Calibrations for the focal plane assembly
Here are shown a collection of calibrations that are all in some way linked to the image and imaging properties in the focal plane
- the shape of the instantaneous field of view (IFOV);
- the optical transfer function
- the evolution of the photometric responses
The shape of the IFOV is relevant for the analysis of double stars. The shape has been determined during the commissioning phase. It refers to mean responses. There are quite clear indications that the effective IFOV for the modulation amplitude is significantly different, and that for a final analysis of the double star a further calibration has to be attempted.
The optical transfer function (OTF) provides a description of the evolution of the calibration parameters for the Hipparcos single star "image", which is a modulated signal described by a first and second harmonic. This information is again crucial in the analysis of double and multiple images.
The evolution of the photometric responses over the mission is provided for the mean flux and the modulation of the first harmonic. It reflects a variety of effects, from the darkening of the image dissector tube optics, to re-focussing. The photometric response was characterised by a more or less central circular feature, represented by a set of radial components.
The medium and small scale distortions show the grid geometry after correction for the ideal projection. These distortions are almost entirely related to the manufacturing of the grid and very basic properties of the telescope, and show no significant change over the mission.