The LOI instrument

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3.2.1 Wavelengths and bandpasses

The LOI wavelength and bandpass is chosen to be the same as the 500 nm channel of the SPM. This allows the instruments to be calibrated to each other.

Filter: 500 nm

Bandwidth: 5 nm

Effective Bandpass: The spectral profile can be found on the ftp server: It is an ascii file called LOIAndover.dat with 2 columns (wavelength, transmission).

Effective opacity

/optical depth: To be derived from the bandpass

3.2.2 Optical System and Detector

Telescope: Ritchey-Chrétien with a 1300-mm focal length, diameter 55 mm, central obstruction 25 mm.

Detector: Deep diffused silicon photodiode with 12 scientific and 4 guiding pixels as given in Figure LOI-1. The crosstalk between pixel is smaller than 10-4.

Power on detector: 0.8 mW/cm2

Detector stability

Temperature sensitivity: about 50-100 ppm/K

Aging of temperature sensitivity: drift of -400 ppm/K to be controlled with a ground copy of the flight detector

Filter degradation

Temperature sensitivity: to be measured

UV sensitivity: No degradation after 1 year of solar UV (below 400 nm)

Particle sensitivity: 5% relative transmittance drop after a 7-year SOHO dose.

Surface contamination: temperature of the filter should be about 10 0C higher than the structure, and should prevent condensation of contaminants.

Angular sensitivity: Unknown

Figure LOI-1

3.2.3 Guiding System and diameter measurements

Guider stability and range: 0.1" and +/- 8.5'

Thermal stability : 0.2'/K

Hysteresis: 10%

Voltage sensitivity: 16' for 650 Volts

Aging: unknown

High voltage supply: -150 to 450 Volts

Servo loop aging: unknown

Guiding power Spectrum:

EW axis: 5 "/[[radical]]Hz @ 0 Hz, 3.2 "/[[radical]]Hz @ 9000 uHz, linear dependence in the power spectrum, [[sigma]]=0.4".

NS axis: 3.16 "/[[radical]]Hz @ 0 Hz, 2.23 "/[[radical]]Hz @ 9000 uHz, linear dependence in the power spectrum, [[sigma]]=0.26"

Diameter Measurements: Spectrum rather flat 3.2 "/[[radical]]Hz, [[sigma]]=0.17".

The above power spectra are based on ground-based measurements. They are essentially limited by the atmospheric seeing and its associated transparency changes. These spectra should, hopefully, be better in space.

3.2.4 Noise levels

Electronics stability

VF converters: 1 ppm/K

Instrumental Noise

Short Term (5-min range): <0.5 ppm2/uHz (ground-based measurements, limited by the atmosphere)

Dark current noise: See Fig SPM-I

Long Term: unknown. Most of the noise contribution will be due to the temperature fluctuations on the filter and detector.

3.2.4 Observing sequence

Please refer to the VIRGO Operation manual.

Last updated by Thierry Appourchaux on Thursday December 11, 1997 at 13:14