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

The 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.

TESS 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.

The 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.

A 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).

For 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.

With 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.