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Omega Centauri Globular Cluster



The field shown in image D is near the centre of the bright southern globular cluster Omega Centauri. It measures 12 × 12 arcseconds square and covers about 1/16 of the area of a CCD frame, obtained with the NTT on the night of "First Light''. It can easily be identified near the left edge of image C. The upper left picture is an enlargement of a photographic plate obtained in 1984 with the ESO Schmidt telescope under atmospheric seeing conditions mediocre by La Silla standards (2 arcsec). Next (upper right) follows an excellent photographic plate obtained at the Cassegrain focus of the ESO 3.6 m telescope in 1977 with seeing 1arcsecond. A 10 sec unfiltered CCD exposure was made with the NTT at the moment of "First Light'' on March 23, 1989. Part of it is shown below; to the lower left in the "raw" version in which the width of the stellar images (at half maximum intensity) is 0.33 arcseconds, a value never before achieved with a large, ground-based telescope. To the lower right, the same frame is shown after "sharpening'' by advanced image processing; the resolution is now improved to 0.18 arcseconds. The stellar images are noticeably sharper and faint stars are much better visible.

The first (upper left) picture is an enlargement of a photographic plate obtained in 1984 with the ESO Schmidt telescope under seeing conditions mediocre by La Silla standards (2 arcsec). The exposure time was 10 min on unsensitized, blue-sensitive IIIa-J emulsion behind a GG495 filter (spectral range 500 - 540 nm). The original image scale is 67.5 arcsec/mm; i.e. the field shown corresponds to 0.18 x 0.18 mm2 on the original 30 x 30 cm2 plate. In other words, about 2.6 million fields of this size are contained on the Schmidt plate; see also the indication of the field on image A. Next (upper right) follows an excellent photographic plate obtained at the Cassegrain focus of the ESO 3.6 m telescope in 1977. The exposure lasted 6 min 15 sec and the seeing was 1 arcsecond. The emulsion was IIIa-J and no filter was used (spectral range 300 - 540 nm). The image scale is 7.2 arcsec/mm; on the original 6 x 6 cm2 plate this field measures 17 x 17mm2.

A 10 sec unfiltered CCD exposure was made with the NTT at the moment of "First Light" on March 23, 1989; a small part of it is shown here in two versions. The first (lower left) is the "raw'' 100 x 100 pixel2 image (pixel size 0.123 arcseconds). The Full Width at Half Maximum (FWHM), as measured directly on the stellar images in the frame is 0.33 arcseconds. To this value, the NTT optics contributed perhaps 0.15 arcseconds, so that the actual, atmospherically induced seeing may have been better than 0.3 arcseconds, a spectacular value, even by La Silla standards. At the lower right, the same frame is shown after "sharpening" by advanced image processing. For this, the frame was subjected to deconvolution with a point spread function, which was empirically constructed from 50 profiles of uncontaminated stellar images and at the same time resampled at 1/5 of the pixel size in both directions. The FWHM is now improved to 0.18 arcseconds; the stellar images are noticeably sharper and faint stars are much better visible. To facilitate the comparison, the intensity scale is the same in both NTT frames.

The image processing was made by Dietrich Baade at the ESO MIDAS facility in Garching with an algorithm developed by Leon Lucy. About 3 hours VAX 8600 CPU time was needed to perform 20 iterations; this time can of course be significantly reduced with other computers, optimized for "number-crunching". The improvement in resolution is dramatic, as illustrated for instance by the triple star, just right of the field centre. The distance between the two components which are closest to each other, is only 0.79 arcseconds. The Schmidt picture does not indicate any multiplicity, the 3.6 m barely resolves the system, while the NTT shows the three components, well detached from each other. Note also the resolution of the double system near the lower border, here the distance is 0.59 arcseconds.

Credit:
ESO

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