|
|||
| Joint Astronomy Centre |
|
|
|
| Show document only |
SCUBA Update
As reported in the last JCMT newsletter SCUBA has had a long-standing problem with vibrationally-
induced microphonics from the closed-cycle cooler (CCC). These levels have been reduced significantly
over the last year or so, but residual features still remain in the signal band of the instrument (these
features would significantly degrade the achievable signal-to-noise, particularly for the "raster-scan"
observing mode). As a result of this it was decided to remove the CCC, and replace it with a liquid nitrogen
can. This modification means that the internal optics will be at a higher temperature, but the main radiation
shield will be significantly colder than previously. The net effect is that the background photon noise on
the bolometer arrays is increased by only 1% — thereby having minimal effect on the receiver
performance. The new nitrogen can will be tested in mid-August, when for the first time the instrument
will be complete (both arrays and the photometric pixels will be installed).In recent months the performance of the instrument in the laboratory has been most encouraging (with the CCC switched off!). We are now routinely recording data through the data acquisition system, using the dedicated SCUBA user-interface and observing queue. The observing modes are gradually being commissioned in the lab; "jiggle-map" and "flat-field" have recently been successfully demonstrated. Optical tests using a telescope simulator have shown that the beam profiles at 850 microns are very close to theoretical, and not significantly distorted at the edges of the array. Jiggle-maps of a point source (essentially equivalent to beam map on the telescope) are nice and circular, again, even at the edges of the array. We've even managed to image a extended source — a cut-out, assymmetric "K" shape! This was important to demonstrate jiggle-map of an extended source (about 1 arc minute), and also to check that field distortion corrections had been applied correctly. Another highlight has been the successful demonstration of the internal calibrator. This corrects for variations in bolometer sensitivity, due to their construction, and also due to their non- uniform response to background power (caused by changes in sky emissivity) and source brightness. SCUBA is now scheduled for delivery to the JAC just before Christmas of this year. There will be a period of 4-5 weeks in Hilo when the the instrument will be assembled and checked-out before going up the mountain. This period will also be used for JAC staff training. We estimate that it will require up to a further 4 weeks to fully install SCUBA on the telescope and complete the necessary pre- commissioning system checks. Astronomical commissioning is expected to begin in early March of next year. Wayne Holland, JAC SCUBA Project Team
|