Paul N. Robb

Selection Of Optical Glasses

A new method of selecting glass combinations for the correction of paraxial chromatic aberration in optical systems has been developed. This new method corrects axial color for at least three wavelengths using two different types of glass and at least four wavelengths using three different types of glass. Certain combinations can be found that are corrected for as many as five wavelengths, using either two or three different types of glass. The Abbe number and relative partial dispersions are not utilized in this new approach; instead, the dispersion of the glass is characterized using Buchdahls chromatic coordinate. The Buchdahl model and the mathematical methods employed are described briefly. Thick air-spaced doublet and triplet lenses are presented as examples of designs that demonstrate color correction at three, four, and five wavelengths and are corrected for spherical aberration and spherochromatism as well. A more complex lens design is illustrated by a Petzval objective with a 24-in. focal length, a focal ratio of f/3.5, and a 12-deg field, which is corrected for both focal shift and spherochromatism and is diffraction-limited over the spectral region from 0.44 to 0.9 μm.

2.7-meter-diameter silicon carbide primary mirror for the SOFIA telescope

The Stratospheric Observatory for Infrared Astronomy (SOFIA) will be a 2.5-m clear-aperture telescope mounted in an open cavity in a modified Boeing 747 SP aircraft. SOFIA represents the next generation of the NASA Ames Research Centers infrared astronomy program. The existing airborne infrared telescope, the Kuiper Airborne Observatory (KAO), is a 0.91-m-aperture telescope flown on a Lockheed C-141 aircraft. The SOFIA telescope will have approximately eight times the sensitivity and three times the resolution of the KAO, and will be able to detect all of the far-infrared point sources detected by the Infrared Astronomical Satellite in 1983. A number of studies have been performed on the design of a large-aperture telescope capable of operating in the environment of an aircraft flying at 41,000 ft at Mach 0.85 while looking at astronomical sources through an open port. SOFIA poses a number of serious technical challenges for both the telescope designer and the system designer. This paper addresses one of these challenges, namely, the design of the telescopes primary mirror. Using new Russian technology will permit the fabrication of a lightweight, 2.7-m-diameter, f/1.3, primary mirror made of silicon carbide. The mirror and its graphite-aluminum mount will weight 650 kg, will not require any kind of active figure control or gravity sag compensation, will have a thermal time constant less than any other material, and will meet or exceed all of the requirements for the SOFIA mission.

Three-mirror anastigmatic telescope with a 60-cm aperture diameter and mirrors made of silicon carbide

This paper describes a telescope with an aperture diameter of 60 cm, for which the mirrors and mirror mounts are being fabricated. The telescope is a three-mirror anastigmat with an offset field and consists of three powered aspheric mirrors made of silicon carbide and two folding flats made of silicon. The mirrors and mirror mounts are being fabricated at the Vavilov State Optical Institute in St. Petersburg, Russia, as part of a collaborative program with the Lockheed Martin Palo Alto Research Laboratories. The designs of the mirrors and mounts, and the results of interferometric tests of the mirrors, are discussed.

Interferometric measurements of silicon carbide mirrors at liquid helium temperature

This paper presents the results of interferometric tests of two silicon carbide mirrors tested at room temperature and 6 K. The first mirror has a spherical f/1.73 surface, a diameter of 170 mm, and is of solid, plano-concave construction. The other mirror, a plano measuring 308 mm by 210 mm, is of lightweighted, closed-back construction. The mirrors were manufactured by the Vavilov State Optical Institute, St. Petersburg, Russia, and were loaned to Lockheed for these tests. Optical tests on both mirrors were performed using the Lockheed cryogenic optical test facility at liquid helium temperature and a Zygo Mark II interferometer. There was no change in the surface figure of the mirrors, within the test uncertainty of approximately plus or minus 0.02 waves at 0.6328-micrometer wavelength.

Refractive indices of liquids in the infrared spectral region

Liquids have not found widespread application in optical design for a number of reasons, one of which is the fact that index of refraction data for liquids is available for only very few compounds, and the data that do exist are only in the visible spectral region. In this paper we report on the results of our joint project for measuring the refractive index of liquids in the infrared (IR) spectral region. A number of liquids have transmission passbands that extend well into the IR, to wavelengths of 10 micrometers and beyond. Many of these liquids also have abnormal dispersion and are suitable for low-cost apochromatic lens designs in the infrared. Using these materials, design options that significantly reduce the cost and weight of IR imaging systems become available. These measurements have been made at the Vavilov State Optical Institute, Saint Petersburg, Russia, in collaboration with the Lockheed Palo Alto Research Laboratory.

