To allow scientists to view specific parts of the electromagnetic spectrum the WFPC2 featured a rotating wheel which moves different optical filters into the lightpath (between the WFPC2's aperture and the CCD detectors).
The 48 filter elements included: As predicted, over the course of its mission the WFPC2 experienced degradation of the CCDs, resulting in defective ("hot") pixels.
The telescope's operators performed 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 is compared.
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.