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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 systematic deformations of a panel were estimated (in 1996) to be 12 and 13 micron, respectively. The dish is regularly adjusted for seasonal thermal effects using holography. The rms of the residual surface errors after an adjustment is made is typically 24 microns, as measured by holography using the new RxH3 receiver. This number does include the imperfections in the individual panels, in contrast to results from RxH2 before 2002. Some older information about the surface until the first adjustments with RxH3 is given here. In November 2002 the new panel analyses software was first used to derive moves files from RxH3 maps to adjust the surface. Since then we the rms at nighttime was improved from about 35 micron to about 24 micron. On 18 December this improvement was confirmed by comparing SCUBA FCF values with the 'old' and 'new' surface at 450 and 850 micron. Currenly we use 160 GHz maps to derive moves files since they proved to be more accurate (rms about 5 micron lower) than 80 GHz maps (which take less time to obtain). In March 2003 we tried to move 30 motors with certain error status, and they appear indeed to move. This caused improvement of an additional few micron in the rms. There remained 35 adjusters which cannot be moved. However there are systematic deviations in the derived moves that are probably time (temperature) dependent - see e.g. here. This will be investigated as more maps are being obtained. In Autumn 2003 improvements were made in the adjuster electronics which resulted in a repair of the lines of faulty adjusters in sectors 2 and 7, some of which had large deviations. The panels with these adjusters could be moved. However there are now two new series of adjusters (in sectors 4 and 12) which have phase errors, but these panels have only small deviations. At the moment (April 2005), 30 motors cannot be moved, 4 of which have large errors (>60 micron) and 5 have medium large errors (30-60 micron). February 2005. Insulation and fans cooling the centerbeams in the cabin were repaired. This improved the surface accuracy at the beginning and end of the night. See link below. In October 2005, RxH3 was removed from the cabin because of modifications necessary for SCUBA2. RxH3 was returned in March 2007, but the first useful maps could only be obtained in September 2007. Likely the surface and surface changes changed because the cabin door was removed as well as RxB3. The cabin is much colder and the temperature stabilization of RxH3 is problematic. On 26 September the surface rms was measured to be 25.7 micron, and an adjustment was made on 10 October. On 13 October 2007 an rms of 22.3 micron was measured, the best rms measured with RxH3 until now.
The surface of other telescopes:
HHT
SMA
GBT
KOSMA:
holography
photogrammetry
LMT
IRAM 30m
Thermal control
Effelsberg |
