AUTHORS : Gerhardt R. Meurer
DETECTOR : SBC
Measure the global dark rate of the Solar Blind Channel and determine what quantities govern it (e.g. MAMA tube temperature). Monitor temporal evolution of the dark rate. Present an accumulated long exposure dark image and discuss its features. Locate and measure hot pixels.
Table 1 lists the available dark frame images that have been obtained with the SBC as of June 1999. The data were obtained for a variety of purposes. These include (1) as part of functional and aliveness tests, (2) as background comparison images for other calibartion goals, and (3) explicitly for determining the dark rate and dark structure.
I. A "Super Dark" Image
During the first thermal vacuum campaign (Feb-Mar 1999), ten one hour long dark images were obtained (see Table 1). The data were taken during the flat fielding phase of the campaign during which the thermal environment of the instrument was constant. All data were obtained after the SBC had been reached an equilibrium temperature of 31°C. An IDL program combine_sbcdrk.pro was used to combine the images and to locate hot pixels.
Figure 1 : SBC "Super Dark" image
|FITS||GIF||GIF thumbnail: 2x rebin||GIF thumbnail: 4x rebin|
Dark structure. Figure 1 shows the "Super Dark" image displayed with square root scaling and 2x rebinning. A gzipped fits version of the full resolution image can be obtained by clicking on the fits button below the image. The image is normalized by the exposure time to give counts/pixel/hour. The following features are noticeable in the Super Dark:
II. Global dark rate trends
MAMA Tube Temperature and MCP current. The global dark rates from Table 1 are plotted against MAMA tube Temperature TSBC and MCP current IMCP in Fig. 2.
Figure 2 : Dark rate vs Temperature and Current
Strong dependencies are seen in both cases. The lines in Fig. 2 correspond to a factor of ten change in the dark rate for a change in TSBC of 16.6°C (solid line) or 6.4°C (dashed line). In terms of IMCP, a factor of ten change in the dark rate corresponds to a change of 7.5 mA (solid line) or 3.38 mA (dashed line). The plots are very similar in appearance because IMCP and TSBC are strongly correlated. However, the relationship between dark rate and IMCP appears to be somewhat cleaner than the relationship with TSBC. In both plots, there are strong deviations from the (eye-fitted) lines. These amount to deviations on the of ~0.8 dex in some cases. The cause of these deviations is being investigated and is not yet known.
Temporal variations The strong dependence of the dark rate on TSBC and IMCP can easily mask any second order effects. Therefore, Fig. 3 plots dark rate over only a very limited range in time (measured realtive to the first exposure taken on 6 Mar 1999, UT 5:11). This interval of time covers most of the flat field phase of the first thermal vacuum campaign. During this interval, there was no cycling of the SBC power, and TSBC = 30.8 ± 0.2 °C and IMCP = 64.47 ± 0.15 mA were fairly steady.
At first blush, as in Fig. 3a, there is an apparent 10% decline in the global count rate over 100 hours. However, much of this may be due to the IMCP dependency. When this is removed, using the steeper of the two slopes shown in Fig. 2b, then the temporal correlation is much less obvious - maybe a 5% effect with about 5% intrinsic scatter.
Figure 3 : Dark rate versus time
|a. Raw global count rate.||b. Global count rate corrected for IMCP dependency.|