Next, something completely different: a big-sensor camera

Poisonberry, originally uploaded by jiihaa.

Dry, originally uploaded by jiihaa.

What would you think of a camera which has a 6.7 meter aperture and a 3.2 gigapixel sensor? Handy to have, hmmm...

The specifications of the LSST (Large Synoptic Survey Telescope) are fascinating. In fact, the LSST Science Book is a fascinating read in other parts also, not only the camera specs. But it is a hefty download at over 50 megabytes.

Here are some quotes from the book. First, on Optics and Telescope Design:

The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 square degrees, and will be devoted to a ten-year imaging survey over 20,000 square degrees south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures [...]

[...] the standard visit time in a given field is only 34 seconds, quite short for most telescopes. The time required to reorient the telescope must also be short to keep the fraction of time spent in motion below 20% [...] The motion time for a nominal 3.5◦ elevation move and a 7◦ azimuth move is five seconds. In two seconds, a shaped control profile will move the telescope, which will then settle down to less than 0.1′′ pointing error in three seconds.

The mount uses 400 horsepower in the azimuth drive system and 50 horsepower in the elevation system. There are four motors per axis configured in two sets of opposing pairs to eliminate hysteresis in the system. Direct drive systems were judged overly complicated and too excessive, so the LSST design has each motor working through a multi-stage gear reduction, with power applied through helical gear sets. The 300-ton azimuth assembly and 151-ton elevation assembly are supported on hydrostatic bearings.

Next, a few words about the camera:

The LSST camera [...] contains a 3.2-gigapixel focal plane array [...] comprised of 189 4 K × 4 K CCD sensors with 10 μm pixels. The focal plane is 0.64 m in diameter, and covers 9.6 deg2 field-of-view [...] The CCD sensors are deep depletion, back-illuminated devices with a highly segmented architecture, 16 channels each, that enable the entire array to be read out in two seconds [...]

A total of 189 4 K × 4 K sensors are required to cover the 3200 cm2 focal plane. To maintain high throughput, the sensors are mounted in four-side buttable packages and are positioned in close proximity to one another with gaps of less than a few hundred μm. The resulting “fill factor,” i.e., the fraction of the focal plane covered by pixels, is 93%.