Wide Field and Planetary Camera 2
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The Wide Field and Planetary Camera 2 (WFPC2) is a camera installed on the Hubble Space Telescope. It was installed by servicing mission 1 (STS-61) in 1993, replacing the telescope's original Wide Field and Planetary Camera (WF/PC). It was used to image the Hubble Deep Field in 1995, and the Hourglass Nebula and Egg Nebula in 1996.
The charge-coupled devices (CCDs) in the WFPC2 detect electromagnetic radiation in a range from 120 nm to 1100 nm. This includes the 380 nm to 780 nm of the visible spectrum, all of the near ultraviolet (and a small part of the extreme ultraviolet band) and most of the near infrared band. The sensitivity distribution of these CCDs is roughly normal, with a peak around 700 nm and concomitantly very poor sensitivity at the extremes of the CCDs' operating range. WFPC2 features four identical CCD detectors, each 800x800 pixels. 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 affords 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-scientific JPEG 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 features a rotating wheel which moves different optical filters into the lightpath (between the WFPC2's aperture and the CCD detectors). The 48 filter elements include:
- A polarizing filter.
- A graduated filter, featuring a wide range of very narrowband filters. By positioning the target object at a precise part of the field, the operator can use an accurately picked narrowband filter (albeit over a very small part of the field).
- A number of different optical filters, allowing the operator to choose from a variety of different response characteristics.
As predicted, over the course of its mission the WFPC2 has 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 built by NASA's Jet Propulsion Laboratory, which also built the predecessor WFPC ("WFPC1") camera originally launched with Hubble in 1990. WFPC2 contains its own internal corrective optics to fix the spherical aberration in the Hubble telescope's primary mirror. A replacement unit, Wide Field Camera 3, built by NASA Goddard Space Flight Center, is planned for deployment on SM4 (service mission #4). WFC3 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. SM4 has now been approved.
