Optical System

The Ultraviolet Imager Optical system uses a high speed (f/2.9) three mirror off axis design resulting in a circular 8 degree full field of view. This design provides an unobscured optical aperture, excellent baffling, flat filed, provision for optical filter insertion and general compactness. Each of the three mirrors are portion of a conic section and all share a common optical axis. The mirrors made of aluminum which underwent an extensive thermal cycling, plating, machining, and polishing process. They are overcoated with a far ultraviolet reflective MgF2/Al coating. In order to minimize scattering, very smooth surfaces are required. The surface roughness achieved on these mirrors is less than 20 Angstroms rms. Internal baffling is incorporated to further reduce stray light scattering. The image surface is essentially flat over the 8 degrees diameter of the filed of view (an 18 mm diameter image plane).

For more information on the optical design of the UVI see the following paper:

• Johnson, R. B., 'Wide Field of View Three-Mirror Telescopes Having a Common Optical Axis', Opt. Eng., vol. 27, p. 1046, (1988).

• The optical design for the UVI was conceived by Barry Johnson. Refinements in the optical design were made by Drs. M. Krim and A. Nonemacher of Hughes Danbury. The final optical design was done by Dr. Chen Feng while at the Center for Applied Optics fo the the University of Alabama in Huntsville.

Photographs

• 3-D CAD rendering of the UVI optical design (22K)
• The UVI Primary/Secondary mirror housing and internal baffling assembly (47K)
• A view of the UVI optical bench side (195K)
• A view of the UVI optical bench top (131K)

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Filter Design

Each of the five UVI filters are comprised of three reflective and one transmissive element. Each element is a dielectric multilayer stack. The design of the filters results in a significant reduction of the visible light passing through while maintaining a high throughput for the wavelengths of interest. The wavelengths short of the region of interest are primarily absorbed by the materials in the dielectric stack and/or the transmissive element. The wavelengths long of the region of interest are primarily absorbed by the substrate on the reflective elements. For more information on the filter design, refer to the following paper:

• Muamer Zukic, Douglas G. Torr, Jongmin Kim, James F. Spann, and Marsha R. Torr, Filters for the International Solar Terrestrial Physics Mission far-ultraviolet imager, Opt. Eng., 32, 3069, 1993.

A significant effort was put forth in evaluating materials to be used in the filters. Selection of materials is difficult in the wavelength region from 115 nm to 200 nm because of the limited number of materials which have are transmitting (small extinction coefficient). Of those materials that are transmitting, many are susceptible to radiation effects and become less transparent when exposed. We conducted tests to determine which materials are suitable for a radiation environment and provide a proper high or low index of refraction required for the multilayer design. For more information on the materials, refer the following three papers:

• M. Zukic, D. G. Torr, J. F. Spann, and M. R. Torr, Vacuum Ultraviolet Thin Films, Part I Optical Constants of BaF2, CaF2, LaF3, MgF2, Al2O2, HfO2, and SiO2, Appl. Opt., 29, 28, 4284, 1990.
• M. Zukic, D. G. Torr, J. F. Spann, and M. R. Torr, Vacuum Ultraviolet Thin Films, Part II Vacuum Ultraviolet All-Dielectric Narrowband Filters, Appl. Opt., 29, 28, 4293, 1990.
• Charles E. Keffer, Marsha R. Torr, Muamer Zukic, James F. Spann, Douglas G. Torr, and Jongmin Kim, Radiation Damage Effects in Far Ultraviolet Filters and Substrates, Appl. Opt., 33, 6041, 1994.
  

The pioneering effort of the filter design was headed by Dr. Muamer Zukic while at the Center for Applied Optics at the University of Alabama in Huntsville. He is currently a Research Scientist at Jaycor, Huntsville, AL, phone: (256) 837-9100. The development of the filters was a joint effort between the University of Alabama in Huntsville and the Marshall Space Flight Center. The key individuals in this effort were Dr. M. Zukic, Dr. J. F. Spann, Dr. Kim, and Dr. Charles Keffer.

Photographs

• The UVI filter box (70K)
• The UVI filter wheel assembly (124K)
• The UVI filterwheel mounted on the optical bench (135K)

• Filter Performance

Because of the unique design of the UVI filters, the required bandwidth and peak throughput is achieved. The filter's performance allows the separation of various auroral emissions for the first time using a 2 dimensional imager. The analysis of various images taken with the filters allows for quantitative assessment of the total energy flux into the auroral regions and the characteristic energy. The peak wavelength and bandwidth of each filter is listed below.

Filter	Peak Wavelength	Bandwidth (includes response of CsI photocathode)	Throughput Plot (filter elements only)
ALL	- - -	- - -	Summary Plot
1304	130.4 nm	40 nm FWHM	1304 filter plot
1356	135.6 nm	50 nm FWHM	1356 filter plot
LBH short	150.0 nm	141 - 158 nm (10% of peak)	LBH short filter plot
LBH long	170.0 nm	164 - 178 nm (10% of peak)	LBH long filter plot
Solar	180.0 nm	176 - 190 nm (10% of peak)	Solar filter plot

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Detector System

The Ultraviolet Imager detector system uses an intensified, passively cooled CCD array. The analog and digital detector boards are located in close proximity to the CCD. The design of the image tube incorporates an opaque CsI photocathode deposited on the input chevron microchannel plates. A fast P31 phosphor is used to reduce decay times. Radiation hardened, Cesium doped fiber optics couple the image tube to the CCD. Correlated double sampling techniques are incorporated in the electronic hardware design to reduce digital to analog noise sources in the data. To reduce pick up noise, the high voltage power supply is located near the image tube. To protect against discharges, the high voltage power supply, the high voltage leads to the image tube and the image tube are entirely potted using Conap EN-11 potting material.

• The key individuals responsible for the design, assembly, and testing of the detectors are Alex Tejada, Hassan Dougani, Mike Clem, Chuck Fellows, Tim Baldridge, and Larry Savage. The manufacturer of the CCD is Thomson CSF. The manufacturer of the image tube is SAIC, San Diego, CA. The manufacturer of the high voltage power supply is K&M.

Photographs

• UVI detector boards, HVPS, and image tube during assembly (89K)

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Mechanisms

The UVI has three mechanisms, all of them located in the camera.

• The filter wheel mechanism's purpose is to position the commanded filter in the optical path. The mechanism uses a dual winding, permanent magnet, mu-metal shielded stepper motor with a harmonic drive output. Discrete encoders to identify filter wheel positions are mounted on the same structure as the motor. These include optical fiducials and micro switches.
• The folding mirror mechanism's purpose is to position a mirror in the optical path so that an image is either incident on the primary detector or reflected onto the secondary detector. A push-push mechanism mounted on the output of a paraffin actuator is used to toggle the spring loaded mirror between two positions.
• The aperture door mechanism's purpose is to open and close the aperture door. A push-push mechanism mounted on the output of a paraffin actuator is used to toggle the spring loaded aperture door between two positions. Optical fiducial encoders and discrete microswitches are used to determine the door's position.

• The mechanisms were designed by Dr. Anees Ahmad of the Center for Applied Optics at the University of Alabama in Huntsville, and George Loughhead. The assembly and testing of the mechanisms was performed by Chuck Fellows and Larry Savage. The manufacturer of the stepper motor is Schaeffer Magnetics, Inc., of Chatsworth, CA. The manufacturer of the paraffin actuators is Starsys Research, Boulder, CO.

Photographs

• The UVI filter wheel motor and primary mirror (83K)
• The UVI folding mirror mechanism assembly (83K)
• The UVI aperture door mechanism assembly (85K)

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