Performance
As predicted, over the course of its mission the WFPC2 experienced degradation of the CCDs, resulting in defective ("hot") pixels. The telescope's operators perform monthly calibration tests to catalog these; with the WFPC's aperture closed a number of long exposures are taken, and pixels which differ significantly from near black are flagged. To avoid false positives caused by cosmic rays tripping a given pixel, the output of different calibration shots are compared. Pixels which are consistently "hot" are recorded, and astronomers who analyse raw WFPC2 images receive a list of these pixels. Typically astronomers adjust their photo-processing software to ignore these bad pixels.
WFPC2 was largely superseded for broad-band imaging by the Advanced Camera for Surveys, installed during servicing mission 3B in 2002. However, the early 2007 failure of ACS resulted in WFPC2 returning to its role as Hubble's primary visible light camera. WFPC2 was removed from HST during Servicing Mission 4 in May 2009, for return to Earth and eventual museum display. It was replaced by Wide Field Camera 3, which features two UV/visible detecting CCDs, each 2048x4096 pixels, and a separate IR CCD of 1024 x 1024, capable of receiving infrared radiation up to 1700 nm.
Read more about this topic: Wide Field And Planetary Camera 2
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