Space Telescope Science Institute   4.5.4 SBC Signal-to-Noise Ratio Limitations  4.5.6 SBC Nonlinearity

4.5.5 SBC Flatfield


Figure 4.17: MAMA flat field.

 

The SBC requires two types of flat fields: the "pixel-to-pixel flats" (or P-flats) which take care of the high-frequency structures in the detector sensitivity, and the "low-order flats" (or L-flats) which handle the low-frequency components. Current P-flats were recently derived by Bohlin & Mack (ACS ISR 2005-04) using the on-board deuterium lamp, and found to be independent of wavelength. The P-flat in Figure 4.17 shows the effect of the disabled broken anode (rows 601 to 605) and of the shadow of the repeller wire (around column 577).

Low-frequency flatfield corrections for the SBC imaging modes have recently been derived using multiple exposures of the globular cluster NGC6681 (Mack, et. al., ISR in prep.). Variations of ±6% (full range) were found for the F115LP and F125LP filters, ±8% for the F140LP and F150LP filters, and ±14% for the F165LP filter. (The F122M filter was not included in this analysis due to lack of sufficient data). The L-flat shows a similar general pattern in all filters, with the required correction increasing with wavelength. While performing the L-flat analysis, a decline in the UV sensitivity with time was detected at a level of 2 to 4%/yr over the first 1.6 years of operation. The sensitivity appears to have leveled off after this time. A slight sensitivity loss as a function of temperature (~0.1%/degree) was also discovered. These effects were detected for the STIS FUV-MAMA detector (see STIS ISRs 2003-01 and 2004-04).


 4.5.4 SBC Signal-to-Noise Ratio Limitations  4.5.6 SBC Nonlinearity
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