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Extreme Ultraviolet Deep-Sky Survey

NSSDC ID: 1992-031A-02
Mission Name: EUVE
Principal Investigator: Dr. C. Stuart Bowyer

Description

The EUVE science archive contains data products obtained from the EUVE Spectrometer. The following is a description of the spectrometer, its associated optics and detector.

The EUVE Spectrometer ^^^^^^^^^^^^^^^^^^^^^

The Spectrometer is a three-fold symmetric slitless objective design based on variable line space grazing incidence reflection gratings. Photon images are accumulated simultaneously in three bandpasses with effective spectral resolutions of 200-400 in 3 bandpasses from 70-760 A. The Spectrometer and Deep Survey instruments share the DS/S mirror. The regions of the mirror devoted to the Spectrometer and Deep Survey are defined at the front aperture, which is an annulus divided into six segments. Each of the Spectrometer channels receives a beam of light from one of three alternating segments. This division gives each channel a geometric area of 75 cm^2. After the mirror, each converging beam then strikes one of three gratings which focus the spectra onto three detectors, arranged in a circle around the central Deep Survey detector.

The throughput of the EUVE Spectrometer is determined by the combined effects of the mirrors' and gratings' coating reflectivities, which are functions of both wavelength and grazing angle, the filter transmissions, and the quantum efficiency functions of the detector photocathode materials.

Collimators and Sky Background ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In order to achieve good spectral resolution, any EUV spectrometer must be designed to limit the effect of diffuse sky radiation. The meduim and long wavelength channels of the EUVE Spectrometer have wire-grid collimators placed directly after the aperture before the mirror, which limit the grazing angles of the incident light to exclude some of the sky background. They consist of 15 etched molybdenum grids, spaced exponentially and held in a thermally stable claw structure, also of molybdenum. The transmission profile of the stack is triangular in the dispersion direction, and limits the beam to 20 arc minutes FWHM.

The transmission of each collimator assembly was tested in visible light. The collimator relative transmissions were measured in the EUV by comparing the Spectrometer throughputs, measured as a function of off-axis angle, before and after installation of the collimators in the medium and long wavelength channels. Alignment to the boresight of the instrument was also determined. Both collimators function as designed, with peak transmissions of 64.2% and 65.4% in the medium and long wavelength channels, respectively.

Variable Line Space Gratings ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The EUVE Spectrometer incorporates plane diffraction gratings with continuously varying line spacing, placed in the converging beam of the telescope to diffract the light as it approaches the focus. Like concave gratings, they obviate the use of other focusing optics after dispersion. Unlike uniformly spaced rulings, variable line space gratings can produce nearly stigmatic spectra using straight, conventionally ruled grooves.

The gratings are blazed for use in the first inside order. "Inside" will be used to mean diffracted orders at angles between the surface normal and the specular direction, and will be referred to with a minus sign when represented numerically, e.g. -1st order. The gratings cover three overlapping bandpasses; short wavelengths from 70 to 190 A, medium wavelengths from 140 to 380 A, and long wavelengths from 280 to 760 A. The groove densities range from 415 to 3550 grooves/mm. The gratings were ruled by Hitachi, Inc. at the Naka optical works in Japan. The short wavelength grating is coated with rhodium to optimize the reflectivity between 70 and 190 A. The medium and long wavelength gratings have platinum surface coatings.

Spectrometer Filters ^^^^^^^^^^^^^^^^^^^^ Thin film filters, a few thousand A thick, completely cover each detector. They define broad bandpasses while screening out bright geocoronal and interplanetary lines such as Lyman alpha radiation and some higher orders of diffraction. The materials are Lexan and boron in the short wavelength, aluminum and carbon in the medium, and aluminum in the long wavelength channel.

The two longer wavelength filters have an off-axis quadrant(1) of material which covers the same bandpass as one of the shorter channels. At these positions, which correspond to off-axis angles of approximately 0.5 deg, some wavelengths that would normally lie in the shorter channel's range appear in the longer wavelength channel in second order (n = -2), and are passed by the alternate filter. Wavelengths from parts of the shorter bandpass that overlap the longer channel also appear in first order. These off-axis locations are configured to be used as backups to duplicate the short and medium channels, should either of these detectors fail.

Microchannel Plate Detectors ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ All the EUVE detectors are Microchannel plate (MCP) detectors. MCP detectors are electron-amplification devices that provide two-dimensional imaging and time-tagging of individual EUV photon events.

Each detector employs a biased stack of three porous quartz MCP's with a channel length-to-diameter ratio of approximately 80:1. The stack acts as an electron multiplier, and is backed by a conducting anode, partitioned into a graduated "wedge, strip, and zigzag" pattern. The top plate has an applied photocathode of potassium bromide (KBr), to enhance the photoelectric response at EUV wavelengths. When a photon excites the front surface, a bias of 4-5 kV causes cascading electrons to form a cloud of 2-3x10E7 electrons which then strikes the divided anode.

Event positions (X,Y) are calculated by on-board instrument software (ISW) from the division of the charge cloud among the wedge, strip, and zigzag areas of the anode. The detectors record positions 0-2047 in each dimension, and a single pixel is about 29x29 mc. This results in a pixel size of roughly 4.25 sec when remapped to the sky.

All the detectors are equipped with four stimpulser, or "stim" pins, which periodically excite the anode at standard positions, and are used to monitor position stability. The detectors have been placed at the sagittal intersection to produce good imaging over the whole detector, rather than optimized spectral focus at one point.

Facts in Brief

Mass: 3300.0 kg
Power (avg): 1100.0 W
Bit rate (avg): 0.016 bps

Funding Agency

  • NASA-Office of Space Science (United States)

Disciplines

  • Astronomy: Ultraviolet
  • Solar Physics: Ultraviolet

Additional Information

Questions or comments about this experiment can be directed to: Coordinated Request and User Support Office.

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. C. Stuart BowyerPrincipal InvestigatorUniversity of California, Berkeleybowyer@ssl.berkeley.edu
Dr. Michael L. LamptonOther InvestigatorUniversity of California, Berkeleymlampton@ssl.berkeley.edu
Dr. Roger F. MalinaOther InvestigatorUniversity of California, Berkeleyrmalina@ssl.berkeley.edu
Mr. Henry D. HeetderksOther InvestigatorUniversity of California, Berkeleyhdh@ssl.berkeley.edu
Dr. D. LangleyOther InvestigatorUniversity of California, Berkeley 

Selected References

EUVE guest observer program handbook, EUVE Guest Observer Center, NASA NRA 92-OSSA-5, Appendix G, Jan. 1992.

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