Starry Starry Night
The stars used to navigate our spacecraft are numerous and mostly faint and unremarkable, except of course to the navigation team. We do however, use bright stars to calibrate our camera and IR spectrometer and some of them are familiar to the night sky observer and can be seen without a telescope.
Our bright calibration stars include Vega (a Lyrae), the 5th brightest star in the sky, Achernar (a Eridini), the 9th brightest, and Canopus (a Carina) the second brightest after Sirius. Canopus is best observed from the Southern Hemisphere, but it can also be seen from the southern US.
How do we calibrate? We observe these well-studied stars of known brightness. We then can place the data numbers produced by the instruments on a scale that represents the energy emitted by the stars.
The imaging cameras also look at star clusters to determine the extent of any geometric distortion introduced by the camera optics and the structures on which they are mounted. We plan to observe some of the Messier objects to determine the degree of geometric distortion that exists in the cameras under the cold conditions of space. Currently on our target list are four star clusters including: M11, in the constellation Scutum, M48, an open cluster in Hydra, M29, an open cluster in Cygnus and M41, an open cluster in Canis Major. Distortion corrections are important to arrive at an accurate measurement of the size and shape of the comet and the crater formed by the impactor spacecraft.
Lucy McFadden, University of Maryland