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The 20 cm Twin Astrographic Telescope was equipped with yellow and blue Ross-type lenses (focal ratio f/10) for this project. The entire northern hemisphere down to $-15^{\circ}$ declination has been observed with this telescope (1977-86) while located in Washington DC to a limiting magnitude of approximately 12. About 90\% of a total of $5500$ plates have been measured thus far on the USNO Starscan measuring machine.
For the first version of the TAC, positions for about 700,000 stars from 2354 yellow and 2153 blue plates have been obtained by conventional plate adjustment. The IRS was used as the reference star catalog, and the mean number of reference stars per plate is 22.
The mean sigma in each coordinate of the plate adjustment with a linear model plus plate tilt terms and third order optical distortion correction pre--applied is 208 mas (milli arc seconds) for the yellow plates and 210 mas for the blue plates. A field distortion pattern has been determined and applied for the yellow and blue lens plates separately.
Magnitude dependent systematic errors have been investigated in detail. For the yellow lens the magnitude equation (TAC-IRS) was found to depend on the epoch as well as on declination. Regions of approximately constant magnitude terms have been identified.
The first preliminary TAC was constructed by averaging the individual positions of overlapping plates, separately for the blue and yellow lens exposures. Comparing individual positions of field stars on overlapping plates, the mean position error of a single image of a 10th magnitude field star is found to be about 160 mas. External comparisons with the ACRS and PPM catalogs are in progress.
After combining the data of all yellow and blue plates the complete TAC will be used for the SDSS (Sloan Digital Sky Survey). Proper motions will be obtained in combination with the Astrographic Catalog (AC) being compiled at the USNO.
