Hubble Space Telescope

The Hubble Space Telescope is a telescope in orbit around the Earth. Its position outside the Earth's atmosphere allows it to take extremely sharp images, and since its launch in 1990, it has become one of the most important telescopes in the history of astronomy. It has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics.

Flawed mirror

Within weeks of the launch of the telescope, the images returned showed that there was a serious problem with the optical system. Although the first images appeared to be sharper than ground-based images, the telescope failed to achieve a final sharp focus, and the best image quality obtained was drastically lower than expected. Images of point sources spread out over a radius of more than one arcsecond, instead of having a point spread function concentrated within a circle 0.1 arcsec in diameter as had been specified in the design criteria {{ref|Burrows}}.

Related Topics:
Point source - Point spread function

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Analysis of the flawed images showed that the cause of the problem must be that the primary mirror had been ground to the wrong shape. Although it was probably the most accurately figured mirror ever made, with variations from the prescribed curve of no more than 1/20 of the wavelength of light, it was too flat at the edges. The mirror was barely 2 micrometres out from the required shape, but the difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edges of a mirror reaches a different focus to the light reflecting off the centre. The aberration meant that images from the Space Telescope were only marginally better than the best images obtainable from the ground.

Related Topics:
Wavelength - Micrometre - Spherical aberration - Focus

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Origin of the problem

Working backwards from images of point sources, astronomers determined that the conic constant of the mirror was −1.0139, instead of the intended −1.00229. The same number was also derived by analysing the null correctors (instruments which accurately measure the curvature of a polished surface) used by Perkin-Elmer to figure the mirror, as well as by analysing interferograms obtained during ground testing of the mirror.

Related Topics:
Conic constant - Null corrector - Interferogram

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A commission was established to determine how the error could have arisen and was headed by Lew Allen, director of the Jet Propulsion Laboratory. The Allen Commission found that the null corrector used by Perkin-Elmer had been incorrectly calibrated, as a spot on a metering scale where an end cap had worn away was wrongly believed to be a valid scale. The null corrector had then been wrongly spaced by 1.3 mm.

Related Topics:
Lew Allen - Jet Propulsion Laboratory

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During the polishing of the mirror, Perkin-Elmer had analysed its surface with two other null correctors, both of which (correctly) indicated that the mirror was suffering from spherical aberration. These tests were specifically designed to eliminate the possibility of major optical aberrations. Against written quality guidelines the company ignored these test results as it believed that the two null correctors were less accurate than the primary device which was reporting that the mirror was perfectly figured.

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The commission blamed the failings primarily on Perkin-Elmer. Relations between NASA and the optics company had been severely strained during the telescope construction due to frequent schedule slippage and cost overruns. NASA found that Perkin-Elmer had not regarded the telescope mirror as a crucial part of their business and were also secure in the knowledge that NASA could not take its business elsewhere once the polishing had begun. While the commission heavily criticised Perkin-Elmer for these managerial failings, NASA was also criticised for not picking up on the quality control shortcomings such as relying totally on test results from a single instrument. {{ref|selecteddocs}}

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Design of a solution

The flaw meant that Hubble could obtain data about as good as that achievable with a large ground-based telescope on a night of good seeing, but at a vastly greater cost. NASA and the telescope became the butt of many jokes, and the project was popularly regarded as a white elephant. However, the design of the telescope had always incorporated servicing missions, and astronomers immediately began to seek potential solutions to the problem which could be applied at the first servicing mission, scheduled for 1993.

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While Kodak had ground a back-up mirror for Hubble, it would have been impossible to replace the mirror in orbit, or bring the telescope temporarily back to Earth for a refit. Instead, the fact that the mirror had been ground so precisely to the wrong shape led to the design of new optical components with exactly the same error but in the opposite sense, to be added to the telescope at the servicing mission, effectively acting as 'spectacles' to correct the spherical aberration.

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Because of the way the instruments were designed, two different sets of correctors were required. The design of the Wide Field and Planetary Camera (WFPC) included four relay mirrors to direct light onto the four separate charge-coupled device (CCD) chips making up the camera, and so the relay mirrors on the replacement Wide Field and Planetary Camera 2 could be figured to correct the aberration. However, the other instruments lacked any intermediate surfaces which could be figured in this way, and so required an external correction device.

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COSTAR

The system designed to correct the spherical aberration for light focussed at the FOC, FOS and GHRS was called the "Corrective Optics Space Telescope Axial Replacement" (COSTAR) and consisted essentially of two mirrors in the light path, one of which would be figured to correct the aberration {{ref|Jedrzejewski}}. To fit the COSTAR system onto the telescope, one of the other instruments had to be removed, and astronomers selected the High Speed Photometer to be sacrificed.

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During the first three years of the Hubble mission, before the optical corrections could be fitted, the telescope still carried out a large number of observations. Spectroscopic observations in particular were not too badly affected by the aberration, but many imaging projects were cancelled as the space telescope no longer gave decisive advantages over ground-based observations. Despite the setbacks, the first three years saw numerous scientific advances as astronomers worked to optimise the results obtained using sophisticated image processing techniques.

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