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


OFFSET_X 		


OFFSET_Y 		


POS_SIGMA 		The RMS error in position


RA_LOW 		Image range on sky


RA_HIGH 		


DEC_LOW 		


DEC_HIGH 		


ZP_OFFSET 		Magnitude offset


ZP_SIGMA 		Uncertainty in offset


M_LIM 		Limiting magnitude


FNAME 		Name of file


RAC 		Coordinate of center


DECC 		


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