On Tue, Oct 11, 2011 at 2:06 AM, David Mann <[email protected]> wrote:
> I wonder what kind of optics they're going to use on this thing. > > http://www.bbc.co.uk/news/science-environment-15242383 Here you go: http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=40129 The Gaia mission is a successor to the Hipparcos mission. The main goal is to do high-precision astrometry of stars in our galaxy. Astrometry is the measurement of a star's position on the sky (analogous to precisely surveying geographic features on the earth). The specs indicate that Gaia will measure the positions of the brightest stars to better than 7 microarcseconds. Through astrometry, you can determine the distance to a star through parallax (its shift against the background, due to perspective changes as the earth moves in its orbit), and through repeated observations you can determine the star's motion in space, to understand the dynamics of stars in the galaxy. The optical design is interesting. It consists of two telescopes, each with a 1.45m × 0.5m aperture. The telescopes are pointed 106.5 degrees apart on the sky, but project their images onto the same focal plane. This allows precise measurement of the angles (i.e. relative positions) between stars in the two telescope fields. Measuring the relative positions of stars over such a large baseline results in a better astrometric solution than trying to chain together a bunch of short-distance measurements that all come from single fields of view. If you've ever stitched a panorama, you know the idea: you want your control points to be widely separated (not bunched together) to determine the solution. Also worthy of note is that the sensors (CCDs) operate in "time-delayed integration" (TDI) mode, also known as "drift scanning". When you read out a CCD, you physically "clock" the accumulated charge (electrons) from row to row, until it reaches the last row of the device, at which time the charge in the row is shifted out, pixel by pixel, into an amplifier. In TDI mode, you "clock" the CCD rows continuously, at some slow, fixed rate. On Gaia, the spacecraft scans the sky (like panning a camera), so that the image of any star moves down the CCD at the same rate that the charge is being clocked. This setup allows constant integration (exposure) on the sky, without taking breaks to read out the CCDs. The downside is that your exposure time is basically fixed (set by the time it takes an object to cross the focal plane, at your scan rate). On ground-based telescopes, TDI mode is usually designed so that you "park" the telescope without tracking, and read out the CCD at the same rate that the stars cross the field of view due to the earth's rotation (the "sidereal rate"). -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
