The Hubble Space Telescope is a Ritchie-Chrétien optical design with a main primary concave mirror followed by a
convex secondary. The focus is determined by the position of each of these two mirrors. The truss containing them is
made of graphite epoxy which has very low thermal expansion. Nevertheless, temperature variations do cause the mirror
separation to vary by several microns within an orbit. Additionally, outgassing of water vapor causes a long-term
shrinkage which soon after launch in 1990 varied by more than 2 microns per month. This necessitated adjusting the
position of the secondary mirror every few months. Currently this rate is greatly reduced and adjustments are needed less
than once per year.
The focus is monitored monthly to continually assess the need for such adjustments. The measurements have been used
to develop models to predict the focus at times between measurements to assist in the analysis of observations. Detailed
focus knowledge is of value in photometry, coronagraphy and image deconvolution. The various focus models that have
been applied so far are described with an evaluation of their performance. Continuing attempts to refine the model will
We describe software which models the Point Spread Function of the James Webb Space Telescope. The software is
designed to be expandable to incorporate optical and instrument data as they become available. An initial model of the
detector used in the Near Infra-red Camera has been used to generate realistic stellar images.
The ACS solar blind channel (SBC) is a photon-counting MAMA detector capable of producing two-dimensional imaging in the UV at wavelengths 1150-1700 Angstroms, with a field of view (FOV) of 31" × 35". We describe the on-orbit performance of the ACS/SBC from an analysis of data obtained from the service mission observatory verification (SMOV) programs. Our summary includes assessment of the point-source image quality and point spread function (PSF) over the SBC FOV, the dark current measurements, the characteristics of the flat fields, fold analysis, throughput, and the UV sensitivity monitor to check for contamination. Where appropriate, a comparison with pre-launch calibration data will also be made.
The off-axis location of the Advanced Camera for Surveys causes strong geometric distortion in all detectors -- the Wide Field Camera (WFC), High Resolution Camera (HRC), and Solar Blind Camera (SBC). Dithered observations of rich star cluster fields are used to calibrate the distortion. We describe the observations obtained, the algorithms used to perform the calibrations and the accuracy achieved. We present our best current calibration of the geometric distortion of each of the detectors.
The Hubble Space Telescope Calibration Database System collects and organizes data used in calibration of the many operating modes of the on-board scientific instruments. During the period form July 1995 to January 1997 the calibration data base system underwent a major redesign. The existing system had performed well since 1990 but some shortcomings were becoming apparent. The advent of two new science instruments, one of which has a very large number of operational modes, promised major complications. The new design operates with far fewer database tables yet provides extra functions. The tracking of replacement files has been improved, the maintenance of documentation has been simplified, and the process for installing data automated and streamlined. Additionally, various scripts have been written to perform checks on currently installed and historic data. This has resulted in a very efficient and reliable installation process which accommodates the new instruments and supports new data formats. It has also allowed us to detect and correct some discrepancies in the existing data that arose from occasional errors in the earlier manual procedures.
Contact scientists for the Hubble Space Telescope examine observation products and report problems and shortcomings in the data. We have developed a database to hold this information and a series of World-Wide Web pages to facilitate reporting directly to this database. Over time, different characteristics of observations have been identified which are essentially signatures of each instrument that cannot be calibrated out. These have been classified as anomalies with which each contact scientist is very familiar. It is part of their task to identify these anomalies and flag them. Depending on the nature and severity of the anomaly, the principal investigator is contacted and the anomaly brought to their attention. To maintain a permanent record and to support others who retrieve data from the archive, we created a database for storing the contact scientists data quality assessments, and a WWW tool to facilitate the assessment process. The tool and database have been in routine use since early December 1996.
We discuss the requirements and design of the calibration data base which supports the processing of Hubble Space Telescope observations. We describe procedures for ensuring that, despite the continuing changes to software and hardware, calibration files to match the instrument configuration for any observation are provided when needed.
The advantages of photon-counting detectors such as the Ranicon over other detectors in space-based astronomy in the area of time-resolved imaging and spectroscopy are discussed. Details of a system to record the positions and absolute arrival times of individual photons are described, with emphasis on the time-tag module and detector electronics and data collection system. The use of a GPS system for accurate absolute timing when synchronizing observations from different observatories is suggested.
A data-collection system for use in astronomical observations with the resistive-anode detectors (Ranicons) described by Paresce et al. (1979 and 1988) is briefly characterized. Analog signals from the Ranicon pass through 12-bit A/D converters and (along with time information from a digital clock) into a 64-bit FIFO buffer, and the photon positions are calculated by a high-speed arithmetic-logic system capable of an event rate of 30 kHz (so that, by Poisson statistics, only about 21 percent of events will be missed). Results from successful timing tests of the system on the 2.2-m telescope at ESO are presented graphically.