MAPS - Minnesota Automated Plate Scanner Catalog of the POSS I


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'astrom' Table Parameters

The 'astrom' database table stores information describing the astrometric fit to each POSS I field in the MAPS Catalog. The following astrometric fit parameters are stored for each field in the MAPS Catalog:

Field Name Definition
possfield MAPS POSS I Field Name
possnumber MAPS POSS I Field Number
catalog Astrometric catalog used for the data reduction. Either 'lick' for the Lick Northern Proper Motion Catalog or 'acrs' for the Astrographic Catalog of Reference Stars (ACRS).
epoch Epoch used for astrometric fit. Always '1950'.
x_zpc, y_zpc The horizontal and vertical zero point corrections applied to data before astrometric reduction (in microns).
init_rot Initial rotation of astrometric fit (in degrees).
num_cal_stars The number of calibration stars used in astrometric calibration.
x_ctr, y_ctr The center of the plate (in ERE).
ra_ctr, decl_ctr The center of the plate in equatorial B1950.0 coordinates on the FK4 system used for the astrometric fit.
raJ_ctr, declJ_ctr The center of the plate in ecliptic J2000.0 coordinates on the FK5 system (precessed from ra_ctr, decl_ctr).
l_ctr, b_ctr The center of the plate in Galactic coordinates on the lII system (precessed from ra_ctr, decl_ctr).
rarms, declrms The RMS of the Right Ascension and Declination fit (in arcseconds).
dx, x_x, x_y, x_xr2, dy, y_x, y_y, y_yr2 These are the coefficients of the plate solution used (corresponding to dX, X|x, X|y, X|xr, dY, Y|x, Y|y, Y|yr2 in the equations below). The plate model that maps these modified coordinates, (x,y) to standard coordinates on the celestial sphere (i.e. the focal-length-normalized gnomic projection, (X,Y), of RA and Dec) is given by:
  X = dX + (X|x)*x + (X|y)*y + (X|xr2)*(r^2)  where r^2 = x^2 + y^2
  Y = dY + (Y|x)*x + (Y|y)*y + (Y|yr2)*(r^2)
dX and dY are given in microns, and represent the difference between the true and nominal plate centers. The remaining terms are dimensionless derivatives. Note that one cannot apply the plate solution coefficients directly to the plate positions in the MAPS catalog because the steps of correcting for distortion, recentering, rotating, flipping, and normalization by focal length are all performed internally in the data reduction before this astrometric fit is applied.