AUTHORS : A.R. Martel and G. Hartig
The main goal is to measure and characterize the global count rates of the internal deuterium (D2) calibration lamp through the SBC filters . Degradation in the lamp's output with time is also investigated. This is an update of the TB/TV 1 deuterium lamp analysis of McCann & Meurer (Mar 1999).
All the SBC D2 frames were acquired during the Thermal Balance/Vacuum 3 campaign. The configuration of the instrument is detailed in these Web pages. An ND1.3 filter is located in front of the D2 lamp to restrict its global count rate to <300000 counts/sec on the SBC detector (in TB/TV 1 (Feb 1999), an ND2 filter was in place). The dataset consists of 2x100 sec exposures through the F165LP filter acquired on Jul 16-17 as part of the Comprehensive Imaging Functional Test in Cold Soak#2 (MEB sides 1 and 2) and of 100 sec exposures through the F115LP, F125LP, F140LP, F150LP, PR130L, and PR110L filters and a 500 sec frame through F122M (ID 32020-32026), all imaged on Jul 18. A sequence of fifteen 1000 sec exposures through F115LP was also acquired on Jul 18 to construct a deep F115LP flat field and to monitor for changes in the lamp's output with time.
The results are tabulated in Table 1. The MAMA tube temperature is listed in Col. 4 and from it, the global dark rate in Col. 5 was evaluated from the fit in Fig. 5 of SBC Darks : TB/TV 3 (Jul 2001), which was derived for darks with an MCP voltage of -2304.52 V, as for the majority of the D2 flat fields. ID 32037 is ignored since the D2 lamp automatically turned off during this exposure. In Col. 6, the net global count rates of the SBC frames are tabulated by simply summing the counts of all 1024x1024 pixels, including the warm and hot pixels, and subtracting the dark rate of Col. 5 (the dark rates are insignificant (<0.02%) compared to the observed flat field rates and could be safely ignored). The "local" count rate in Col. 7 is calculated by dividing the global count rate by 1024x1024 pixels. These new, ND1.3 global count rates are a factor of ~3.03 greater than those listed in McCann & Meurer : Table 1, which were measured with an ND2 filter. This is a smaller factor than the model predictions of 3.72 (McCann & Meurer : SBC Throughput Model) between the fluxes through these two neutral density filters.
In Fig. 1, the MAMA tube temperature is plotted as a function of time. The curve is nearly identical to the one shown in Fig. 3 of SBC Darks : TB/TV 3 (Jul 2001). After ~4.25 hours, the temperature reaches ~30.6 C, and the heat pipes turn on, resulting in a drop and leveling off of the temperature. In Fig. 2, the relative count rates through the F115LP filter are plotted as a function of time to evaluate the degradation of the D2 lamp flux. The spike in count rates at 3.6 hours is the result of turning the lamp back on (ID 32038) after it automatically turned off during exposure ID 32037 (the ACS tungsten and D2 lamps can be in continuous operation for only 4.25 hours). Over ~4.5 hours, the lamp's flux degraded by ~0.7%, significantly better than the value measured in TV/TB 1 : 0.92% in 1 hour (McCann & Meurer (Mar 1999)). In Fig. 3, the co-added F115LP flat field (~4 hours) is shown. The observed structured is discussed in detail by Meurer et al. (1999).
Table 1 : TB/TV 3 Deuterium Flat Fields
|GLOBAL DARK RATE
Note : The MCP voltage (MCPVOLT) is held constant at -2304.52 V for all the images except for the F150LP (ID 32023) and the two F165LP (ID 31613,31697) images which are set at -2303.90 V
CEI SPECIFICATION :
In Table 4-10 of the Contract End Item Specification (Part II, Nov 1995), the nominal global count rate at 10% roll-off is >300000 counts/sec. All the measured global rates are <200000 counts/sec. The flat fields were all acquired over 1-2 days so no weekly or monthly stability specifications can be verified. Further CEI specifications are discussed in Meurer et al. (1999).
The global count rates of the internal deuterium lamp with ND1.3 were measured for each filter on the SBC wheel. All the rates are <200000 counts/sec. A sequence of flats through F115LP shows a drop of ~0.7% in the count rates over ~4.5 hours.