The second source of photometric standards available at the WHT are the
unpublished cluster sequences of L. Davis (NOAO). These are sequences
of 20-30 stars near galactic clusters, with typically 15 
V
20 and
0.5 (B-V) 1.5, taken at the 0.9m Kitt Peak
telescope.
These fields are approximately
4.6 x 7 arcmin in size, and will thus fit within
the 7.2 arcmin field of the TEK2 chip at prime focus.
Since the stars in these
fields are generally fainter than the
Landolt standards, they can be used with longer exposure times.
However, observers should restrict themselves to stars brighter
than V 
18.5, as the photometric errors increase rapidly for
stars fainter than this.
Observers are reminded that these are unpublished data and are
available on a user-risk basis. Because of the limited color coverage
and small number of repeat observations for some field/filter
combinations, observers should not rely solely on these standards,
but should supplement with other standards, e.g. the Landolt standards
described above (in particular, there are few blue stars
[(B-V) 0.5] in these cluster sequences).
Finder charts and data
for these sequences are given in the next section below, and may also be
found in the above-mentioned green binders.