Authors: Gabriel Perdue, Marco Sirianni, Gerhardt Meurer, William Jon McCann
Detector: HRC - Flight Build #1
To calculate accurately the gain (e-/DN) of the HRC flight build #1 (7161LXD01-01) device.
The images used were taken in the Spring of 1999 (on Thursday 11 March, Friday 12 March, and Saturday 13 March) at GSFC during the Thermal Vacuum testing period. In this particular series, first pixel response (FPR) tests were used for the bulk of the gain calculations. In a few instances, ordinary flat fields are used.
The gain for each series of paired images was calculated using the photon transfer method. An IDL program, pt.pro, written by Marco Sirianni was used to perform the calculations. The data required for this test consists of sequence of frames paired by exposure time. Ideally the sequence is illuminated over the entire dynamic range, although this was not done in practice for all the cases presented here. Two different exposure methods were used in the test: Data taken with the First Pixel Response (FPR) readout method were obtained with the C Amplifier as part of CTE monitoring and characterization tests, while simple flat fields were with the B Amplifier in order to monitor basic CCD behavior. The procedure first performs a bias subtraction on the series. Then, to account for (and remove) flat fields variations for each pair of images they are subtracted from each other. Using the resulting image, the variance is computed. Then the mean of each pair is found and used to calculate the average signal. To remove local dependancy from the calcualtion, multiple 40x40 pixels subarrays are selected from across the chip and the statistics are found in each region. Finally, before the actual photon transfer calculation takes place, deviant (to greater than three sigma values from the mean) pixel values are replaced with the median value of the entire subarray. This "cleaning loop" is repeated three times. The results are then plotted as points by signal across the x-axis and variance across the y-axis. A best fit line is then found, the slope of which, below saturation, is the reciprocal of the conversion factor (the gain) in e-/DN. In addition, the program provides a histogram of the gains found in each of the different subsections of the chip. A similar program, acsgain.pro, written by Jon McCann, was used to verify some of the results.
The results are presented in Tables 1 though 3. Note: in Table 1, the first of the paired images is a serial readout and the second of the pair is a parallel readout. Because the electronic gain (uV/DN) used in this test was not given, only the system gain (e-/DN) will be reported. Also, note that the results found using pt.pro and those found with acsgain.pro are agree within ~2%.