4 Calibration

The calibration system can be configured using the -cal option. For SCUBA this option can be used to decide how to obtain the sky opacity, which gains to use and which bolometers should be turned off.

Jiggle map and photometry observations are automatically calibrated by the pipline (maps can be calibrated in Jy/beam or Jy/arcsec${}^2$ by configuring the recipe).

The arguments to -cal should be comma-separated keyword=value pairs. The recognised keywords are:

• gains

  This keyword controls the way that gain values are determined. The options are:

  default:

  Use generic values for the gain (e.g. use 240 Jy/V at 850 microns). This is the default option.

  index:

  Derive gains by using the gains index file. The index file is updated every time a calibration observation is reduced (e.g. photometry on a planet). The nearest calibration (in time) will be used for the gain determination. An error will occur if no calibration observation has been taken (or reduced) before an observation is reduced.

• tausys

  This keyword controls the behaviour of the tau correction. The options are:

  CSO:

  Derive taus for all wavelengths by using the CSO tau stored in the header of each frame. This only works if the CSO tau is being updated.

  skydip:

  Derive taus by using the skydip index file. The index file is updated every time a skydip observation is reduced. The nearest skydip (in time) will be used for the extinction correction. If no skydip observation has been taken (or reduced) before an observation is reduced the CSO tau value will be used. Warnings are issued if the selected skydip was taken more than 3 hours from the current observation. index is an allowed synonym for skydip.

  a number:

  If a number is given it is assumed to be a CSO tau value. A value of 0.0 will turn off extinction correction.

  850skydip:

  Derive taus by using the 850 skydip values. Tau values from other filters are all derived from the 850 value using the standard tau ratios. If no suitable skydip can be found the CSO tau value will be used. Warnings are issued if the selected skydip was taken more than 3 hours from the current observation.

  dipinterp & 850dipinterp:

  Same as skydip and 850skydip except values either side of the current observation are interpolated to find the current tau. This is not the same as specifying two tau values to the SURF extinction task since that will calculate the interpolated tau throughout the time of the observation rather than just calculating the value for the start. If only one value can be found then that value is used; if no values are found then CSO will be used. Warnings are issued if skydips were taken more than 3 hours from the current observation.

  csofit:

  Derive the CSO tau (and hence the tau for the specified filter) by using a polynomial fit to the CSO data for each night. This is the most accurate method of determining the opacity but is only available for nights between April 1997 and February 2001 (more fits will be provided as they become available). This method has the added advantage that photometrically unstable nights will not have a fit and therefore will not be processed (useful when processing large numbers of observations automatically).

• badbols

  This keyword controls the selection of bad bolometers (i.e. bolometers turned off by the pipeline). The options are:

  index:

  Uses an index file containing bad bolometers. The index file is written by the SCUBA_NOISE recipe and contains a list of all bolometers that had noise (from REFLECTOR observations) greater than the specified threshold limit (currently the default threshold is 100 nV).

  file:

  Uses a file badbol.lis found in ORAC_DATA_OUT. The file should contain a single line containing a space-separated list of bolometer names⁵. The file may contain a line such as:

    A1 E2 H7 I5

  This is the default system.

  a list of bolometers:

  Finally, it is possible to specify a list of bolometer names. This list should be colon-separated, e.g.: ‘H7:A2:b3’

Here are some examples:

Derive opacities from the index file but use the default gains.

Use a constant value for the opacity and use the derived gains from the index file.

Use the 850 micron skydips either side of an observation to derive all taus. Use the standard gain values.

Turn off bolometers a3 and c14.