Observing at the James Clerk Maxwell Telescope (JCMT)

What I’m meant to be observing

I am meant to be observing for the Cosmology Legacy Survey (CLS) using the Submillimetre Common-User Bolometer Array (SCUBA). The CLS is a survey of high redshift galaxies at wavelengths of 450 and 850 microns. Unfortunately the CLS needs the best weather conditions, and we have pretty poor weather this week.

The way surveys work is that lots of astronomers help out taking the observations and the results are shared by the community. So I won’t necessarily be using the data that I obtain. The telescope is actually controlled by an experienced operator who works permanently at the site. The attending astronomers are just there to help make decisions about what to observe and also for safety reasons  – there must always be two people at the telescope. Typically visiting astronomers will observe for about 5 nights, although it varies. They then go home and the next visiting astronomer takes over. This system has the added advantage of giving students experience of working at a telescope.

Weather Conditions

There are a number of factors that effect our observing. We can’t observe if humidity is too high, 85% is a rough guide, or if there are strong winds. Assuming the humidity and wind speed are ok we then have to consider the optical depth. This is a measure of how much radiation is actually reaching the telescope. The sub millimetre wavelengths that we use are affected by the amount of water vapour in the atmosphere. The more water vapour, the poorer the optical depth and the poorer the quality of the observations. We measure optical depth using something called tau – this is defined as -ln(I/Io) where I is the observed intensity of the radiation and Io it the actual intensity of the radiation. So we are measuring how much of the radiation gets through the atmosphere. As we are taking the natural log of the ratio, the smaller tau the better.

For CLS we need a tau of 0.1 or less, but so far we haven’t had anything this low.

What we are actually observing

JCMT operates a queuing system. Time on the telescope is divided up between the major surveys (of which CLS is the biggest), but if conditions are not right for observing that survey we look at the queue to see what we can observe instead. This takes into account

a) the priority awarded to the project,
b) whether the target is visible (Most targets can only be observed at certain times of the year and of the night),
c) the recommended tau for the observation

Our best tau values this week have allowed us to take some observations for two other SCUBA2 surveys – The SCUBA-2 “All-Sky” Survey (SASSy) and The Nearby Galaxies Survey (NGS).

However, much of the time the tau has been too high for these projects too and so we have been observing with the heterodyne instruments –  HARP and Receiver A. These measure spectral lines.

Targets can often be observed for a short period in good weather or a long period in bad weather. The best weather is generally reserved for the high priority surveys and so one tactic is for smaller projects to request longer observations in poorer weather. This allows them to get telescope time when the tau is too high for the surveys.

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