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cobj Files

Once the object list has been calibrated against the USNO A2.0 catalog, we are in a position to know how to map the image coordinates to the sky, and we can fill in all of the image file headers. The following fields now have meaningful values that define a (very rough) WCS rotation matrix to perform this conversion: PC001001, PC002002, PC001002, and PC002001. These numbers are used by ds9 to calculate world coordinate system units for display, but are not accurate enough for real analysis.

A cobj FITS file has a primary header and two data extensions. The primary header is taken directly from the corrected image. The first data extension contains the calibrated object list. The second data extension contains the ``calibration structure,'' or ``cal'' structure.

The calibrated object list at extension 1 is an array of structures, one element per object, each element of which contains the following fields:

RA          Right Ascention (deg)

DEC Declination (deg)
X Horizontal Pixel coordinate
Y Verticle Pixel coordinate
M Calibrated magnitude
MERR Uncertainty in above
FLAGS Same as in sobj file
RFLAGS Additional ROTSE flags (See Table C.2)

The cal structure at extension 2 contains a single structure with much information. The bulk of the cal structure consists of the corrected image header keywords converted from ASCII to a binary structure. In addition are several important keywords from the sobj header. Finally are calibration diagnostic values described here:

BADPIXFILE       The name of the bad pixel file

NMATCH The number of matched objects
POS_SIGMA The RMS error in position
RA_LOW Image range on sky
ZP_OFFSET Magnitude offset
ZP_SIGMA Uncertainty in offset
M_LIM Limiting magnitude
FNAME Name of file
RAC Coordinate of center
KX Third order rotation matrix
KY to convert coords
SAT_MAG Saturation magnitude
NOBJ_BADPIX Objects that fall on bad pixels
SKY 64$\times$64 array for sky bkgd

Table C.2: Possible values for the RFLAGS bitmask field in the calibrated object list structures.
Bit value Tag Name Meaning
1 HOTPIX Object falls on hot pixel in this observation
2 USNOCAT Object is present in USNO Catalog
4 ASTEROID Object is most likely an asteroid
8 BADPOS Position centroid for this observation is too far from mean position
16 NOTEMPL Relphot subframe failed to identify sufficient template objects
32 PHOTSDEV Relphot subframe RMS correction is large
64 BADIMAGE A catch-all image-wide problem flag

next up previous contents index
Next: Match Structures Up: ROTSE-III Data Products Previous: sobj Files   Contents   Index
Rotse Pager 2003-05-20