  We present the results of SCUBA2 observations at 450 micron and 850 micron of
the field lensed by the massive cluster A370. With a total survey area > 100
arcmin2 and 1 sigma sensitivities of 3.92 and 0.82 mJy/beam at 450 and 850
micron respectively, we find a secure sample of 20 sources at 450 micron and 26
sources at 850 micron with a signal-to-noise ratio > 4. Using the latest
lensing model of A370 and Monte Carlo simulations, we derive the number counts
at both wavelengths. The 450 micron number counts probe a factor of four deeper
than the counts recently obtained from the Herschel Space Telescope at similar
wavelengths, and we estimate that ~47-61% of the 450 micron extragalactic
background light (EBL) resolved into individual sources with 450 micron fluxes
greater than 4.5 mJy. The faint 450 micron sources in the 4 sigma sample have
positional accuracies of 3 arcseconds, while brighter sources (signal-to-noise
> 6 sigma) are good to 1.4 arcseconds. Using the deep radio map (1 sigma ~ 6
uJy) we find that the percentage of submillimeter sources having secure radio
counterparts is 85% for 450 micron sources with intrinsic fluxes > 6 mJy and
67% for 850 micron sources with intrinsic fluxes > 4 mJy. We also find that 67%
of the > 4 sigma 450 micron sources are detected at 850 micron, while the
recovery rate at 450 micron of > 4 sigma 850 micron sources is 54%. Combined
with the source redshifts estimated using millimetric flux ratios, the
recovered rate is consistent with the scenario where both 450 micron and 20 cm
emission preferentially select lower redshift dusty sources, while 850 micron
emission traces a higher fraction of dusty sources at higher redshifts.
[Abridge]
