The Infrared Speckle Interferometer

The first version of the Infrared Speckle Interferometer was developed by ESO in late 1979, for use with the ESO 3.6-metre telescope at La Silla Observatory. During the first year of operation the prototype instrument, in a preliminary state, made it possible to record sufficient results to prove its feasibility. The first astrophysically usable results from this instrument were obtained in 1981.

The Infrared Speckle Interferometer was based on the ESO 3.6-metre Infrared Photometer using a specific high frequency boosted preamplifier, additional apertures and the f/35 wobbling secondary mirror in a slit-scanning mode. These additions were developed at ESO with the joint support of the Lyon Observatory, and the Meudon Observatory in France.

The instrument functions were based on the speckle imaging technique, which in infrared, was using short-exposure one-dimensional (1D) images to “freeze” the variation of atmospheric turbulence, theoretically increasing the resolution up to the diffraction limit of the telescope. Exposures were obtained by digitally converting the signal in perfect synchronisation with the atmospheric scanning, the rate of which could be tuned over a wide range so it was always “freezing the seeing”.

The Infrared Speckle Interferometer had optimised sensitivity, dynamics and frequency response for observations from 2200 nm to 4800 nm, with various slits adapted to the resolutions at these wavelengths. The filter was selected between a set of broad-band photometric filters and two circular variable filters (CVF) — interference filters designed such that the wavelength transmitted varies as a function of the position of the filter. It could achieve the full resolution of the ESO 3.6-metre telescope and store instantaneous 1D images of the objects under study taken in the direction of the scanning. Two observing modes were available to achieve either maximum resolution for bright objects or lower resolution of faint objects. Observing with the maximum resolution mode meant that the adopted slit width and step as well as a  fast scanning rate allowed very compact objects to be measured. Observing in imaging mode meant the use of a large slit and a slower scanning rate in order to measure faint objects.

The theoretical limiting magnitudes were 7.7 at 2200 nm, and 4.0 at 4800 nm under good seeing conditions. The performance of the Speckle Interferometer was, for a given seeing, a compromise between sensitivity and resolution, i.e. the wider the slit, the higher the limiting magnitude and the lower the resolution.

The Infrared Speckle Interferometer was remotely controlled through a specific acquisition programme, available on a computer terminal, in a very similar manner to that already used at the ESO 3.6-metre telescope. An online simplified version of the data treatment software was also available to the observer and an offline version developed for HP 1000 systems could be used for the final data reduction process.

This prototype version of the Infrared Speckle Interferometer, which was never available as a general-user instrument was replaced by a new version in 1984 (see The Infrared Specklegraph).