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Camera Design

How big are the CCD cameras?

The four PanSTARRS cameras will each be the largest digital cameras ever built. Each camera will have about 1.4 billion pixels spread over an area about 40 centimeters square. For comparison, a typical domestic digital camera contains about 5 million pixels on a chip a few millimeters across.

How many CCDs are there in the focal plane?

The focal plane of each camera contains an almost complete 64 x 64 array of CCD devices, each containing approximately 600 x 600 pixels, for a total of about 1.4 gigapixels. The CCDs themselves employ the innovative technology called "orthogonal transfer", which is described below.

The individual CCD cells are grouped in 8 x 8 arrays on a single silicon chip called an orthogonal transfer array (OTA) , which measures about 5 cm square. There are a total of 60 OTAs in the focal plane of each telescope (The four corner OTAs are omitted because they are too far from the opic axis of the telescope to collect useful data)

The first Gigapixel camera was shipped to Haleakala in August 2007 and successfully mounted on the PS1 telescope.

Why so many CCDs?

Several reasons:

  1. Small CCDs can be read out more quickly than large ones.
  2. A manufacturing defect usually cripples a single CCD. By dividing the focal plane into a large number of CCD devices we limit the damage caused by a chip faults. The ability to make good use of slightly imperfect chips results in a very large saving of both cost and manufacturing time. One of the reasons we use four cameras is to mitigate the effect of chip defects.
  3. Bright stars can saturate CCDs very quickly. CCDs which include a bright star image can be set to read out very fast, with no ill-effects on the neighboring CCDs.
  4. The CCDs all use orthogonal transfer technology (see next section) that reduces blurring by the earth's atmosphere.

What is an Orthogonal Transfer CCD?

An Orthogonal Transfer Charge Coupled Device (OTCCD) is a device that allows for image motion compensation in the focal plane itself. During an exposure, selected bright stars have their positions rapidly monitored in order to calculate the immediate effects of atmospheric phase fluctuations. In a traditional "tip-tilt" adaptive optics system, these position errors are fed back to a small mirror whose angle is rapidly adjusted to compensate for the atmospheric disturbance. An OTCCD achieves the same goal by electronically shifting the image within the CCD itself rather than by moving a mirror

What filters will you be using in the cameras?

Pan-STARRS is primarily sensitive to visible light, though observations can be extended slightly into the infrared passbands. Each camera will include an identical set of 5 to 6 optical filters that can be remotely positioned in front of the focal plane.

The searches for asteroids and potentially hazardous objects may use a wide filter ("g+r+i") that covers most of the visible waveband from 0.5 to 0.8 microns. This filter provides maximum sensitivity for detecting solar system objects. Surveys for stars and galaxies are much more valuable when they include color information, so Pan-STARRS will include standard photometric g-, r-, i-, and z-band filters, as used in the Sloan Digital Sky Survey. The excellent near-infrared response of the Pan-STARRS detectors means that we can also make use of a y-band filter (1.0 microns).

What is the main source of noise in the images?

We anticipate that the read noise in the orthogonal transfer CCDs will be about 5 electrons and the sky background will be about 7 electrons per pixel with the broadband filter. Thus, sky noise will dominate read noise in exposures of 15 seconds or more.

Next: Data Handling

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