EIS Software Note No. 6: Warm and hot pixels on the EIS CCDs

EIS has two 2048x1024 pixel CCDs, one for each of the two wavelength channels. The CCDs were provided by the company e2v and are of device type 42-20. Prior to launch it was expected that the EIS CCDs would experience hot pixels as these are well known defects where individual pixels on the CCDs have higher than normal rates of charge leakage. These are due to either defects in the silicon, or radiation damage experienced in a space environment (in the case of EIS the most likely radiation damage is due to the SAA –- South Atlantic Anomaly, which the Hinode satellite passes through periodically every orbit). The CCD manufacturer e2v provides a criterion to identify hot pixels which is detailed in Section 4.1 below. A defect that was not expected but identified approximately 9 months into the mission was the accumulation of warm pixels, which are similar to hot pixels but have a lower signal level and are much greater in number. To identify these warm pixels it was found that no formal criterion was available (as for the identification of hot pixels) and so a ‘bespoke’ criterion was defined, which is detailed in Section 4.1. The large number of warm pixels on the EIS CCDs represent perhaps the single biggest instrumental issue affecting EIS data analysis. The present Software Note discusses some properties of warm pixels and how they are flagged for data analysis. EIS Software Note 13 discusses how scientists should deal with bad pixels in their data analysis.