Corrective Optics Space Telescope Axial Replacement

Optical correction instrument

The Corrective Optics Space Telescope Axial Replacement (COSTAR) is an optical correction instrument designed and built by NASA. It was created to correct the spherical aberration of the Hubble Space Telescopes primary mirror, which incorrectly focused light upon the Faint Object Camera (FOC), Faint Object Spectrograph (FOS), and Goddard High Resolution Spectrograph (GHRS) instruments. It was flown via shuttle to the telescope in the servicing mission STS 61, on December 2, 1993, and successfully installed over a period of eleven days.

Once it had been identified in 1990 that the primary mirror in the recently launched Hubble Space Telescope (HST) was defective due to it having been ground to the wrong shape, engineers at NASA came under immense pressure to fix the problem. The incorrect shape of the mirror introduced severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its center. The effect of the flaw on scientific observations depended on the particular observation—the core of the aberrated point spread function was sharp enough to permit high resolution observations of bright objects, and spectroscopy of point sources was affected only through a sensitivity loss. However, the loss of light to the large, out of focus halo severely reduced the usefulness of the telescope for faint objects or high contrast imaging. This meant nearly all the cosmological programs were essentially impossible, since they required observation of exceptionally faint objects.

When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WFPC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS). Since it was too difficult to bring the HST back to earth for repairs the engineers considered everything from replacing the telescope's secondary mirror by sending a spacewalking astronaut into the telescope's optical tube, to installing a circular shade around the opening of the tube, which would reduce the aperture and improving the focus by blocking out the outer regions of the primary mirror. It was eventually determined that with the HST still in orbit that they could replace the WFPC with the improved Wide Field and Planetary Camera 2 which would incorporate corrective optics. A crisis meeting of the European Space Agency was held in Germany to discuss the issues with the HST. Among the attendees was James H. Crocker, a senior optical engineer at the Ball Aerospace Corporation. While taking a shower in his German hotel one morning he noticed that the showerhead travelled on a vertical rod and it could be clamped on the rod at different heights and angles. The maid had left the showerhead at the base of the rod and positioned flat against the wall, which meant it was taking up very little space until Crocker loosened the clamp and moved it to the position he wanted. The idea came to him that they could mount the required corrective components on such a device that would allow them to be inserted into the tube before being folded out on robotic arms to the required position to intercept the beams of light from the secondary mirror, correct and then focus them on the various scientific instruments. 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, which was the least important of the four axial detectors. A total of ten correction mirrors with diameters ranging in size from approximately 18 to 24 mm were used as the Faint Object Camera and Faint Object Spectrograph each had two apertures for each of their two measuring channels whereas the Goddard High Resolution Spectrograph only had one aperture for both of its channels. The original WFPC was replaced by the WFPC 2 during the same mission.