SCUBA comes to JCMT!

The good news...

SCUBA was delivered to the JAC from ROE in early April and, after a very successful assembly and cooldown in Hilo, during which time local staff were trained in assembly and operation of all aspects of the instrument, it was transported to JCMT where it was reassembled and mounted onto the telescope. There were no major problems, a tribute to the careful planning and hard work of the team and the local staff. Base temperature of 75 mK was reached on 14th June. Tests at this time revealed that there were only 2 dead pixels out of 131 and 2 more that had very high noise, leaving 127 operational pixels out of 131, or 97%.

There was a three-week period of daytime testing before first light was obtained on July 8th. The images obtained that night were not terribly exciting, just Uranus and Jupiter, and were somewhat distorted due to a timing mismatch between SCUBA and the chopping secondary mirror. However, such teething problems were rapidly sorted out and in figure 1 we present what was the first 'proper' image of Mars taken a little while later. Mars is effectively a point source in the SCUBA beam so that its image represents a beam map of the centre pixel on the LONG-wave array.

Figure 1: Mars beam map

Things up to this point had gone so well that some of us had a feeling that something had to go wrong son, and sure enough it did! A wiring failure inside the cryostat led to the loss of signals from the SHORT-wave array the very day after first light! Fortunately, we did obtain enough information on the first night to allow us to estimate that the sensitivity of the SHORT array at 450 microns was close to that predicted.

Over a month since first light very successful progress has been made in commissioning the different observing modes of SCUBA and ironing out the inevitable problems and bugs in such a complicated system (and one so radically different from what has been used before). We are now able to offer service observing during the latter part of the extended semester 96B. There will be a break in the commissioning programme for September when SCUBA will be warmed up and opened to fix the wiring fault that caused the loss of the SHORT-wave array and a few other problems.

Pointing, focussing, skydipping, point-source photometry and jiggle-mapping of sources of size less than the field-of-view of the instrument have all been successfully commissioned. At the time of writing, we are starting the procedure of flat-fielding the LONG-wave array. In figure 2 we show a map of the galactic star formation region NGC 2071 which took just over 5 minutes of integration on a wet night and in figure 3 we show an image of W 48, another galactic star-forming region. Readers should note that these images are not properly flat-fielded or calibrated, and are basically just semi-raw data.

Figure 2: SCUBA image of the NGC 2071 region.

The not-so-good news...

As mentioned above, no measurements could be made with the SHORT array after the first night. This is due to a wiring failure which will be fixed when the instrument is warmed up at the end of August. Some channels also seem to be sensitive to vibration from the chopping secondary mirror. We think we have identified the area of the wiring that is sensitive and intend to fix this as well when SCUBA is open in September.

The sensitivity of the LONG-wave array is not as good at present as originally hoped. This is due to an excess of background radiation which we believe to be caused by a leak in a blocking filter, a new one of which is being built at QMW and which should also be replaced during the warmup in September. A similar problem had previously been found with the 3 photometric pixels and fixed.

Figure 3: SCUBA image of the W 48 region.

What's next?

As described above, there is a warmup scheduled for the end of August at which time it is hoped to fix the dead channels, reduce the sensitivity to vibration, and reduce the background on the LONG-wave array. After reassembly and cooldown, commissioning wil begin again in early October, when we will begin flat- fielding the SHORT array and also begin work on the more complex observing modes, such as scanning the array over large areas.

Service observations will be performed during the extended semester 96B in December, January and February. The call for proposals has already been sent out and the relevant technical details of SCUBA are reprinted in the following article.

Walter K Gear, JAC, SCUBA Project Scientist