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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Reducing SCUBA Data at the James Clerk Maxwell
Telescope
T. Jenness and J. F. Lightfoot 1
Joint Astronomy Centre, 660 N. A`oh•ok•u Place, University Park, Hilo,
HI 96720, USA
Abstract. The Submillimetre Common­User Bolometer Array (SCUBA)
is now operational at the James Clerk Maxwell Telescope (JCMT). This
paper describes the SCUBA User Reduction Facility (SURF) data reduc­
tion software that has been developed for use with SCUBA.
1. Introduction
The Submillimetre Common­User Bolometer Array (SCUBA) is the recently
commissioned instrument on the James Clerk Maxwell Telescope (JCMT) 2 on
Mauna Kea. SCUBA is a bolometer array which means that for the first time
JCMT has imaging capabilities in the submillimetre.
In order to maximize the number of bolometers in the array, they were
packed on a hexagonal (rather than rectangular) grid. This means that the
software has to reconstruct the data into an image. Moreover, because the
bolometers are larger than half the beam width, the image cannot be fully
sampled in a single exposure. In order to fully sample the image, the secondary
mirror is ``jiggled'' to several adjacent positions and the software has to keep
track of what sky position is associated with each data measurement. Finally,
because of how SCUBA is mounted (on the Nasmyth platform), the sky rotates
as the telescope tracks; this is also something the software must correct for.
2. SCUBA Observing modes
There are three major SCUBA observing modes, and these need some individual
treatment from SURF, the Scuba User Reduction Facility, which is run o#­line
by the user:
Photometry: This mode is used to measure the flux of a point source. In its
simplest guise the observation involves pointing a single bolometer at the
source, measuring the signal, chopping and nodding to reduce the e#ect of
sky emission, and integrating to build up the signal­to­noise. SCUBA also
1 Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, United Kingdom
2 http://www.jach.hawaii.edu/
216

Reducing SCUBA Data at the James Clerk Maxwell Telescope 217
allows for 2 or 3 bolometer photometry (chopping on the array), simulta­
neous photometry using the long and short wave arrays, and jiggling on
source to reduce the e#ects of seeing.
Jiggle­mapping: This is the main imaging mode for sources which are smaller
than the array (i.e. less than about 2 arcmin). As the SCUBA arrays are
not fully­sampled and not on a rectangular grid, images can not be taken in
the same way as for an optical CCD array. At least 16 individual secondary
mirror, or `jiggle', positions (each of 1 second) are required to make a fully
sampled image (64 jiggles are required if both the long and short wave
arrays are being used simultaneously). The SURF data reduction package
must take these data, combine them and regrid them onto a rectangular
grid.
Scan­mapping: Scan mapping, as the name suggests, is performed by the tele­
scope scanning across an area of sky while simultaneously chopping. Dur­
ing the scan, care is taken that enough bolometers measure the same patch
of sky so that the image is fully sampled, so it does not require jiggling
or additional scans. This mode is suitable for sources that are extended.
SURF combines the two beams that result from the chopping into a single­
beam map.
3. SURF -- The SCUBA User Reduction Facility
The real­time SCUBA observing system first demodulates data in the trans­
puters -- it takes data at 128Hz but only supplies data every second. It also
provides a `quick­look' display of the data based on some rudimentary data
reduction done in the transputers.
The demodulated data is then ``raw data'' as far as SURF, the o#­line
system, is concerned. SURF aims to provide publication quality data reduction
and currently requires user input and feedback.
The `quick­look' display provided by the transputers can also be eaves­
dropped by a remote observer using a WORF system (Economou et al 1996;
Jenness et al 1997).
SURF (like the real­time system) is written in the Starlink 3 software envi­
ronment and therefore runs on Sun Solaris, Digital Unix and Linux. Computa­
tionally intensive routines are written in fortran as ADAM tasks that form
part of a monolith. Applications such as data reduction pipelines (calling multi­
ple tasks from the monolith), string handling routines (e.g., generating observ­
ing logs) and small data reduction tasks were more suited to modern scripting
languages than to fortran and it was decided to use the perl programming
language (Wall et al. 1996) for these.
Since the software is written in the Starlink environment, Starlink's N­
Dimensional Data Format (NDF) is used for all file I/O. This format provides for
error arrays, bad pixel masks, storage of history information and a hierarchical
extension mechanism and SURF makes full use of these features. In order to
3 http://star­www.rl.ac.uk/

