LWS Glossary 

  • Dark Current (DC) measurement - A DC measurement is preformed at the beginning and at the end of each AOT (there can be more for Fabry-Perot AOTs). The interchange wheel (refer to the LWS manual) is set to Fabry-Perot position and the etalon is misaligned; in this condition, apart from a 'straylight' contribution which has been yet firmly estimated, no flux should be collected by the detectors and hence their signal provides a measure of the DC. The duration of this measure has changed during the mission, along with the internal Illuminator Flash Sequence (sometimes denoted as 'Stimulated Flashes').

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  • Grating Position (often referred to as LVDT) - A number ranging from 0 to 4095. It is the readout of the encoder (Linear Voltage Displacement Transducer) giving the inclination angle of the grating; this value is directly related to the input-beam^gratin angle which on turn translates into wavelength (via a calibration file).

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  • Illuminator Flashes - There are 5 resistors (Illuminators) placed in front of the 10 LWS detectors (indicated by the numbers from 1 to 5 in the figure below; refer to the LWS user manual): they are switched on ('flashed') in sequence and signal from the 10 detectors is collected. There is no optics feeding flux from these devices onto the detectors so their absolute flux cannot be measured: they are used for relative calibration purposes only.

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    During an AOT a flash sequence is performed both at the beginning and at the end of the observation. Given that the overall LWS flux calibration is referred to Uranus as primary calibrator and that Uranus cannot obviously be observed during each AOT, the illuminator flash sequences performed in the user's AOT are directly ratioed to an analogous Reference flash sequence done when Uranus was observed: a set of 10 Absolute Responsivity Correction (ARC) factors (one per detector) is hence derived to be applied to the data in the reduction process. Flash sequence has been changed during ISO mission as more knowledge was being acquired about detector behaviour. In the OLD Scheme (Launch - Rev. ???) the 5 illuminators were switched-on in sequence at two power levels (low and high); after realizing that the 'high' power flash was perturbing detectors for the following tens of seconds, a NEW scheme was adopted (Rev. ??? on) in which the 5 illuminators are switched on in 1 single sequence at a power level which is intermediate with respect to the 2 power levels used in the OLD scheme.


     

  • ITK (Instrument Time Key) - This is a measure of time in units that can be converted into seconds dividing it by 214 . ITK is unique throughout the revolution, i.e. it is reset to 0 at the beginning of each ISO revolution.

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  • OLP (Off Line Processing) - Otherwise referred to as 'The Pipeline', it is the automatic reduction process (the user may refer to the ISO Data User Manual) that produces the calibrated spectrum (LSAN or SWAA files for LWS and SWS respectively).

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  • Plot Window - In all ISAP and few LIA applications the plot area allows interactive use with the cursor, featuring the following options:
  • Relative Spectral Response Function (RSRF) - It is the instrumental transmission profile; it consists of a set of 10 curves (1 per detector) which provide the conversion table from Photocurrent (Amps) to Flux (W/cm2/um):


  • This set of curbes is extracted from the calibration files available under ISAP Version 1.6a
     
  • Time Responsivity Drift - The responsivity of the 10 LWS detectors is known to increase with time within each revolution, although it is believed to stay nearly costant on a revolution by revolution basis. The plot below shows the photocurrent as a function of time (ITK) for detector SW4 from the start to the end of a revolution:

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