Raw astronomical data is normally processed through some kind of data reduction system to calibrate the data and combine small observations into larger ones with more areal coverage and/or deeper sensitivity. At the JCMT, the ORAC-DR pipeline is used to calibrate and combine raw data into different kinds of data products, such as images and IQU polarization datacubes. Each major observing mode at the JCMT has an associated ORAC-DR "recipe" that controls the data reduction.

The astronomical products available through the "Search for Processed Data" page are fully calibrated FITS files with complete WCS information, suitable for harvesting into a virtual observatory. The existing pipeline cannot process all of the older JCMT data into such products, but it is anticipated that more products will be produced as effort is available to develop the software.

Currently, observations taken in the following SCUBA observing modes can be grouped together into associations and processed into products. Although the grouping logic implemented in ORAC-DR is sometimes quite intricate, the majority of the associations can be defined for the various SCUBA observation modes as follows:

• JIGMAP - Jiggle maps: Defined by
  • Observing Mode = "map"
  • Sample Mode = "jiggle"
  • Filter in ("350n:750n", "450n:850n","450w:850w")
  Group by :
  • Project ID
  • Target Name (JCMT Object Name)
  • Filter
  • Chop Throw
  The chop throw affects the apparent calibration. The products are 2-D intensity images.

• JIGPHOTMAP - Jiggle maps with the "photometric" detectors: Defined by:
  • Observing Mode = "map"
  • Sample Mode = "jiggle"
  • Filter in ("350o:ap","850s:phot","850w:phot")
  Group by :
  • Project ID
  • Target Name (JCMT Object Name)
  • Filter
  • Chop Throw
  The chop throw affects the apparent calibration. The products are 2-D intensity images.

• EM2SCAN - Emerson II scan maps: Defined by:
  • Observing Mode = "map"
  • Sample Mode = "raster"
  • Chop Coord = "lo"
  Group by :
  • Project ID
  • Target Name (JCMT Object Name)
  • Filter
  There must be at least 4 different combinations of chop throw and position angle for the reduction to succeed. The products are 2-D intensity images.

• EKHSCAN - Emerson-Klein-Haslam scan maps: Defined by:
  • Observing Mode = "map"
  • Sample Mode = "raster"
  • Chop Coord != "lo"
  Group by :
  • Project ID
  • Target Name (JCMT Object Name)
  • Filter
  There must be at least 4 different combinations of chop throw and position angle for the reduction to succeed. The products are 2-D intensity images.

• JIGPOLMAP - Polarization jiggle maps: Defined by:
  • Polarimetry = "Yes"
  • Sample Mode = "jiggle"
  or use
  • Observing Mode in ("Polarization Mapping","Polarization Photometry")
  • Sample Mode = "jiggle"
  Group by :
  • Project ID
  • Target Name (JCMT Object Name)
  • Filter
  • Chop Throw
  The chop throw affects the apparent calibration. The product is an IQU polarized intensity datacube.

Query Results can optionally list the data reduction recipe in the column "DR Recipe", where SCUBA recipes customized for use at the CADC have names of the form SCUBA_*_CADC. (The CADC customization consists primarily in adding FITS headers needed by the archive.)

The Calibration Method column on the "Query Results" page shows the calibration method used by ORAC-DR, and can be used as quality indicator for the calibration. Whenever possible, SCUBA groups have been processed using the polynomial fits to the Caltech Submillimeter Observatory's Tau Dipper measurements to correct for the opacity of the atmosphere. Raw data files for which the polynomial fits are available are considered to be reliably "calibratable". Although it is an exaggeration to describe all of the raw data files for which the polynomial fits are not available as "uncalibratable", such data require considerably more care to reduce properly and carry a larger systematic error.

If there were enough valid members of a SCUBA association to produce such a calibrated data product then any association members that do NOT have CSO tau values available were excluded the final processed group. Such products are flagged with "Calibration Method" = CSO_FIT and are considered to have reliable calibrations.

Associations that do not have any "calibratable" members were still processed, but are flagged with "Calibration Method" = CSO to warn the user that the calibration is less reliable. In some cases the atmosphere was too unstable to be fitted with a polynomial, but the data are still usable with a larger systematic uncertainty in the calibration. In other cases the calibration is wildly off, in one notable case by a factor of 100,000. Such products are still present in the archive because they may be useful for positional measurements. Let the user beware!

Users who wish to see only "calibratable" or "uncalibratable" associations can specify the criterion Calibratable = "Yes" or "No" on the "Search for Processed Data" page.

Archive users who want to combine these "uncalibratable" data files with the calibratable members will need to retrieve the entire association using the association definitions given above and process the files themselves. Care should be taken to verify that the CSO tau meter was updating regularly, or to use one of the other calibration methods using skydips that are available with ORAC-DR.

For more details about SCUBA data processing see the ORAC-DR SCUBA data reduction manual and the ORAC-DR home page. The publication Towards the automated reduction and calibration of SCUBA data from the James Clerk Maxwell Telescope gives details about the current calibration techniques employed to derive the flux values provided in some of the archive data products.