4.8 Dark Backgrounds

Low dark noise is one of the benefits of MPP, since inverted phase operation suppresses the dominant source of CCD dark noise production (Si-SiO₂ surface states). The remaining source of dark noise, thermal generation in the silicon bulk, is determined by the quality of the silicon used in chip fabrication. The intrinsic dark rate of WFPC2 CCDs is <0.01 e^- pixel^-1 s^-1 at temperatures below -80°C.

Figure 4.5: Average Dark Rates vs. CCD Row.

The temperature set-points for the WFPC2 TEC coolers are: -88, -83, -77, -70, -50, -40, -30 and -20 °C. The corresponding approximate median dark rates are given in Table 4.2. For instrument health and safety reasons, GOs cannot command temperature changes.