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Session 14 - Milky Way Galaxy.
Display session, Monday, January 15
North Banquet Hall, Convention Center

[14.04] CCD Astrometry with the USNO Redlens Astrograph

N. Zacharias (USRA,USNO), T. J. Rafferty (USNO)

Since the mid--1970s, the US Naval Observatory has operated an eight-inch double astrograph using lenses corrected for the yellow and blue spectral bandpass respectively. The telescope has recently been modified using the yellow lens as the guidescope and replacing the blue lens with a new five--element red lens. Built originally to use 8" by 10" photographic plates with a field of 5 by 5.5 degrees, the astrograph has been equiped with a Kodak 1552x1024 pixel CCD camera (pixel size of 9 \mu m x 9 \mu m) and 16 bit readout. The camera uses a spectral bandpass of 570 to 650 nm and has a field size of 23'x15' (1 pixel = 0.9 arcsec). A 120 second guided exposure saturates for stars of magnitude R \approx 8.5. Precise positions for stars up to R \approx 6 can be obtained, using a diffraction grating in front of the lens. The images are well sampled with a typical FWHM of 2.5 pixels.

Images are fitted with two-dimensional Gaussian profiles. The fit precision increases from below 0.02 pixel for medium bright stars (\approx 8 \ to \ 13 mag) to 0.1 pixel at R \approx 15.5 . The position precision, as obtained by frame to frame transformations, is \le 20 mas for the medium bright stars. The external position accuracy is better than 30 mas, as compared to reference stars obtained photographically with a different instrument.

Currently the equipment is used for small projects and tests. The large, flat, usable field of the red lens of over five degrees in diameter offers the possiblity of using larger and multiple CCD cameras for a direct link to Hipparcos stars.

Program listing for Monday