[Comm2011] astrometry in corners / field rotation

[Konrad Kuijken]

Dear all,

I have done a simple, by-hand analysis of the orientation of the  
corner CCDs, to try to answer the questions
  - is there a problem with the telescope rotator angle preset
  - does the plate scale vary

On each ccd I took two stars from SAO, and compared their RA/DEC with  
the pixel positions, using it to derive a plate scale and a rotation.  
I only looked at equatorial fields, in r band, to avoid any coordinate  
system curvature.
Quick conclusion:
  - telescope preset seems to be OK to within measurement error of  
about 0.001 rad (0.05 deg)
  - plate scale of each chip seems to be constant to within 1 part in  
1000 (again, probably just the measurement+catalogue error)

  This is certainly stable enough to make it very surprising that we  
seem to be having to pin down the predicted guide star positions!

attached the measurements (based on a selection of equatorial fields,  
r band, exposure 30-90 seconds).

Note that I took this brute-force approach because it is not obvious  
to me that the astrometric solutions we derive in astrowise give a  
clean CD matrix: the astrometry fit uses a higher-order polynomial and  
so linear-order coefficiens may be contaminated by the quadratic and  
cubic terms.

Cheers, have a good weekend, talk to you after my vacation,
Koen


in this table, for each corner ccd the x and y pixel positions of  
star1, and its catalogue position in hms and dms; then same for star2  
(near top & bottom of chip)

# exposure ccd x1 y1 rah1 rams1 dec1 x2 y2 rah2 rams2 dec2       (
06-09T0914 65 642 182 21 3410.529 +00303 980 3957 21 3415.276 +01629
06-09T0914 72 1085 3493 21 3750.102 01453 750 377 21 3745.434 00348
06-09T0914 89 980 4027 21 3415.053 010253 1040 580 21 3415.97 05037
06-09T0914 96 1049 4089 21 3749.423 10309 1255 619 21 3752.44 05049
06-13T0159 65 884 4041 15 3541.645 -05514 1269 362 15 3547.177 -10820
06-13T0159 72 1012 3387 15 3916.774 -05730 944 373 15 3915.900 -10814
06-13T0159 89 1133 3939 15 3544.995 -00928 1062 859 15 3543.978 -02026
06-13T0159 96 977 4096 15 3916.133 -00852 1123 618 15 3918.298 -02114
06-30T0039 65 1167 3782 15 3717.47 -03055 853 51 15 3713.022 -04411
06-30T0039 72 1055 3559 15 4049.15 -03140 941 292 15 4047.645 -04316
06-30T0039 89 921 4167.5 15 3713.873 01635 864 575 15 3712.974 00349
06-30T0039 96 1126 3967 15 4050.11 01554 1049 395 15 4048.994 00313
06-30T2320 65 1391 3662 12 4029.93 -04745 916 182 12 4023.225 -10007
06-30T2320 72 1192 3936 12 4400.336 -04642 903 202 12 4356.393 -05959
06-30T2320 89 1176 3979 12 4026.705 -00029 1121 542 12 4025.994 -01243
06-30T2320 96 914 3999 12 4356.21 -00022 1001 655 12 4357.53 -01215
07-01T0406 65 963 3961 18 4001.514 00647 822 381 18 3959.674 -00557
07-01T0406 72 1050 3950 18 4336.154 00648 1058 222 18 4336.408 -00627
07-01T0406 89 938 4097 18 4001.066 05324 883 260 18 4000.35 03945
07-01T0406 96 1076 3413 18 4336.403 05102 1043 374 18 4335.971 04014

     exposure              ccd       deg/pix         "/pix         rot  
(rad)

   06-09T0914          65   5.931e-05      0.2135   -0.001193
   06-09T0914          72   5.928e-05      0.2134   -0.002215
   06-09T0914          89   5.931e-05      0.2135    -0.00128
   06-09T0914          96   5.924e-05      0.2133   -0.001792

   06-13T0159          65   5.935e-05      0.2136   -0.000926
   06-13T0159          72   5.935e-05      0.2137   -0.002202
   06-13T0159          89   5.934e-05      0.2136   0.0001302
   06-13T0159          96   5.926e-05      0.2133   -0.001789

   06-30T0039          65   5.926e-05      0.2133   -0.000339
   06-30T0039          72   5.918e-05       0.213   -0.002461
   06-30T0039          89   5.923e-05      0.2132    0.001739
   06-30T0039          96   5.918e-05       0.213   0.0004459

   06-30T2320          65   5.923e-05      0.2132  -0.0009465
   06-30T2320          72   5.929e-05      0.2134   -0.003184
   06-30T2320          89   5.932e-05      0.2136    -0.00147
   06-30T2320          96   5.923e-05      0.2132   -0.001752

   07-01T0406          65   5.928e-05      0.2134    -0.00326
   07-01T0406          72   5.924e-05      0.2133   -0.002643
   07-01T0406          89   5.929e-05      0.2134   -0.001217
   07-01T0406          96   5.923e-05      0.2132  -0.0008559



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