Planet. Space Sci., Vol. 42, No. 8, 627-629, 1994 pp. Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-0633/94 $7.00-t-0.00 0032-0633(94)00118-9 Comet and asteroid astrometry with the Nordic Optical Telescope (NOT)-a progress report Ove Christian Dahl and Kaare Aksnes Institute of Theoretical Astrophysics, University of Oslo, Norway Received 8 July 1993 ; revised 17 May 1994 ; accepted 17 May 1994 Scientific justification The long term goal of this project is to track faint comets and asteroids over as large segments of their orbits as possible in order to learn more about their dynamics and physics. Until a few years ago, astrometry of such objects was based entirely on photography. Today the use of a CCD camera offers great advantages in terms of sensi- tivity, efficiency, and speed of operation during the observ- ing as well as the reduction process. Attached to a big telescope like the 2.5 m NOT, a CCD camera makes comets and asteroids observable out to beyond Neptune (e.g. the asteroid 1992QBl discovered by Jewitt and Luu (1992) and observed at -41 AU). This allows more pre- cise comet orbits to be calculated than has been hitherto possible, which, in turn, will yield better knowledge about how the non-gravitational forces and the outgassing change with a comet’s distance from the Sun. The appear- ance of comets at large distances may also shed light on the relationship between comets and asteroids, some of which are suspected of being extinct comets (most recently the asteroid (4015) 1979VA which Bowel1 (1992) has traced back to the comet Wilson-Harrington (1949 III)). One major drawback of the CCD is its small size which will limit the Field Of View (FOV) to encompass typically only a few well-measured reference stars. For this reason and also because the geometrical fidelity of a CCD may be in doubt, a preliminary program has been undertaken Correspondence to : 0. C. Dahl aimed at testing the astrometric quality of the available NOT/CCD combinations. We report here on the results of this test. Observing and reduction procedures The Nordic Optical Telescope has been operating since September 1989, financed by the governments of Denmark, Sweden, Finland and Norway. It is located 2382 m above sea level on the rim of a large volcanic crater on La Palma, Canary Islands. The telescope has a 2.56 m aperture, Cassegrain focus and alt-azimuth mount- ing. The primary mirror has a focal ratio off/2 and the combined system has a focal ratio off/l 1 with an unvig- netted, well-corrected field of 25 arcmin diameter. The alt- azimuth mounting causes a field rotation which is elim- inated by rotating the telescope. The observations were carried out with two CCD’s in the Aarhus-Tromss Low Dispersion Spectrograph (LDS) (Thomsen, 1990 ; Grundahl, 1992). Four runs over a total of 10 partial nights have been conducted: in December 1991, March 1992, and January and February 1993. Dur- ing the first two runs, only the secondary LDS channel was usable with a blue-sensitive Tk512 CCD having a scale of about 0.5 arcsec pixel-’ and a FOV of about 4.3 arcmin by 4.3 arcmin. During the last two runs, also the primary channel was used with a red-sensitive P8603 CCD with 0.86 arcsec pixel-’ and a FOV of about 5.5 arcmin by 8.3 arcmin. Because of vignetting, only a circular field of about 5.5 arcmin diameter was accessible. This CCD was reported to have been mounted at a 2.“1 tilt to com- pensate for chromatic aberration in the camera lenses. This produces a small, nearly linear change in the scale over the CCD, but the plate constants in the astrometric solution should automatically correct for this. It was found that exposure times in the range 30 s to 6 min were usually sufficient to record objects of total magnitudes between 18 and 22. Fig. 1 is an example of a