the closest match, giving a predicted count rate for the A2 mirror of 1.3 x 107counts s-1 for a V0 = 0 star. Multiplying this count rate by 10-0.45.13 gives 1.2 105 counts s-1. Reddening will decrease the counts slightly; calculation of the scale factor indicates that you should multiply by 0.93, giving a new count rate of 1.1 105 counts s-1. The dead-time correction factor estimates that only 83% of those counts will be detected, so one would expect approximately 94000 counts s-1 with this star. In fact, when m Col was observed early in Cycle 4, 19,600 counts were obtained in 0.2 seconds with the A2 mirror, which works out to 98,000 counts s-1, which is within 5% of the calculated value.

Second, consider a very red star such as Aldebaran:

Using the table, you would see that BD-1o3113 has a similar spectral type (K5III) and color (1.61). The calculated count rate for mirror A2 is then counts per second. The adjustment for apparent magnitude is , which yields a count rate of 530 per second, or 210 in 0.2 seconds. Early in Cycle 4 a Tau was acquired with mirror A2 and the count rate seen was 246 in 0.2 seconds, within 15% of the predicted value.