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Astron. Astrophys. 338, 132-138 (1998)

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3. Data analysis procedure

The observations were carried out using the MPE infrared high resolution camera, SHARP (Hofmann et al. 1993), at the 3.60-m telescope of the European Southern Observatory (ESO) at La Silla, Chile. SHARP uses a 256[FORMULA]256 pixel NICMOS 3 detector array developed by Rockwell International Corporation with the parameters summarized in Table 3. This array is split into four 128[FORMULA]128 quadrants, each of them having an independent data acquisition channel. A system of digital signal processors allows continuous read-out of the array as fast as 10 Hz, as well as on-line image processing for quick-look purpose. With an image scale of [FORMULA] per pixel the camera is used for diffraction limited imaging at J (1.25µm), H (1.65µm) and K (2.2µm) bands.


Table 3. Camera parameters (Böker et al. 1994)

In order to eliminate instrumental effects (flat field, sky, dead pixels, ...), data cubes of hundreds of exposure frames have been stored with the observed object centered in two opposite quadrants from one data cube to the other.

The image processing was done using the MPE InfraRed Speckle Interferometry reduction software IRSI (Eckart & Duhoux 1990). Standard infrared data reduction software was applied to each individual frame: quadrant extraction, dead pixel removal, sky subtraction, flat fielding. Then, these frames have been co-added using the Shift-and-Add algorithm. The same processing has been applied both to the program targets and to the reference stars. Finally, a deconvolution process derived from the Lucy algorithm has been used to show the low level structures in the dust shells (Lucy 1974). The lowest contour of each reconstructed map has been limited to 0.1% of the maximum of intensity, in agreement with the dynamic range of 1000:1 of the calibrators.

The orientation of the maps on the sky is based on the well-known bipolar morphology of the Red Rectangle nebula. We have compared our 2.2µm COME-ON+ reconstructed map of the Red Rectangle with an angular resolution of [FORMULA] to the map we previously obtained by infrared speckle interferometry at the 3.60-m CFH Telescope with a comparable resolution in the same spectral band (Cruzalèbes et al. 1996). The overall shape, i.e. a North-South extension of the nebula with a position angle of about [FORMULA] to the east, is similar. The North-Eastern lobe is also dominant at this wavelength. Moreover, recent ESO-ADONIS observations that we carried out in 1997 January confirmed the right orientation of our previous COME-ON+ maps, mainly thanks to the confirmation of the presence of the North-Eastern clump of dust at a distance of [FORMULA] of the central star of VY CMa (Monnier et al. 1998).

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998