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The TESS Instruments:

  • Field of View: 10deg X 10deg with 0.5deg resolution across the Limb
  • Spectral Resolution: 10 angstroms from 800-1400 angstroms
  • Sensitivity: High Efficiency F3 SEIS gives 20 counts per second/rayleigh each
  • Size: Compact (5.9X9.7inches) low mass system for small satellite applications

TESS and PhotometersThe TESS detector is the main detector of the TERRIERS payload, we have 4 of them for operation during the night, and one for operation during the day.

4 Night TESSTESS is a Single element imaging spectrograph (SEIS) designed for both high wavelength and high spatial resolution (in one dimension). It is a one bounce diffractive system which combines the spectral properties of a Rowland mount spectrograph with the imaging (spatial resolution) properties of a Wadsworth through the use of a toroidal diffraction grating. No primary optics are necessary, making the system especially attractive for use in extreme ultraviolet (EUV) where low reflectivity of common optical coatings can severely limit instrument sensitivity.

TESS - side viewThe TESS design is the best compromise between spacial imaging and high sensitivity through the use of the single element imaging spectrograph (SEIS) (A paper is available on the SEIS grating - Adobe Acrobat format). The system consists of five basically identical spectrographs; four identical nightglow instruments (for redundancy and added sensitivity), and one with a smaller slit to reduce sensitivity and increase spectral resolution for daytime operation. The nightglow complement is mounted on a single optical bench (that also forms the interface plate to the spacecraft), while the dayglow instrument mounts opposite the photometers on a second optical bench which is parallel to the nightglow optical bench.

Instrument Parameters / Design Decisions for TESS
FeatureNight(Day) ParameterReasoning
Number4(1)redundancy, high sensitivity
MountSEISlarge field of view, spatial resolution; minor modifications to standard configuration
Slit Size.5(.1)x40(10)mmhigh throughput
Surface FigureToroidalSEIS
Line Density3200/mmallows aperture opening w/o loss of spectral resolution
Blank Size30x75mmsmaller grating is more cost effective
Ruled Area25x65mm
Radii150x290.4mmallows smaller sized instrument
Blaze Wavelength900 angstromsflat response over bandpass
Reflective CoatingSiCstable; high reflectance over bandpass
TypeMCP w/wedge-striprad. hard heritage
PhotocathodeKBrhigh efficiency; flatten wavelength response; visible blind
Format.20x.20mmreliable efficient; long
 256x256 pixelselectronics heritage

Night TESSA summary of the design decisions for TESS are given above. We chose a SEIS, because its imaging capabilities and large field of view offer the highest possible throughput with high spatial resolution (.4 degrees) in a very small configuration (150mm Rowland circle).

Installing the Night TESSFor doing high resolution tomography (20 km resolution) with the expected signals (.5--50 R), it is extremely important to maximize throughput. Even though a single SIS could achieve the specified performance, at a similar weight, power, and cost four smaller independent spectrographs that yield the same performance were baselined to add redundancy and flexibility of operation. Furthermore, this design allowed us to vastly increase the dayside performance by adding a nearly identical dayglow spectrograph for marginal cost.

TESSWith the nightglow sensitivity of 80-100 cps/R over the entire band TESS will be able to achieve a 5 standard deviation detection of a 3 R signal in the nominal 0.083 second integration period (assuming a high 10% background due to scattering and dark noise). Even with one instrument we should be able to achieve some of the science objectives.

26 May 1999
Center for Space Physics
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