Notes on the Astrometry





The plate scale and orientation were first determined by fitting to objects in the APM catalog lying throughout the field. The plate scale was found to be 0.1885"/pixel, consistent with previous determinations of the plate scale in the QUIRC camera. The orientation angle was 0.01107 radians which is also consistent with typical values for QUIRC (0.01269 for B1950).

33 objects in the field with 17 < V < 22.5 were used in the APM fit. The plate eo674 was used where the field is slightly more centred. The dispersion was 0.32" in both RA and DEC directions for a total error of 0.45". This is comparable to expected astrometric errors in the APM catalog, which was verified by comparing the RA and DEC positions in the two APM plates covering the region (eo1427 and eo674). The rms error between the two plates is 0.73" and the average offset was 1.8" in RA and 1.5" in DEC, consistent with APM errors fully dominating the error budget. (The field lies close to the edge of eo1427 where the APM solution may be worse.) The offsets are also consistent with advertised accuracies of the APM absolute astrometry. No systematic effects were visible.

The offset to the VLA system was determined from the 5 radio sources in Fomalont et al. with the smallest DEC and RA errors (0.2") with I < 22 optical counterparts. The offset was 1.68" in the RA direction and -0.59" in the DEC, consistent with the expected APM absolute errors. The RMS errors were 0.16" in the RA and 0.11" in the DEC, consistent with the radio source errors. All magnitudes are corrected 3"-diameter magnitudes offset to total magnitudes.

14 of the sources in Fomalont et al. lie close to an object in the catalog as is summarised in the Table. All but one are consistent with the radio errors, but object 17 is well outside the expected errors. Four objects have no counterpart in the K < 20.0 table.



Identified Radio Sources In Fomalont et al.
Number
in
tables
RA(2000) DEC(2000) z RA
Offset
DEC
Offset
Radio Error
 RA     DEC
1 12 36 43.98 62 12 50.0 0.55600.04 -0.10 0.07 0.6
2 12 36 44.38 62 11 33.3 1.01300.00 -0.28 0.02 0.2
3 12 36 46.33 62 14 4.5 0.96100.00 0.23 0.02 0.2
4 12 36 49.70 62 13 12.8 0.4750 -0.09 1.04 0.100.6
5 12 36 51.71 62 12 20.1 0.2990 -0.04 1.01 0.060.4
6 12 36 55.44 62 13 11.0 0.0000 0.03 0.69 0.110.6
7 12 36 34.46 62 12 12.8 0.4560 0.00 -0.01 0.030.2
8 12 36 34.49 62 12 40.9 1.2190 -0.00 -0.02 0.030.2
12 12 36 52.87 62 14 44.1 0.3210 0.01 -0.00 0.020.2
13 12 36 53.37 62 11 39.6 0.0000 -0.02 0.05 0.060.4
14 12 37 1.59 62 11 45.9 0.0000 -0.00 1.34 0.120.5
15 12 37 8.30 62 10 56.0 0.0000 -0.06 -0.16 0.100.5
17 ?ID 12 37 21.35 62 11 31.0 0.0000 -0.10 -1.26 0.020.2
18 12 37 25.95 62 11 28.8 0.0000 0.01 -0.49 0.050.2