RxB -> RxW relative pointing
RxB -> RxW relative pointing
Old !!! - collimation tweaks should keep offsets
= (0,0) (imc/20040706)
Introduction
Experience of using RxW at D band during 1999 and 2000 showed that there were servere difficulties in pointing
the telescope accurately, something that is particuarly important for D band given the small 8 arcsecond beam.
Only in exceptionally good weather was it possible to point on all but a handful of the brightest targets,
this meant that large slews were necessary to move from a feasible pointing source to the actual source to be
observed which only adds to the pointing errors.
Proceedure
In order to improve this situation we have measured the offset in pointing between receiever RxB and RxW. The
proceedure for using RxW should therefore be:
- Ensure the CBE is looking at mixer A for both receivers.
- Tune RxW as usual to the desired observing frequency (unless this is in a region of poor atmospheric
transmission, in which case tune to 665 GHz).
- Run the fe command and tune RxB to a frequency in the middle of its band (eg. 345.796 GHz)
- Point and focus RxB as normal - make a note of the focus values.
(Use the brightest source you can find as you will need to focus RxW using this).
- Run the fe command again and switch back to RxW (donot reissue the tune command though).
- Change the pointing offsets by -1" Az, -5" El (ie. command: setoff -1 -5).
- Focus RxW - note the offsets.
- Run fe again and go back to RxB.
- Point as normal using RxB on a source close to your actual observing target.
- Run the fe command again and switch back to RxW (donot reissue the tune command though).If necessary tune to your observing frequency.
- Change the focus offsets back to those noted earlier for RxW.
- Change the pointing offsets by -1" Az, -5" El (ie. command: setoff -1 -5).
- You are now ready to start observing.
- Repeat the above proceedure whenever you need to repoint (remember to manually change the focus offsets
when switching between the receivers).
Details of offset measurements
The offset between the two instruments was measured on 16th August 2000 on a night with the CSO Tau ~0.6 for
most of the night. Tests were done on Uranus, W3(OH) and Mars at various stages in the night. In all cases the
offset was measured between the A mixers in both receivers. We also measured the offset between mixer A and
mixer B in RxW. For each measurement the pointings were repeated several times until stable reading were
obtained (usually about 3 attempts).
Several postscript plots of this data are available.
A plot showing all the measurements in the order they were taken (labled 0,1,2,...,8,9,X). The
objects observed were:
- Uranus (Nos. 0,1,2)
- W3(OH) (Nos. 3,4,5,6,7)
- Mars (Nos. 8,9,X)
The weather was at its most stable for the first two observations of Mars. Uranus was also significantly lower
in the sky than the other sources (approx 30deg) which may explain why its points are clustered a little away
from the others.
Another plot shows the direction in which the measurement was taken (ie. measuring from RxB to
RxW - shown by a W - or from RxW to RxB - shown by a B). The scatter appears to be random - good!
An average of all these points produces an offset of -1, -5 which is also close to the points where
subjectively I felt the best data was taken (ie. points 7 and 8 on the plot).
The measeurements of the offset between mixer A and B in RxW were less clear but produced the following
values:
- 0, -2.8
- -1.6, -1.3
- -0.4, 0
- 0.7, 2.3
This produces an averge value of -0.33, -0.45 which given the scatter in the values can be taken as zero
offset (the offset was measured as less than 1 arcsec in the lab whilst the upgraded mixers were being
installed in Feb 2000).
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