PieTown  PANEL   MAP      Adjustment List      Adjustment Plot

1.  Introduction:  HOLOGRAPHY: aips++ glossary
In radio astronomy, a method for refining the feed and panel alignment, thus for improving the aperture efficiency or antenna gain of a radio telescope. By scanning an antenna's beam over a raster around an unresolved radio source, and using another antenna pointing at the same  source as a reference, information is obtained about the amplitude and phase distributions of the signal reflected from the antenna surface. These distributions are used to specify corrections (if needed) for the focus and alignment of the feeds or of the positions of individual panels in the reflector.

For the VLBA, there is no reference antenna nearby, as there is for the VLA in D-array. The scheme used currently is to track a geostationary satellite  beacon of  very narrow bandwidth  (0.7Hz  :  here is a  spectrum), and so get sufficient SNR with a small reference horn mounted on the 25m antenna.

The main antenna signal is picked off above the feed-cone by a mirror & horn, so as to leave the regular set of feeds undisturbed.  The 2 signals go into the VLBA 2cm downconverter, and are mixed down to ~10KHz and filtered by the BBC's to  62KHz bw.  External filters cut the bandwidth to 12.5KHz. The signals are then digitized at 25KHz with 12bits,  transformed by a 32K FFT, and cross-multiplied to make a software FX correlator running on a low-end PC. The PC also logs az/el offsets from the VME antenna computer. Mike Revnell designed and built this in short order.

A 4-hr raster of +- 1.2 degrees gives about 0.7m resolution on the primary dish surface.  The  same PC is used to correct for phase and pointing drifts, and transform the  beam pattern into the aperture amplitude and phase distribution with the AIPS  task HOLGR.
 

2. SCHEMATIC    and gory details are in this  CHECKLIST.
 

3.  Pictures:
Old  hardware,  with reference horn mounted on quadrupod leg,   was used for these  surface-error  maps  of
VLBA_PT .   The 2 images correspond to subreflector positions over the 3mm feed (202 deg) and  almost
diametrically opposite (34 deg).    Note the 2 main reflector panels up ~2mm for testing.

New feed-mounts    were installed 2002 March 13, allowing rotation position every 20 deg.  The reference
feed was also moved closer to the antenna axis, but behind the subreflector.
Repeatability is  now  about 100 microns, seen in these  surface maps

Here is a  table  of  antenna efficiencies and surface errors, to be filled as holography visits the antennas.
 

4.  A parallel effort to  measure and re-surface the subreflector surfaces is headed by Jon Thunborg at the VLA antenna barn.
Measurements and resurfacing on the spare subreflector  are in progress .  (Report, 2002 Jul  22 VLBA test meeting by Thunborg.)