TheWide Field and Planetary Camera 2 (WFPC2) is a camera formerly installed on theHubble Space Telescope. The camera was built by theJet Propulsion Laboratory and is roughly the size of ababy grand piano. It was installed by servicing mission 1 (STS-61) in 1993, replacing the telescope's originalWide Field and Planetary Camera (WF/PC). WFPC2 was used to image theHubble Deep Field in 1995, theEngraved Hourglass Nebula andEgg Nebula in 1996, and theHubble Deep Field South in 1998. DuringSTS-125, WFPC2 was removed and replaced with theWide Field Camera 3 as part of the mission's first spacewalk on May 14, 2009. After returning to Earth, the camera was displayed briefly at theNational Air and Space Museum and theJet Propulsion Laboratory before returning to its final home at the Smithsonian'sNational Air and Space Museum.[1][2]
WFPC2 was built byNASA'sJet Propulsion Laboratory, which also built the predecessorWFPC camera launched with Hubble in 1990. WFPC2 contains internal corrective optics to fix the spherical aberration in the Hubble telescope's primary mirror.
Thecharge-coupled devices (CCDs) in the WFPC2 (designed at JPL and manufactured by Loral) detectedelectromagnetic radiation in a range from 120nm to 1000 nm. This included the 380 nm to 780 nm of thevisible spectrum, all of the nearultraviolet (and a small part of the extreme ultraviolet band) and most of thenear-infrared band. The sensitivity distribution of these CCDs is roughlynormal, with a peak around 700 nm and concomitantly very poor sensitivity at the extremes of the CCDs' operating range. WFPC2 featured four identical CCD detectors, each 800x800pixels. Three of these, arranged in an L-formation, comprise Hubble's Wide Field Camera (WFC). Adjacent to them is the Planetary Camera (PC), a fourth CCD with different (narrower-focused) optics. This afforded a more detailed view over a smaller region of the visual field. WFC and PC images are typically combined, producing the WFPC2's characteristic stairstep image. When distributed as non-scientificJPEG files the PC portion of the image is shown with the same resolution as the WFC portions, but astronomers receive a raw scientific image package which presents the PC image in its native, higher detail.
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'saperture closed a number of long exposures are taken, and pixels which differ significantly from near black are flagged. To avoid false positives caused bycosmic rays tripping a given pixel, the output of different calibration shots is 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 theAdvanced 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 byWide 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.
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