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 one billion pixels spread over an area about 40 centimeters square. For comparison, a typical domestic digital camera contains about 3 million pixels on a chip a few millimeters across.
How many CCDs are there in the focal plane?
Each focal plane consists of an 8 x 8 array of innovative devices
called "Orthogonal Transfer Arrays" (OTA). An OTA is
a single silicon chip which itself consists of an 8 x 8 array of
individually addressable 512 x 512 Othogonal Transfer CCD devices.
Hence the total number of pixels per camera is (8x8)x (8x8)x(512x512)
- about one billion - although a few devices in the corners of
the square array may be excluded from the survey area, as in the
illustration of the complete "gigapixel" camera on the
right.
What is an Othogonal 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 5-6 optical filters that can be remotely positioned in front of the focal plane.
The searches for asteroids and potentially hazardous objects will use a wide filter ("R+V") that covers most of the visible waveband from 0.50 - 0.80 namometers. This filter provides maximum sensitivy 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, to match those used in the Sloan Digital Sky Survey.
What is the main source of noise in the images?
We anticipate that the read noise in the Orthogonal Transfer CCDs will be about 3 electrons, and the sky background will be about 7 electrons per pixel with the broad-band filter. Thus sky noise will dominate read noise in exposures of 15 seconds or more.
Where can I learn more about the Pan-STARRS detectors and cameras?
The LSST designers are counting on Moore's Law to make their CCD cameras and data processing computers feasible 10 years the future. What are you doing that will allow you to build and operate comparable systems in less than 5 years from now?
First, with regard to the CCDs, the OTCCD technology being developed at Lincoln Laboratory is ripe for mass production. As noted above, the field tests of prototype devices have been extremely successful. We are working with their team to find efficient approaches to package the devices to produce the massive mosaic cameras needed by Pan-STARRS. Second, even at the current rate of growth of computer technology, we believe the computing power will be available to handle the massive amounts of data produced when Pan-STARRS is operational.