The surface of the main 15 meter 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. The dish is regularly adjusted for seasonal thermal effects using holography with receiver RxH3. The rms of the surface is typically 24 microns. This number does include the imperfections in the individual panels.
The holographic system consists of:
- A 2-channel source located inside the UKIRT dome and a 4-channel receiver (two frequencies at orthogonal polarizations) in the JCMT receiver cabin.
- Two frequency operation at 80.35 and 160.7 GHz.
- Full-phase measuring system (Main and reference signals at the two frequencies).
- Frequency stepping to eliminate the effects from spurious signal paths (typically we use 16 frequencies with a range of a few 10 MHz).
- Backend with simultaneous high/low gain ranges and cross-correlators.
- Real-time VME based data acquisition system taking samples at a rate of up to 1 kHz with the telescope performing a raster scan at speeds of up to 400 arcseconds per second.
- Time-stamped data samples for better accuracy.
Observations at 80 GHz are done for monitoring purposes (typically 50 minutes per map).
Observations at 160 GHz are done for adjustments (110 minutes per map). The resolution of these maps is about 8cm.
The latest surface map was made on 20160430 and the derived rms was 23.4 micron.
