• 4.1 Introduction
• 4.2 Behavior of the Detector Circuit
• 4.3 Grating Characteristics
• 4.4 Fabry-Perot Characteristics

   
4. Instrument Characteristics

   
4.1 Introduction

In this chapter we give some information on the instrument specific characteristics that have influence on the data. These characteristics will be described with calibration parameters that are determined in the Performance Verification phase. Most of these parameters have also been determined in the laboratory during ground based tests.

   
4.2 Behavior of the Detector Circuit

  

Figure 4.1: Schematic diagram of the integrating amplifier circuit used in the LWS detectors.

The LWS uses photoconductive detectors in which the radiative input power P is converted into a photocurrent I. The detectors are used in an integrating amplifier circuit, shown schematically in figure 4.1, whereby the photocurrent is integrated on the gate of an FET. The output voltage V_out(t) of the FET is read non-destructively at a frequency of 88 Hz and the amplifier is periodically re-set by shorting the gate to earth via switch S1. It is clear from the diagram that, as the voltage on the gate of the FET charges, the effective bias V_in(t) changes across the detector, introducing a non-linearity into the response. The original way this was handled is described in appendix A. Because of the weaknesses of this original method, this theoretical approach is no longer used from version 7 of the OLP pipeline. Currently the photocurrents are extracted using the ' method. This method is described in section 6.3.6. This new method is calibrated empirically against signals of known strength.

   
4.3 Grating Characteristics

The grating characteristics are:

Grating rest position.

This is the position of the grating mechanism when no power is applied to the drive coils. The grating is returned to this position at the end of each measurement with the grating in order to minimize the popwer input to the ISO cryostat.

Grating scan range

The set of positions over which the Grating may be moved is limited by the design of the mechanism (the range of the mounting pivot movement). The Grating Scan range defines this set of positions to be those for which the drive current is less than 12 mA.

Grating spectral element

This is the Full Width Half Maximum of the of the instrument spectral response, in the Grating Mode, to an unresolved spectral line. It determines the resolving power of the instrument in medium resolution observations.

   
4.4 Fabry-Perot Characteristics

Fabry-Perot Optimum Offsets

The Fabry-Perots have one fixed and one moving etalon. The movement of the moving etalon is controlled by three coils each driven by a servo-loop. In order to operate correctly the moving etalon has to be parallel to the fixed one to a very high degree of accuracy - this is achieved by applying a fixed offset to the control signals for two of the coils. These fixed offsets have been optimised during Performance Verification. The offsets are constant over the scan range for FPS, however, for FPL (owing to a fault in the wiring harness) they are constantly altered over the scan range to keep the etalons parallel.

Fabry-Perot Spectral Element Size

This is taken as the full width at half maximum of the Airy profile it varies with wavelength over the band of each Fabry-Perot but the resolving power, defined as , is more or less constant.

Fabry-Perot Miniscans

For extended range wavelength scans on the Fabry-Perots (AOT L03), a small range is scanned at each grating setting, these are termed miniscans. The range over which the Fabry-Perot is scanned for each miniscan is limited to keep the spectrum at the peak of the grating resolution element response.

Fabry-Perot Scan Range

The range over which the Fabry-Perot is scanned for L04 is expressed in numbers of resolution elements - the maximum range is 15 resolution elements.