calibration results by item stray light AUTHORS : A.R. Martel and G. Hartig
GOAL :
The goal is to determine the location of light leaks in the ACS enclosure and their contribution on the HRC and WFC detectors.
LOCATION AND DATE :
All the images were acquired at GSFC/SSDIF on Sep 6, 2001.
METHOD AND DATA :
ACS is in square configuration in the handling dolly (Fig. 1). The vortex chillers maintain the WFC#4 detector at a temperature of -77 C and the HRC#1 at -83 C. The light leak test must be performed late in the evening when all technicians and engineers working on other hardware have left the clean room. To achieve a totally dark environment, the ceiling lights are turned off, the curtains on the second floor mezzanine are drawn, and black cardboard squares are placed over all the windows from the outside (Fig. 2). For ACS, the lumalloy blanket is removed, the aperture cover is left on, and the internal CALDOOR is deployed. In the aft bulkhead, only the outboard vent (furthest from the optical axis) is covered with the correct flight panel. The inboard vent is shut and the middle vent is hidden by the dolly.
For each seam of the ACS enclosure, 60 sec HRC and WFC frames are acquired in EXTERNAL mode through the CLEAR/CLEAR filters at gain=1 e-/DN and bias offset of 3. During each exposure, G. Hartig and M. Erickson manually shine a fiber light along each seam at a distance of ~6 inches from the enclosure. Meanwhile, from the control room, A. Martel counts down the integration time at 10 sec intervals and relates these numbers to Hartig and Erickson via intercom so they can adjust their scanning rate appropriately. For the Sep. 6 test, the "Micro-Controle" lamp #154 (HST property #G100923) was used at maximum intensity. Its absolute output will be measured with a radiometer at BATC.
RESULTS :
Reference 60 sec dark frames were subtracted from the HRC and WFC frames of each seam scan. The bias levels were adjusted with the virtual overscans. The resultant images are compiled in Table 1. Only the WFC image ID 34122, illuminated through the outboard vent in the aft bulkhead, shows evidence of contaminating light : there is a shallow gradient from the AC edge (high) to the BD edge (low). This is also confirmed by the greater contrast between the virtual overscan and the data region in the AC half of the image. In Fig. 4, ID 34122 is displayed on a tighter linear scale than in Table 1 and in Fig. 5, a cut along the rows is shown. The gradient is ~2.3 counts from edge-to-edge or ~0.1% of the median of the original frame (~2475 DN).
Table 1 : 60 sec HRC and WFC frames
| SEAM | ID | DETECTOR | COMMENT |
|---|---|---|---|
| -V3 Outboard | 34115 | HRC | |
| 34116 | WFC | ||
| +V2 Inboard | 34117 | HRC | |
| 34118 | WFC | ||
| -V2 Outboard | 34119 | HRC | |
| 34120 | WFC | ||
| Aft Bulkhead | 34121 | HRC | |
| 34122 | WFC | Outboard vent. Gradient from AC to BD edges. | |
| +V3 Inboard | 34123 | HRC | |
| 34124 | WFC | ||
| Front Bulkhead | 34125 | HRC | |
| 34126 | WFC | ||
Note : The HRC frames are shown on a linear scale in a range of -10 to +10 DN while the WFC frames are displayed from -25 to +25 DN. These values correspond to ~1% of the median of the original frames (~984 DN for HRC and ~2475 DN for WFC).
CEI SPECIFICATION :
There is no formal CEI specification.
CONCLUSION :
Evidence for light leak in the ACS enclosure was found only for the outboard vent of the aft bulkhead in the WFC optical path (ID 34122). No other seam showed light contamination at the 1% level. This test will be repeated in Nov 2001 with ACS configured in RAS/HOMS (BATC), in particular in the aft section of the instrument.
UPDATE :
On Nov 7, 2001, with ACS in RAS/HOMS at BATC, additional light leak measurements were taken for the three vents, now in their proper configurations, in a similar fashion as the GSFC tests. No stray light was detected through the vents in both the HRC and WFC channels.
