Telescope VT-40/500 General features VT-40/500 is a compact 500-mm telescope delivering 4° in diameter flat field of view (80.5 mm in the focal plane) of diffraction-limited images. The telescope was designed by Dr. V.Yu.Terebizh1, and has been built by G.V.Borisov2, as a basic instrument for a number of observational programs, in particular: deep sky surveys; search for comets and asteroids; investigation of the objects of large angular size; investigation of transient phenomenon, e.g., gamma-ray bursts; search for satellites of the Earth and theirs debris in surrounding space. A few similar telescopes 220 mm and 350 mm in diameter were made last years and are working in Russian and Ukrainian observatories (Fig. 1), the 500-mm telescopes are now in progress. Fig. 1. First telescope of the series, VT-06/220. Specifications Basic specifications of the telescope are given in Table 1. Those values need only a few comments concerning matching the telescope optics to detectors and images blurred by the Earth's atmosphere. Table 1. Specifications of VT-40/500 Parameter Value Aperture diameter 500 mm Focal length 1150 mm f-number 2.30 Scale in the image plane 5.58 μm/arcsec Optimized spectral range 0.45 - 0.85 μm Angular diameter of the flat field of view 4°.0 Linear diameter of the flat field of view 80.5 mm Back focal length 75 mm Linear central obscuration 0.32 Fraction of obscured light in the axial beam 11% Fraction of vignetted rays at the edge of the field 11% Transmission of optics > 80% Angular size of 9-m CCD pixel 1".6 Angular size of 13.5- m CCD pixel 2".4 Length of the tube ~1100 mm It is an important feature of the telescope that it is optimally matched to the most frequently used nowadays charge coupled devices (CCD). For example, the diagonal of CCD with 4096 x 4096 of 13.5-μm pixels has length 78.2 mm, while the linear size of the telescope field of view is 80.5 mm. As it is known, the atmosphere usually blurs the stars images up to 1".5 - 2".5. According to Table 1, this value corresponds to 9.0 - 13.5 μm in the image plane, i.e., to the most popular size of the CCD's pixel. For example, Kodak KAF-16801E has 4096 x 4096 of 9.0-μm pixels, Marconi CCD 42-40 has 2048 x 2048 of 13.5-μm pixels. If optics provides images that are not significantly larger than one pixel, the telescope and the detector are optimally matched for observations of faint objects in wide field. That is just the case, according to the next section. Image quality The spot diagrams3 of the VT-40/500 are depicted in Fig. 2 for the most important wavelength range 0.45 - 0.85 μm (integral light; images in the narrow spectral bands are better). The values of the root-mean-square (RMS) and geometrical (GEO) radiuses of the images of a point-like source are given at the bottom of Fig. 2. Fig. 2. Spot diagrams of the telescope VT-40/500 in the integral light 0.45 - 0.85 μm. Box width corresponds to 13.5-μm CCD pixel. Field angles, 0; 0°.5; 1°.0; 1°.32; 1°.75 and 2°.0, are depicted at the top of each image; the corresponding off-axis distances on the detector are depicted below (0; 10.0 mm; 20.1 mm; 26.5 mm; 35.2 mm and 40.3 mm). Fig. 3. Spot diagrams of the ROTSE-III telescope in the range 0.45 - 0.80 μm The field angles, 0; 0°.5, 1°.0 and 1°.32 (19.95 mm), are the same as the first four field angles for the VT-40/500 (see Fig. 2) Fig. 3 shows, for comparison, the spot diagrams in the somewhat narrower wavelengths range for the known modern survey optical system - the ROTSE-III telescope (joint project of U.S.A. and Australia). The aperture diameter of the ROTSE-III is 450 mm, the diameter of the angular field of view 2w = 2°.64 (39.9 mm in the focal plane). The depicted field angles for ROTSE-III are equal to the first four field angles of the VT-40/500. It follows, from consideration of Figs 2 and 3, that ROTSE-III has worse images even in the 1.52 times narrower field of view than VT-40/500. Fig. 4 demonstrates that the system provides the diffraction-limited images almost in all field of view, i.e., the theoretically best image quality. Fig. 4. RMS radius of the image of a point-like source (m) vs. field angle (degrees) for various wavelengths (black curved line - in polychromatic light, horizontal straight line - diffraction limit). Some other important image properties are illustrated by Fig's 5-7. Fig. 5. VT-40/500: Encircled energy in polychromatic light for various field angles. D80 varies from 4.9 μm (0".88) at the center of field up to 8.6 μm (1".56) at its edge. Fig. 6. VT-40/500: Modulation Transfer Function. Fig. 7. VT-40/500: Vignetting plot. 1 Moscow State University, Sternberg Astronomical Institute; Crimean Astrophysical Observatory, Ukraine E-mail Home page: http://www.terebizh.ru/V.Yu.T 2 Moscow State University, Sternberg Astronomical Institute; E-mail 3 Distribution of light rays in the focal plane.   As one can see, VT-40/500 is a new multi-purpose wide-field telescope of top performance. Размещен 26 октября 2006.