Physics – Optics
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999eso..pres....2.&link_type=abstract
ESO Press Release, 01/1999
Physics
Optics
Scientific paper
The newest astronomical instrument at the La Silla observatory is a super-camera with no less than sixty-seven million image elements. It represents the outcome of a joint project between the European Southern Observatory (ESO) , the Max-Planck-Institut für Astronomie (MPI-A) in Heidelberg (Germany) and the Osservatorio Astronomico di Capodimonte (OAC) near Naples (Italy), and was installed at the 2.2-m MPG/ESO telescope in December 1998. Following careful adjustment and testing, it has now produced the first spectacular test images.
With a field size larger than the Full Moon, the new digital Wide Field Imager is able to obtain detailed views of extended celestial objects to very faint magnitudes. It is the first of a new generation of survey facilities at ESO with which a variety of large-scale searches will soon be made over extended regions of the southern sky.
These programmes will lead to the discovery of particularly interesting and unusual (rare) celestial objects that may then be studied with large telescopes like the VLT at Paranal. This will in turn allow astronomers to penetrate deeper and deeper into the many secrets of the Universe. More light + larger fields = more information!
The larger a telescope is, the more light - and hence information about the Universe and its constituents - it can collect. This simple truth represents the main reason for building ESO's Very Large Telescope (VLT) at the Paranal Observatory.
However, the information-gathering power of astronomical equipment can also be increased by using a larger detector with more image elements (pixels) , thus permitting the simultaneous recording of images of larger sky fields (or more details in the same field). It is for similar reasons that many professional photographers prefer larger-format cameras and/or wide-angle lenses to the more conventional ones. The Wide Field Imager at the 2.2-m telescope
Because of technological limitations, the sizes of detectors most commonly in use in optical astronomical instruments - the "Charge-Coupled Devices (CCD's)" - are currently restricted to about 4000 x 4000 pixels. For the time being, the only possible way towards even larger detector areas is by assembling mosaics of CCD's.
ESO , MPI-A and OAC have therefore undertaken a joint project to build a new and large astronomical camera with a mosaic of CCD's. This new Wide Field Imager (WFI) comprises eight CCD's with high sensitivity from the ultraviolet to the infrared spectral domain, each with 2046 x 4098 pixels. Mounted behind an advanced optical system at the Cassegrain focus of the 2.2-m telescope of the Max-Planck-Gesellschaft (MPG) at ESO's La Silla Observatory in Chile, the combined 8184 x 8196 = 67,076,064 pixels cover a square field-of-view with an edge of more than half a degree (over 30 arcmin) [1].
Compared to the viewing field of the human eye, this may still appear small, but in the domain of astronomical instrumentation, it is indeed a large step forward. For comparison, the largest field-of-view with the FORS1 instrument at the VLT is about 7 arcmin. Moreover, the level of detail detectable with the WFI (theoretical image sharpness) exceeds what is possible with the naked eye by a factor of about 10,000.
The WFI project was completed in only two years in response to a recommendation to ESO by the "La Silla 2000" Working Group and the Scientific-Technical Committee (STC) to offer this type of instrument to the community. The MPI-A proposed to build such an instrument for the MPG/ESO 2.2-m telescope and a joint project was soon established. A team of astronomers from the three institutions is responsible for the initial work with the WFI at La Silla.
