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Axis Offset Corrections

Experiments released between 2010-03-01 and 2010-08-31 were affected by a bug in the model generation software (calc/calcif2/calcserver) that caused the Hobart telescope to be treated as an ALT-AZ mount, rather than the correct XY-EW mount. The mount was also incorrectly labelled as 'ALAZ' in the antenna table. This means that the axis offset correction applied in the correlator model is incorrect. The effect is comparable to a position error of several metres. This will be detrimental to any experiments relying on phase referencing, either for relative astrometry or weak source detection.

This error can be fixed in AIPS, following the recipe below. You should apply this fix before doing any other phase calibration.

Step 1

Update your AIPS installation! A proper description of the XY-EW axis offset was only added to AIPS on 1 September 2010, so you should first make sure your AIPS installation is up-to-date before proceeding.

Step 2

Inspect your antenna table in AIPS. If the mount type for Hobart is given as 'ALAZ' then you have been affected by the bug and should proceed to correct it.

go prtan

Note the antenna number for Hobart. For the remainder of this recipe, I will assume Hobart is antenna 3. Substitute the correct antenna number if it is different in your dataset.

Step 3

First you need to remove the 8.195m ALTAZ axis offset that was erroneously applied during correlation:

task 'clcor'
getn N                      (N = catalogue number of your uv data file)
antennas = 3,0              (assuming Hobart is antenna 3 in your dataset)
clcorprm = -8.195, 0        (note the - sign to remove the existing offset)
opcode = 'ANAX'

Inspect the antenna table with 'prtan' and check that Hobart is now listed as having an axis offset of 0m.

Step 4

Edit the antenna table to set the correct mount type for Hobart.

task 'tabed'
getn N
bcount = 3                  (again assuming Hobart is antenna 3)
ecount = 3                  (ditto)
inext = 'AN'
aparm = 5,0,0,6
optype = 'REPL'
keyval = 3,0                (here '3' is the key value for XY-EW mount, not the antenna number)

Step 5

Now re-apply the axis offset, this time for the correct mount type (as edited above)

task 'clcor'
getn N                      
antennas = 3,0             
clcorprm = 8.195, 0         (note the axis offset is now positive)
opcode = 'ANAX'

Step 6

Inspect the antenna table again. You should see Hobart listed as an EW mount with an axis offset of 8.195m.

go prtan

Continue with your standard calibration. Note that CLCOR will modify the 'AN' table and create a new, modified, 'CL' table each time you run it. You should now have at least 3 'CL' tables. Be sure to use this highest 'CL' table as the starting point for all ensuing calibration. You should find that the rate solutions in FRING vary only very slowly with time (at least for sources with good a priori positions).

correlator/axis_offset.txt · Last modified: 2010/09/01 19:17 by cormac