New Galactic Neighbor

Dan Yocum writes "The Sloan Digital Sky Survey reveals a new Milky Way neighbor: a galaxy so big we couldn't see it before. A huge but very faint structure, containing hundreds of thousands of stars spread over an area nearly 5,000 times the size of a full moon, has been discovered and mapped by astronomers of the Sloan Digital Sky Survey."

Comments

Some SDSS info

[Michael Woodhams]

I was a graduate student at the Astrophysical Sciences deptarment at Princeton when they were planning and starting to build the SDSS. A few interesting facts:

Some very clever optics (designed by James Gunn) went into the telescope. Normal telescopes do not produce the large field of view required. There were existing specialized telescopes which did (Schmidt cameras) but they have the imaging plane in the wrong place.

The main camera uses 30 2k x 2k CCDs, cooled by liquid nitrogen. At the time (early '90s) these cost on the order of $200k per chip.

The camera works in "drift scan" mode: the telescope moves such that the images of the stars drift along the columns of detectors in the CCDs. The packets of charge are shifted along the CCDs at the same rate - so instead of producing distinct individual frames, it continuously outputs data along an ever-lengthening strip along the sky. As I recall, the data rate is about 8Mbyte/s.

The camera spends rather more time on spectroscopy than imaging. (The imaging is primarily about selecting targets for the spectroscopy.) The spectrograph does 640 objects at a time. A computer-drilled plate is (manually) plugged with fibre optic cables in the right positions for that field of sky.