Expected performance

The efficiency () of the FTS can be calculated as follows:

 

2|l|Element reflectivity comments

A Beamsplitter 0.45 ( Douglas 1992 -- private communication)

B Mirror coating >0.9 TecOpics -101

C Detector QE >0.35 (Schaefer et al. 1990, Delamere et al. 1990)

= (2A)² B C

> 16%

In the case of bandwidth-limited observations the signal-to-noise ratio can be expressed in the form:   where m is the modulation depth(typically 60%), F_p the flux at the spectral region of interest (photons/ s / unit wavelength), is the spectral resolution, the scan time, and F_t the total flux seen by the detector (photons/s). There is thus a compromise to be reached between time to achieve a given signal-to-noise ratio in the spectrum, and spectral coverage. This will depend in detail on the on the observation strategy, and should be considered during the assessment study.

An imaging UV FTS is an ambitious instrument, but the tremendous scientific reward from such an instrument is worth the effort of further study during the assessment phase. However, if a more detailed study should prove such an instrument unfeasible, we recommend that both a broad band UV imaging photometer, and imaging line photometers be present to regain some of the atmospheric information.