FGE Optics

The key high-technology optical elements are the light-weight, ultra-low-expansion primary mirror, the single-crystal silicon secondary, the graphite-epoxy telescope tube, a tunable lithium-niobate optical filter (Rust 1994), and an image motion compensator based on a novel silicon retina (Strohbehn et al. 1993).

The graphite-epoxy telescope structure provides high thermal stability.

The heat absorbed by the silicon secondary mirror during the first flight was removed with a heat pipe. The secondary never got above 37 C, which is amazing, considering that the intensity of the incident solar radiation on it was 60 times normal. At a pressure of 3 mb, even the most pessimistic models predict no air turbulence until the mirror temperature exceeds 80 C. Indeed, the images show no evidence of "seeing." The heat pipe broke during the landing. For the next flight another method will be used to remove the heat from the secondary mirror: heat conductive ribbons will be attached to the back side of the secondary mount and connected to the heat shields protecting the telescope spider arms. The heat will be absorbed by the heat shields and radiated into space.

The first graphic shows the part of the FGE optics before the beam enters the Optical Pressure Vessel (OPV). This graphic displays the optical setup inside the OPV (the beam enters the OPV from behind, perpendicular to the paper plane), and here the pointing telescope optical layout is shown.

Fabry-Perot Etalon

The FGE etalon consists of a 75-mm-diameter wafer of lithium niobate polished to a flatness of about one-hundredth of a wave. When voltage is applied to the indiumtin-oxide coatings on the crystal faces, an electric field is impressed on the etalon cavity that alters the index of refraction of the lithium niobate, which alters the optical path length in direct proportion. Thus, the wavelength passed by the filter is proportional to voltage. On the first flight, the filter had a passband of about 160 mÅ and, in conjunction with the blocking filters, it could be tuned to any of four wavelengths: 6563 Å (Ha), 6122 Å (Ca I), 6302 Å (Fe I), or 6249 Å (continuum).

We are working with the supplier, CSIRO in Australia, to perfect a "universal filter." They have just delivered a new etalon with a full aperture finesse of ~ 27 and a spectral range of 5000 Å to 7000 Å. Such a high finesse translates into a 0.125 Å passband. We will use this filter either singly or in combination with another on the next flight. Two etalons in series yield a 0.1 Å passband. They will require blockers that are ~20 Å bandwidth. The existing 1.5 Å blockers have proven to be quite sensitive to temperature and aging, and the broader filters will solve this problem.

An image obtained with the Fabry-Perot through the Target Selector Telescope in H\alpha during testing in Albuquerque, NM, is available here.