We present maps , source lists , and derived number counts from the largest , unbiassed , extragalactic sub-mm survey so far undertaken with the SCUBA camera on the JCMT .
Our maps are located in two regions of sky ( ELAIS N2 and Lockman-Hole E ) and cover 260 arcmin ^ { 2 } , to a typical rms noise level of \sigma _ { 850 } \simeq 2.5 Â mJy/beam .
We have reduced the data using both the standard JCMT SURF procedures , and our own IDL-based pipeline which produces zero-footprint maps and noise images .
The uncorrelated noise maps produced by the latter approach have enabled us to apply a maximum-likelihood method to measure the statistical significance of each peak in our maps , leading to properly-quantified errors on the flux density of all potential sources .

We detect 19 sources with S/NÂ > 4 , and 38 with S/NÂ > 3.5 .
To assess both the completeness of this survey , and the impact of source confusion as a function of flux density we have applied our source-extraction algorithm to a series of simulated images .
The result is a new estimate of the sub-mm source counts over the flux-density range S _ { 850 } \simeq 5 - 15 Â mJy , which we compare with estimates derived by other workers , and with the predictions of a number of models .

Our best estimate of the cumulative source count at S _ { 850 } > 8 Â mJy is 320 ^ { +80 } _ { -100 } per square degree .
Assuming that the majority of sources lie at z > 1.5 , this result implies that the co-moving number density of high-redshift galaxies forming stars at a rate in excess of 1000 ~ { } { M _ { \odot } yr ^ { -1 } } is \simeq 10 ^ { -5 } ~ { } { Mpc ^ { -3 } } , with only a weak dependence on the precise redshift distribution .
This number density corresponds to the number density of massive ellipticals with L > 3 - 4 L ^ { \star } in the present-day universe , and is also the same as the co-moving number density of comparably massive , passively-evolving objects in the redshift band 1 < z < 2 inferred from recent surveys of extremely red objects .
Thus the bright sub-mm sources uncovered by this survey can plausibly account for the formation of all present-day massive ellipticals .
Improved redshift constraints , and ultimately an improved measure of sub-mm source clustering can refine or refute this picture .