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

Since the launch of the Hubble Space Telescope (HST) the secondary mirror of the telescope has been moved several times in order to collimate the telescope and also to define a position of best focus. In addition to these moves the focus position changes over time because of water desorption by the graphite epoxy in the metering truss. We report here the focus history of the telescope based on a knowledge of the mirror moves made and an analysis of desorption monitoring data obtained by the faint object camera (FOC) in the f/96 mode and of the routine calibration data obtained by the Wide Field and Planetary Cameras. Focus values were extracted using two different methods. In the first method the distance between the center of the point-spread function (PSF) and the shadows of the pads supporting the HST primary mirror are related to the focus error. In the second method an analytical formula for the PSF with variable aberration coefficients is fitted to the data. Focus positions derived from the two methods show good agreement. The data 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 trend of the earlier data. Long-term variations of the secondary-mirror position of approximately 3-15 mum from the ''best'' focus position have been observed. Variations of the order of 2-5 mum over an orbital period have also been noted. Focus changes resulting from secondary-mirror movements greater than approximately 5 mum change the point-spread function significantly and make deconvolution and quantitative measurements difficult.