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| Joint Astronomy Centre |
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The rms of the latest surface measurement. The JCMT telescope is a 15 meter submm telescope located at the summit of Mauna Kea, Hawai`i. The surface of the main dish consists of 276 panels, each of which can be moved by means of 3 motorized adjusters. The panels were fabricated at RAL and consist of honeycomb structures with thin aluminum surface panels glued on top. Typical rms values for surface roughness and deformations (after manufacturing) range from 10-15 microns. The dish is regularly adjusted for seasonal thermal effects using holography. The rms of the residual surface errors after an adjustment is made was typically 14-18 microns, as measured by holography using the old RxH2 receiver. This number does NOT include systematic errors resulting from the holography system itself, or imperfections in the individual panels such as surface roughness and scalloping. When these errors are also included in the error budget, the rms is expected to be somewhere around 28 microns. On September 9 1999, a life test of the new panel adjuster control system wrecked havoc in sector 11. After realigning many panels that had reached the lower limit of their travel range, large scale distorsions were still too large to be handled by the holography data reduction software. It took, all in all, two weeks of more manual alignments and careful tweeking to bring the dish surface back to a respectable shape. Apart from that mishap however, the surface has been good as of September 1998. DishNow! and BeamsNow! are new additions to the menu. With these, the state of the surface and the predicted beamprofiles can be viewed in almost real-time. Pictures of the surface and beam profiles are updated every 5-6 minutes (this program is not active at the moment). The dish was adjusted on October 7 1999, and remeasured on November 5 1999. No adjustments were made on November 5. No holographic measurements have been made between that date and August 2001 because it was thought that the new receiver RxH3 would be working in April 2000. However this took much longer than anticipated and first tests were made in August 2001. RxH2 measurements showed that the dish had relatively large errors (a total rms of 41 micron), fortunately the largest errors were in the outer ring. An adjustment was made on 5 September 2001, but new RxH2 data (7 September 2001) showed only little improvement. Compared to the previous map a coma term had changed sign, and two panels still showed large deviations (up to about 270 micron). These panels have broken motors and cause unrealistic fits to the RxH2 data. On 25 November we have tried to move 7 motors with moves from the August 15 RxH2 maps larger than 60 micron. These motors had been flagged and it was unknown whether they would move or not. New RxH3 maps made 4 December showed that at least some of them did move (among others the one near the center of the dish), and the surface rms is 66 micron. Omitting the panels in the outer ring (which has a smaller illumination) in the calculation the rms of the dish is 41 micron. We have adjusted the dish on 28 December (using old software to derive adjuster 'moves files', and made new RxH3 maps on 1 January 2002. The total rms improved to 40 micron (26 micron without the outer ring). However the numbers depend on the method of calculation. The above numbers were derived from 'moves files' for the adjusters, as done for the RxH2 maps. The RxH3 maps show structures within the panels and the real rms is larger (see e.g. here for some numbers). A new adjustment was made on 1 February. More recent rms values are in the range 34 - 40 micron.Since then most work has concentrated on making RxH3 more reliable by improvements in hardware and reduction software, on test observations in order to find out which adjusters need repair, and on investigations of standing waves due to the membrane. We now can drive 'moves files' (necessary for dish adjustments) from RxH3 data, and on 4 December 2002 the first adjustment of the dish was made based on RxH3 data, its reduction software and a 160 GHz map. The rms at nighttime improved from about 35 micron to about 25 micron. On 18 December this improvement was confirmed by comparing SCUBA FCF values with the 'old' and 'new' surface at 450 and 850 micron.
Short instructions for RxH3 maps.
Log of RxH3 observations and adjustment (tests).
The surface of other telescopes:
HHT
SMA
GBT
KOSMA:
holography
photogrammetry
LMT
IRAM 30m
Effelsberg |