218 Jenness and Lightfoot
get the most out of using perl it was necessary to write a perl interface to the
fortran NDF and HDS libraries. This module (distributed separately from
SURF) provides complete access to all of the routines in these libraries allowing
for full control of the NDF and HDS structures from perl.
To improve maintainability of the package all source code is kept under revi­
sion control (RCS) and subroutine libraries are shared with the on­line observing
system.
SURF provides the tasks necessary to convert raw demodulated SCUBA
data into a regularly­sampled image:
. flatfielding the array
The di#erent response of each bolometer is removed. Note that the flatfield
remains constant with time and does not need to be re­measured every
night.
. correcting for atmospheric extinction
This task simply calculates the extinction due to the atmosphere by using
the zenith sky opacity (tau) and calculating the elevation of each bolometer
during each 1 second sample.
. removing spikes (manually and in software)
Occasionally spikes are present in the data (instrumental glitches, cosmic
rays etc) and these can be removed manually or by using software.
. removing sky fluctuations from JIGGLE/MAP and PHOTOM data.
The submillimetre sky is very sensitive to sky noise caused by fluctuations
in the emissivity of the atmosphere passing over the telescope. These
variations occur in atmospheric cells that are larger than the array and
the resultant noise is seen to be correlated across the array. At present
this sky noise is removed by analysing bolometers that are known to be
looking at sky and removing this o#set from the data.
. generating a rectangularly sampled image.
As described earlier, SCUBA data is under­sampled (for jiggle data), taken
on an hexagonal grid and subject to sky rotation. SURF calculates the
position of each bolometer during each 1 second sample (in a number
of coordinate systems: RA/Dec, Galactic, Nasmyth, Azimuth­Elevation
or moving centre (planet)) and regrids these data onto a rectangularly
sampled image.
Once the data have been regridded the image can be converted to FITS
(a full astrometry header is provided), if necessary, and analysed in the normal
way.
The latest version of SURF is freely available from the SURF home page 4 .
Additionally the software is distributed by Starlink and available on their CD­
ROM.
4 http://www.jach.hawaii.edu/jcmt sw/scuba/

Reducing SCUBA Data at the James Clerk Maxwell Telescope 219
4. Future
This software is being developed actively at the Joint Astronomy Centre and we
hope to implement the following features in the next few months:
. Improve the automated data reduction by using a perl interface to the
ADAM messaging library (already in development) and the UKIRT ORAC
system (Economou et al. 1998). This will be implemented at the telescope,
triggered by the SCUBA observing system, and o#­line, triggered by the
observer.
. At present sky noise removal has not been implemented for SCAN/MAP
(since, unlike jiggle­map data, there is no concept of a `sky' bolometer).
It is hoped to implement a system for handling sky noise in due course.
. Implementing an Maximum Entropy reconstruction algorithm for SCAN/­
MAP data (e.g., similar to the DBMEM implementation for the single pixel
UKT14 (Richer 1992)) in addition to the standard Emerson, Klein and
Haslam algorithm. A maximum entropy implementation would treat the
reconstruction of the map as a direct linear inversion problem and would
result in an image that is deconvolved from the beam. This technique has
the advantages of enforcing positivity in the image, a proper treatment of
correlations in the two dimensional data and allowing for the possibility of
resolution enhancement for high signal­to­noise data. It is also hoped that
this implementation will be able to handle sky­noise removal by treating
sky­noise as additional free parameters for the reconstruction.
Acknowledgments. TJ would like to thank Frossie Economou for her help­
ful discussions during the development of this software.
References
Economou, F., Bridger, A., Daly, P. N., & Wright, G. S., 1996, in ASP Conf. Ser.,
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volume
Emerson, D. T., Klein, U., & Haslam, C. G. T., 1979, ApJ, 76, 92
Jenness, T., Economou, F., & Tilanus, R. P. J., 1997, in ASP Conf. Ser., Vol. 125,
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