A few other Cameras of this size are available, e.g. at Hawaii, Kitt Peak (USA) and Cerro Tololo (Chile), but this is the first time that a telescope this large has been fully dedicated to wide-field imaging with an 8kx8k CCD. The first WFI images
Various exposures were obtained during the early tests with the WFI in order to arrive at the optimum adjustment of the camera at the telescope. We show here two of these that illustrate the great potential of this new facility. Spiral Galaxy NGC 253
ESO PR Photo 02a/99
ESO PR Photo 02a/99
[Preview - JPEG: 800x850 pix - 205k]
[High-Res - JPEG: 4000 x 4252 pix - 3.0Mb]
ESO PR Photo 02b/99
ESO PR Photo 02b/99
[Preview - JPEG: 800x870 pix - 353k]
[High-Res - JPEG: 2200 x 2393 pix - 2.0Mb]
Caption to PR Photos 02a/99 and 02b/99 : These photos show a sky field around the Spiral Galaxy NGC 253 (Type Sc) seen nearly edge-on. It is located in the southern constellation Sculptor at a distance of about 8 million light-years. The image is the sum of five 5-min exposures through a blue (B-band) optical filtre. They were slightly offset with respect to each other so that the small gaps between the eight CCD's of the mosaic are no longer visible. This image also shows the faint trails of 2 artificial satellites. In PR Photo 02a/99 , the full WFI field-of-view is reproduced, while the sub-field in PR Photo 02b/99 contains some fainter and smaller background galaxies. Many of the quite numerous and small, slightly fuzzy objects are undoubtedly globular clusters of NGC 253.
Technical information: The image processing consisted of de-biassing, flat-fielding, and removal (by interpolation) of some bad columns. The full-width-half-maximum (FWHM) of stellar images is about 1.0 arcsec. PR Photo 02a/99 was rebinned (2x2) to 4kx4k size and sampling 0.48 arcsec/pixel. PR Photo 02b/99 is a subimage of the former, but at the full original sampling of 0.24 arcsec/pixel. It covers about 2kx2k, or about 1/16 of the full field. North is up and East is left. The observations were made on December 17, 1998. The Waning Moon
ESO PR Photo 02c/99
ESO PR Photo 02c/99
[Preview - JPEG: 800 x 1245 pix - 242k]
[High-Res - JPEG: 3000 x 4667 pix - 2.3Mb]
ESO PR Photo 02d/99
ESO PR Photo 02d/99
[Preview - JPEG: 800 x 1003 pix - 394k]
[High-Res - JPEG: 3000 x 3760 pix - 2.1Mb]
ESO PR Photo 02e/99
ESO PR Photo 02e/99
[Preview - JPEG: 800 x 706 pix - 274k]
[High-Res - JPEG: 3000 x 2648 pix - 1.5Mb]
Caption to PR Photos 02c-e/99 : A series of short exposures through a near-infrared filtre was obtained of the waning Moon at sunrise on January 12 (at about 10 hrs UT), i.e. about 5 days before New Moon (24.3 days "old"). As can be seen in PR Photo 02c/99 , the edge of the full field-of-view is about the size of the diameter of the Moon. In addition, two impressive views were extracted from this frame and are here shown at full resolution; 1 pixel is about 470 metres on the surface of the Moon at a distance of just over 400,000 km. PR Photo 02d/99 displays the Mare Humorum area in the south-east quadrant with the crater Gassendi overlapping the northern rim. PR Photo 02d/99 is a view of the plains near the Moon's north-east rim, just eastwards of Sinus Iridum (the large crater in the shadows at the upper right), on the rim of which the crater Bianchini is located. The crater just below the centre is Mairan and the one about halfway between these two and of about the same size is Sharp .
Technical information: Several 0.1 sec exposures were made through a near-infrared filtre (856 nm; FWHM 14 nm) with small offsets were recombined (to cover the gaps between the individual CCD's); otherwise, the image is raw. PR Photo 02c/99 was rebinned (2x2) to 4kx4k size and sampling 0.48 arcsec/pixel. The right-hand side of the picture was cropped in this reproduction to reduce the file size. PR Photos 02d/99 and 02e/99 are subimages of the former, but at the full original sampling of 0.24 arcsec/pixel; they covers about 1000x800 and 900x1050 pixels, or about 1/80 and 1/70 of the full field, respectively. North is up and East is left. The virtues of wide-angle imaging
Wide-angle imaging is one of the most fundamental applications of observational astronomy. Only from (multi-band) observations over large areas of the sky can large-scale structures and rare objects be detected and put in a proper statistical perspective with other objects. Some typic
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