  The calibration of ground-based submillimetre observations has always been a
difficult process. We discuss how to overcome the limitations imposed by the
submillimetre atmosphere. Novel ways to improve line-of-sight opacity estimates
are presented, resulting in tight relations between opacities at different
wavelengths. The submillimetre camera SCUBA, mounted on the JCMT, is the first
large-scale submillimetre array, and as such is ideal for combatting the
effects of the atmosphere. For example, we find that the off-source pixels are
crucial for removing sky-noise. Benefitting from several years of SCUBA
operation, a database of deep SCUBA observations has been constructed to better
understand the nature of sky-noise and the effects of the atmosphere on
instrument sensitivity. This has revealed several results. Firstly, there is
evidence for positive correlations between sky-noise and seeing and sky-noise
and sky opacity. Furthermore, 850-micron and 450-micron sky-noise are clearly
correlated, suggesting that 450-micron data may be used to correct 850-micron
observations for sky-noise. Perhaps most important of all: if off-source
bolometers are used for sky-noise removal, there is no correlation between
instrument sensitivity and chop throw, for chop throws out to 180 arcsec.
Understanding the effects of submillimetre seeing is also important, and we
find that the JCMT beam is not significantly broadened by seeing, nor is there
an obvious correlation between seeing and pointing excursions.
