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E-300 Enhanced Collection System

NASA is studying a proposed Next Generation Space Telescope. The NGST is to be a much larger version of the Hubble Space Telescope (8 to ten meters aperture, versus 2.7 meters). Cost of the NGST is to be a fraction of the HST, ~0.5B versus >$1B. The need is therefore for technologies that can increase the collecting area by ~10X while simultaneously reducing the mission cost by a factor of two or more.

Lightweight optics are of crucial importance in weight and cost reduction. The purpose of the primary mirror in a telescope is to collect and focus the light. The function of the rest of the telescope is to support and point the mirror with precision, and to collect the data by CCD. If the mirror can be made lighter, then the rest of the system (mirror cell, tube, mount, etc) become much lighter also. The telescope costs less and becomes easier to move around. The ratio of telescope weight to primary mirror is quite high. It is about 6 for space telescopes. If the mirror weight can be reduced by a factor x, then the overall telescope weight can be reduced by at least 6x. This is an underestimate since the ratio of system mass to mirror area is not a linear function.

There are a number of competing lightweight optic technologies (lightweight glass, beryllium, silicon carbine, etc). The figure of merit used for comparison is areal density, defined as mirror area divided by mirror mass. Using the lightest material, beryllium (which is toxic), the achievable areal density (ratio of collecting area to mirror weight) is about 20 kilograms per meter squared (20 kg/m2). As a reference point, the areal density of the Hubble Space Telescope primary mirror (glass with egg crate support) is about 180 kg/m2.

The traditional way of making mirrors starts with a piece of glass which is ground and polished to shape and coated with a reflecting surface, usually aluminum. The aluminum film reflects and focuses the light, not the glass. The function of the glass is to keep the aluminum in the proper shape. This traditional technique is limited by the need for a minimum substrate thickness that will not distort or allow print-through of support structures.

The replication technique for mirror production uses a mandrel, usually glass, which is first polished to shape. A piece of graphite fiber reinforced composite (GFRC) material is then applied. After curing the GFRC forms a shell which has the shape of the mandrel. The shell is separated from the mandrel and coated with a reflecting surface. In the replication process the GFRC shell is only as thick as it needs to be to maintain optical shape, typically a few millimeters.

Sources and Methods

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Maintained by Robert Sherman
Originally created by John Pike
Updated Thursday, December 23, 1999 9:10:19 AM