Stability of the optical properties of abnormally dispersive liquids

Abnormal dispersion is found in a large number of optical liquids. These liquids are quite useful in the design of apochromatic lens systems because they are inexpensive and are readily available compared to the expensive abnormal dispersion glasses and crystals more commonly used. Further, there is no practical upper limit on the size of a liquid optical element. However, because the use of these materials as optical elements is quite new, the long term stability of their optical properties after exposure to various environmental conditions must be determined. In this paper, we report on a series of tests performed to determine the optical stability of several abnormal dispersion liquids. The liquids were subjected to: (1) high intensity ultraviolet radiation, (2) high temperatures, and (3) long term exposure to air. Measurements of both optical transmission and refractive index were performed before and after each exposure. The tests were performed on four abnormal dispersion liquids that have been found to be quite useful in the design of apochromatic lenses. The results indicate that long term exposure to either intense ultraviolet radiation or air, produced only negligible changes in the index of refraction. Changes in optical transmission were observed, but they were largely confined to the ultraviolet spectral region. Prolonged exposure to temperatures of 60 degree(s)C did not produce any measurable change in the optical properties. The measurements were performed at the Vavilov State Optical Institute, St. Petersburg, Russia, in collaboration with Lockheed Palo Alto Research Laboratory, Palo Alto, California.

Correlation of chemical structure and refractive index dispersion of abnormal optical liquids

It has been demonstrated recently that high performance apochromatic lenses and objectives can be designed using optical liquids that have abnormal dispersion properties. This has opened the way to low cost apochromatic objectives with optical quality equivalent to lenses using Calcium Fluoride and which are superior in other ways as well (reduced scattering, for example). This paper describes a study on correlating the refractive index dispersion of optical liquids to their chemical structures. The ultimate goal is to establish an understanding of how different chemical moieties affect optical properties of organic liquids.

Influence of chemical composition on the refractive index thermal coefficient of liquids

Liquids have been shown to be very effective in the design of apochromatic lenses. The incorporation of abnormal liquids is complicated by their large thermal coefficients of refractive index (dn/dT). To make a glass-liquid design thermally stable, two liquids with different dn/dT and dispersions are typically used. This paper, which extends previously reported investigations of liquid dn/dT dependencies in the visible spectrum (SPIE Volume 2512, 1995), explores the dependence of dn/DT, abnormal dispersion, and composition for a number of liquids. The optical property measurements were made at the Vavilov State Optical Institute, St. Petersburg, Russia, in collaboration with the Lockheed Martin Advanced Technology Center, Palo Alto, California.

Characterization of abnormal dispersion liquids in the ultraviolet spectral region

Abnormal dispersive liquids have proven quite useful in the design of apochromatic optical systems for the visible and ultraviolet spectral regions. Furthermore, liquids with extended ultraviolet transmission can be very useful in the design of optical systems for use in the ultraviolet, whether or not they are abnormally dispersive. The results of investigations into optical properties of several liquids transparent in the ultraviolet are presented. Intense UV irradiation of the liquids was found to produce changes in both the UV transmission and refractive index. Methods of preventing these changes through the addition of a special chemical were investigated. The optical property measurements were made at the Vavilov State Optical Institute, St. Petersburg, Russia in collaboration with the Lockheed Palo Alto Research Laboratory, Palo Alto, California.

Interferometric measurements of silicon carbide and aluminum mirrors at liquid helium temperature

This paper extends the previously reported results of cryogenic optical testing (SPIE Volume 2543, 1995) by including the results of further reduction of the test data for the 170-mm-diameter silicon carbide mirror and the 178-mm- diameter aluminum mirror. Both mirrors were manufactured by the Vavilov State Optical Institute, St. Petersburg, Russia, for infrared applications and were loaned to LMMS for these tests. Optical tests were performed in the Lockheed Martin cryogenic optical test facility at liquid helium temperatures, using a Zygo Mark II interferometer. The initial surface figures were 0.18 waves and 0.08 waves for the aluminum and the SiC mirrors, respectively, with figure error being given as rms wavefront error at 0.6328-micron wavelength at room temperature. It was found that the maximum change in shape after cooling was between 0.007 and 0.036 waves for the SiC mirror and between 0.017 and 0.062 waves for the aluminum mirror.