FOCUS HISTORY OF THE HUBBLE-SPACE-TELESCOPE - LAUNCH TO MAY 1993

Since the launch of the Hubble Space Telescope (HST) the secondary mir ror of the telescope has been moved several times in order to collimat e the telescope and also to define a position of best focus. In additi on to these moves the focus position changes over time because of wate r desorption by the graphite epoxy in the metering truss. We report he re the focus history of the telescope based on a knowledge of the mirr or moves made and an analysis of desorption monitoring data obtained b y the faint object camera (FOC) in the f/96 mode and of the routine ca libration data obtained by the Wide Field and Planetary Cameras. Focus values were extracted using two different methods. In the first metho d the distance between the center of the point-spread function (PSF) a nd the shadows of the pads supporting the HST primary mirror are relat ed to the focus error. In the second method an analytical formula for the PSF with variable aberration coefficients is fitted to the data. F ocus positions derived from the two methods show good agreement. The d ata show that a desorption of about 83 mum has taken place since 1990 August 16. The desorption has not leveled off as expected from the tre nd of the earlier data. Long-term variations of the secondary-mirror p osition of approximately 3-15 mum from the ''best'' focus position hav e been observed. Variations of the order of 2-5 mum over an orbital pe riod have also been noted. Focus changes resulting from secondary-mirr or movements greater than approximately 5 mum change the point-spread function significantly and make deconvolution and quantitative measure ments difficult.