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IRTF to Get New Secondary Mirror

Diagram of a Cassegrain telescope

The IRTF is a Cassegrain-type telescope. Light initially hits the primary mirror, then is reflected to a convex secondary mirror, which then directs the light to a focus through a hole in the primary mirror. Instruments are mounted at the focus.

Eric Tollestrup, the deputy director of the NASA Infrared Telescope Facility (IRTF), has received a grant from the National Science Foundation to measure the spherical aberration of the IRTF's primary mirror and to fabricate a new secondary mirror that removes the spherical aberration and other aberrations.

The first stage of this project—measuring the aberrations—was completed in spring 2007. The information obtained from this first stage is now being analyzed to derive specifications for the new secondary mirror, which should be installed on the IRTF before the end of 2008. The new secondary mirror will compensate for imperfections in the primary mirror in the way eyeglasses compensate for poor vision, resulting in significantly better image quality.

Spherical aberration is the inability of the mirror to focus all of the incoming light at a single point. According to Tollestrup, "Spherical aberration is a very common problem with telescopes and is usually due to the figure being 'underparabolized'." Telescope primary mirrors are ground to a shape that is part of a sphere first. Then they are ground to their final shape, which in the case of the IRTF should have been deeper in the middle, and higher and steeper on the edges—a paraboloid shape. Tollestrup notes, however, that "at the time the IRTF was delivered, it was made to specification and considered to be as good as the 4-meter telescopes at Kitt Peak (in Arizona) and Cerro Tololo (in Chile)."

The new secondary should also fix other kinds of aberrations in the primary mirror. The process of polishing the mirror introduced "zonal polishing rings" (rings of high and low spots), and "support-pad print-through" caused by imperfect support of the mirror while it was being polished on the polishing table. Tollestrup says, "These aberrations have been known to exist since delivery, but once again were not considered to be out of spec. We hope to minimize these aberrations with the new secondary."

Then there are aberrations that a new secondary mirror will not fix because they are due to poor mirror support in the IRTF mirror cell. With proper maintenance, these are minimized and, Tollestrup says, "A future project will address these."

The Hubble Space Telescope is probably the most famous case of a telescope mirror having spherical aberrations. Shortly after HST became operational in 1990, scientists noticed its images were not as clear as expected. An inquiry revealed a spherical aberration in the primary mirror resulting from a miscalibrated measuring instrument had caused the edges of the mirror to be ground slightly too flatly—unlike the IRTF primary mirror, it had not been ground to meet its specifications. During the first servicing mission in December 1993, astronauts replaced the Wide Field/ Planetary Camera with the Wide Field/Planetary Camera 2, which was designed to compensate for the mirror problem, and installed COSTAR (Corrective Optics Space Telescope Axial Replacement), a set of optics that compensated for the aberration and allowed all of Hubble's instruments to function normally.