USNO-SA 1.0 Astrometric Reference Catalog Extraction

The U.S. Naval Observatory SA1.0 is a spatially sub-sampled version of the USNO-A1.0 Catalog which provides a catalog of 54,787,624 sources intended as an astrometric reference catalog for asteroid hunters and other folks who do not need all of that 10-CDROM, 488,006,860-source catalog.

The size of the sampling pixel is 7.5/285 degrees on a side. It was chosen because it

(a) equally divided the 7.5 degree zone width of USNO-A,
(b) produced a catalog of approximately 55 million entries, which is just about as much as a CD-ROM can hold,
(c) is very close to making a pixel with an area of 2.5 square arcminutes.

The pixels were created in a reasonable manner, but are not exactly square. Please refer to rdpixels.f for the exact details, but the algorithm finds the nearest integer number of pixels of the requested size in a band of declination, and then computes the limits as constant alpha and delta. The zero point for each zone of pixels is offset so that if there were the same number of pixels in adjacent zones, then the centers of one zone would be under/above the edges of the next zone.

The USNO-A catalog is read, and the pixel for each entry is computed. Each new entry is evaluated according to the following rules, which are contained in getqual.f and each.f

(a) If this is the first entry for a pixel, take it.
(b) If the quality of this entry is better (lower) than the current value, take this entry.
(c) If the quality of this entry is the same (equal) to the current value, then choose whichever is closest to the center.

The quality is defined by the following algorithm.

(a) If 16.0 <= m_B <= 19.0 then q = 1
(b) Else if 14.0 <= m_B <= 20.0 then q = 3
(c) Else q = 5

After defining q, a penalty is added for a weird color.

(a) If (m_B-m_R) < 0.5 or (m_B-m_R) > 2.0 then q = q+1

These limits are approximately defined by F0 to K5 spectral type.

The distance from the center of the pixel is not quite the geometric one because of the desire to choose a brighter star in dense regions. The algorithm proceeds as follows.

(a) i = 5*SQRT((x-x0)**2 + (y-y0)**2)/SIZE
This divides the pixel into radial zones from 0 at the center to 7 in the corner. Making an integer means that all entries in a particular zone are assigned the same weight. This radial zone is them modified by

(b) r = 1000*i + 10*m_B
For stars closer to the center of the pixel, any brightness will be chosen over a star in a more distant zone. For stars in the same zone, a brighter star will replace a fainter star. Remember that the quality test is applied first, so really bright or faint stars don't enter into this decision.

The goal of the algorithm was to find a spatially uniform sample of solar type stars, and to choose brighter ones in crowded regions. I hope that this algorithm is a reasonable approximation. Using this algorithm, 54,787,624 stars were selected, leaving approximately 10% of the pixels empty. The breakdown according to quality is as follows. 

   Quality     Number of entries
   -----------------------------
      1           34,454,509
      2            5,170,963
      3            6,014,824
      4            2,264,809
      5            4,745,419
      6            2,137,100
The format is the same as the USNO-A catalog.

-Dave Monet

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