Glossary

chopping The secondary mirror is continuously moved on and off source at approximately 7 Hz in order to remove the sky to zeroth order. This is done in addition to standard jiggling.

demodulation Removal of the chop signal by the transputers. At this time, the raw data can not be accessed, only the demodulated data are stored.

exposure An exposure is the result from a complete set of switches. For example, in a JIGGLE/MAP or PHOTOM observation where the telescope is nodding the source between left and right beams, the data from each nod position is a switch and the reduced result `left switch' - `right switch' say, is an exposure. In a SCAN/MAP observation there is no beam switching so, in this case, an exposure is the same as a switch.

integration An integration means different things for different observations.

For one of the mapping modes it means the data from one fully-sampled coverage of the map area. In a JIGGLE/MAP, where full sampling is achieved by jiggling the secondary mirror, an integration is generally the results from one pass through the complete jiggle pattern. An integration is made up of one or more exposures.

Similarly, an integration for a SCAN/MAP observation is made up of data from the raster scans that cover the map area once.

For PHOTOM observations an integration is usually the average of a 9 point mini-jiggle.

For a SKYDIP observation, an integration is the data from a single revolution of the sector chopper in front of the cryostat window.

jiggle In order to sample an image fully the secondary mirror is moved once a second (whilst chopping) to move the position of the array on the sky; this is called `jiggling.' There are a complete set of jiggle positions for each integration. A PHOTOM observation can also jiggle in order to correct for seeing effects.

measurement A measurement is a group of integrations. Most MAP or PHOTOM observations will consist of only one measurement.

A FOCUS or ALIGN observation consists of five measurements (one for each secondary mirror position). A SKYDIP observation consists of one measurement at each elevation.

nod In order to correct for atmospheric variation the telescope is moved off-source in each exposure so that sky can be measured.

ODF The observation definition file (ODF) is a file containing a list of instructions for an observation with SCUBA.

sub-instrument SCUBA contains bolometer arrays and photometric pixels that can operate at several wavelengths simultaneously. Each of these is called a sub-instrument. They are:

• SHORT - the short wave array containing 91 bolometers
• LONG - the long wave array containing 37 bolometers
• P1100 - the single bolometer optimised for 1100$\mu$m.
• P1350 - the single bolometer optimised for 1350$\mu$m.
• P2000 - the single bolometer optimised for 2000$\mu$m.

switch The switch is the fundamental unit of data-taking in an observation. For example in a JIGGLE/MAP or PHOTOM observation each chunk of jiggle positions measured with the object in the beam of a telescope is a switch. Each scan across the source in a SCAN/MAP observation is also a switch.

tau ($\tau$) Submillimetre extinction is measured using the zenith optical depth, tau or $\tau$, this is a measure of the amount of water vapour present in the atmosphere. For a tau, $\tau$ at a given airmass, $A$, the attenuation due to the atmosphere is given as e$^{-A \tau}$. Note that tau is wavelength dependent and that the value quoted by the Caltech Submillimetre Observatory (CSO) is the $\tau$ at 225 GHz and will therefore be different at the other wavelengths used by SCUBA (see [31] for details of the variation with SCUBA filters; or [32] for details on the variation seen with UKT14).