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JCMT - Secondary Mirror information
The secondary mirror allows the JCMT to focus submillimeter radiation onto detectors built into receivers mounted at the primary focus in the receiver cabin (located below the main mirror) and at the two Nasmyth foci (located on the elevation axis). Aside from providing focal adjustments for the JCMT, the secondary mirror provides the means for eliminating the majority of sky brightness variations originating from the Earth's atmosphere. This is accomplished by vibrating ("chopping" or "nutating") the mirror such that the difference signal between two nearby points on the sky is obtained; since nearby points contain the same contribution from the atmosphere, to a large degree the difference represents the signal from a celestial target in one of the two beam positions. Common chop distances are 60 or 120 arcsec, and typical rates are 1.0 Hz for spectral line observations and 7.8125 Hz for broadband continuum astronomy. The carbon fiber tetrapod support contributes 6% to the telescope spillover losses. The secondary has a shallow cone in the centre - covering the area that corresponds to the shadow of the secondary on the main dish - that prevents radiation being reflected back into the cabin and causing standing waves. It doesn't eliminate all ground pickup, though. SMU Task
Other documents concerning the SMU
Reports on occasional issues (newest at top)
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