Measuring the flatness of focal plane for very large mosaic CCD camera

摘要

Large mosaic multiCCD camera is the key instrument for modern digital skysurvey. DECam is an extremely red sensitive 520 Megapixel camera designed forthe incoming Dark Energy Survey (DES). It is consist of sixty two 4k$\times$2kand twelve 2k x 2k 250-micron thick fully-depleted CCDs, with a focal plane of44 cm in diameter and a field of view of 2.2 square degree. It will be attachedto the Blanco 4-meter telescope at CTIO. The DES will cover 5000 square-degreesof the southern galactic cap in 5 color bands (g, r, i, z, Y) in 5 yearsstarting from 2011. To achieve the science goal of constraining the Dark Energy evolution,stringent requirements are laid down for the design of DECam. Among them, theflatness of the focal plane needs to be controlled within a 60-micron envelopein order to achieve the specified PSF variation limit. It is very challengingto measure the flatness of the focal plane to such precision when it is placedin a high vacuum dewar at 173 K. We developed two image based techniques tomeasure the flatness of the focal plane. By imaging a regular grid of dots onthe focal plane, the CCD offset along the optical axis is converted to thevariation the grid spacings at different positions on the focal plane. Afterextracting the patterns and comparing the change in spacings, we can measurethe flatness to high precision. In method 1, the regular dots are kept in highsub micron precision and cover the whole focal plane. In method 2, no highprecision for the grid is required. Instead, we use a precise XY stage movesthe pattern across the whole focal plane and comparing the variations of thespacing when it is imaged by different CCDs. Simulation and real measurementsshow that the two methods work very well for our purpose, and are in goodagreement with the direct optical measurements.