TESTTA/CCD

The command does a series of tests on a catalogue of flat frames. The flat fields in the catalogue should be grouped in pairs with the same exposure time. Most ideally, one should be of two groups of the order of 8 frames each - the first with increasing exposure times and the second with decreasing exposure times, interleaved with those of the first group. In this way, trends observed in the CCD response that are probably caused by the effect of temperature variations on the light source can be rejected.

The command requires a value for the mean bias level and the standard deviation in the keywords BIASMEAN and BIASSIGM to be filled and hence should be executed after the command TESTB/CCD. If no value or the value zero is found no bias offset will be subtracted.

The whole test is split in three smaller tests, commands TESTT1/CCD to TESTT3/CCD that do the following:

1.

Test T1: Creation of the transfer/linearity table. The table will contain 5 columns: column 1 for the exposure time of the first of each sets (frames 1) (label :Exp_tim1); column 2 the exposure time of the second frames (frames 2) (:Exp_tim2); column 3 the median pixel intensities over the selected frame sections in frames 1 (:Med_cnt1); column 4 the median pixel intensities over the selected area in frames 2 (:Med_cnt2) ; column 5 the variance of the difference of the frames 1 and 2 (:Variance).

2.

Test T2: Determination of linearity curve and the shutter error and the shutter offset. Entries in the linearity table not fullfilling the selection criteria will now be selected out. From the remaining entries in the table a linear fit is done to determine the linearity curve for frames 1 and 2 and the shutter error. Using the linearity data the fractional count rates are plotted against the median counts, applying a shutter offset in the measured exposure times. The real shutter offset is determined by the value for which the fit give the minimum mean residual.

3.

Test T3: Determination of the transfer curve. From the selected entries of the table a linear regression analysis is done to determine the analog to digital conversion factor and the electronic readout noise. The readout noise is determined by the inverse of the slope between the median and the variance multiplied by the sigma of the bias (determined by TESTBA/CCD OR TESTB5/CCD and stored in keyword BIASSIGM).

The complete command produces:

• A table containing the exposure time of the first of each sets (frames 1); the exposure time of the second frames (frames 2); the median pixel intensities over the selected frame sections in frames 1; the median pixel intensities over the selected area in frames 2; the variance of the difference of the frames 1 and 2
• Two linearity curves, expressed as count rate versus true exposure time. The mechanical shutter delay is determined either by linear extrapolation of the normal linearity curve (observed counts versus exposure time), thus assuming the response of the CCD is linear, or by adjusting the exposure times such that the count rate curve is closest to a straight line, thus allowing for a first-order nonlinearity in the response of the CCD.
• A transfer curve (Janesick et al., 1987) generated for any window onto the images obtained.

The linearity and the transfer curves may be generated for any section of the images.