Infrared Specklegraph

The Infrared Specklegraph, an optimised version of the prototype installed in 1979 (see The Infrared Speckle Interferometer), was first tested in September 1984 on the ESO 3.6-metre telescope at La Silla Observatory in Chile. It was designed and integrated under an ESO contract with the Lyon Observatory, the Meudon Observatory and the French National Institute for Astronomy and Geophysics (INAG which later became known as INSU) in France.

The specifically designed LN2-cooled cryostat was equipped with two InSb detectors in order to provide two dedicated channels: one for the signal, the other for guiding. The signal channel had optimised sensitivity, dynamics and frequency response for observations from 1350 nm (J band) to 4600 nm (M band) either in broad band (J to M) or with interference filters (CVFs) and various slits adapted to the resolutions at these wavelengths. The guiding channel was not used since guiding proved to be efficient enough with the signal itself.

The instrument functioning was based on the speckle imaging — which uses short-exposure images called specklegrams — which have an exposure time smaller than the atmospheric variation time, in order to “freeze the seeing”. Because of the lack of infrared arrays at that time, images were obtained by scanning the object with a slit. The Infrared Specklegraph was equipped with a set of slits and a specific high-frequency electronic chain, and was mounted at the infrared f/35 focus on the Infrared Photometer adapter. The f/35 wobbling mirror insured the saw-tooth scanning synchronised with the rapid data acquisition performed by the specklegraph. The observing procedure determined that each scan of the source was followed by a scan of equal length of the sky, for post-compensation of sky noise.

The limiting magnitudes were 7.7 at K and L bands, 5.8 at M band, 4.6 at 3600 nm with a circular variable filter (CVF) and 4.5 at 4600 nm with a CVF.

The Specklegraph together with the f/35 mirror was remotely controlled through a specific acquisition program available on a standard computer terminal, and an online simplified version of the data processing software was also available to the Observer. An offline version developed for HP 1000 systems was available for the final data reduction process.

Between May and August 1987 the Infrared Specklegraph was used to study the infrared emission, especially the light echo, around the vicinity of SN 1987A, and it allowed the first study of a supernova at high spatial resolution in the infrared.

The Infrared Specklegraph was decommissioned in 1995.