Low light level charge‐coupled devices (L3CCDs) have recently been developed, incorporating on‐chip gain. They may be operated to give an effective readout noise of much less than one electron by implementing an on‐chip gain process allowing the detection of individual photons. However, the gain mechanism is stochastic and so introduces significant extra noise into the system. In this paper we examine how best to process the output signal from an L3CCD so as to minimize the contribution of stochastic noise, while still maintaining photometric accuracy. We achieve this by optimizing a transfer function that translates the digitized output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on the photon counting accuracy and the effective signal‐to‐noise ratio. We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about 20 photon pixel−1 and above this there is negligible improvement. Operating L3CCDs at very high speeds will keep the photon flux low, giving the best improvements in signal‐to‐noise ratio.

High-resolution images of T Tau and its infrared companion have been reconstructed from near-and mid-infrared data collected at the Hale 5 m telescope. The near-infrared (1-5 fim) results were obtained by two dimensional speckle imaging and the mid-infrared (10-20 ¡im) results were derived from shift and add procedures applied to slit scans. The spectral energy distributions of the separated components were constructed from 1 to 20 [im data collected in less than half a year (1990 September to 1991 January). The spectral energy distribution of the optical component (T Tau N) is interpreted as containing two distinct constituents, a photosphere and a surrounding disk of circumstellar material. Measurements at a number of infrared wavelengths over the period 1985 December to 1991 January show a 2 mag color-independent change in the brightness of the infrared component (T Tau S). We propose that this may have been caused by an increase in accretion onto T Tau S and model the spectral energy distribution of T Tau S as being dominated by an accretion disk.

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Aims. Most of the known debris discs exhibit cool dust in regions analogous to the Edgeworth-Kuiper Belt. However, a rare subset show hot excess from within a few AU, which moreover is often inferred to be transient from models for planetesimal belt evolution. In this paper we examine 2 such sources to place limits on their location to help distinguish between different interpretations for their origin. Methods. We use MIDI on the VLTI to observe the debris discs around η Corvi and HD69830 using baseline lengths from 44-130 m. New VISIR observations of HD69830 at 18.7 μm are also presented. These observations are compared with disc models to place limits on disc size. Results. The visibility functions measured with MIDI for both sources show significant variation with wavelength across 8-13 μm in a manner consistent with the disc flux being well resolved, notably with a dip at 10-11.5 μm due to the silicate emission feature. The average ratio of visibilities measured between 10-11.5 μm and 8-9 μm is 0.934 ± 0.015 for HD69830 and 0.880 ± 0.013 for η Corvi over the four baselines for each source, a departure of 4 and 9σ from that expected if the discs were unresolved. HD69830 is unresolved by VISIR at 18.7 μm. The combined limits from MIDI and 8 m imaging constrain the warm dust to lie within 0.05-2.4 AU for HD69830 and 0.16-2.98 AU for η Corvi. Conclusions. These results represent the first resolution of dust around main sequence stars using mid-infrared interferometry. The constraints placed on the location of the dust are consistent with radii predicted by SED modelling (1.0 AU for HD69830 and 1.7 AU for η Corvi). Tentative evidence for a common position angle for the dust at 1.7 AU with that at 150 AU around η Corvi, which might be expected if the hot dust is fed from the outer disc, demonstrates the potential of this technique for constraining the origin of the dust and more generally for the study of dust in the terrestrial regions of main sequence stars.

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