The number, frequency and type of photometric standards will depend upon
the scientific programme.
At a bare minimum, one should observe enough standards
to encompass the range in colour and airmass expected for program
objects.
For standard broad bands (UBVRI) there is a set
of positions of photometric standards in the range 11.5<V<16.0 published
by A.U. Landolt (Astronomical Journal 104, 340, 1992), a copy
of which is kept in a folder in the control room. The positions of these
standards are kept in a catalogue on the Telescope Control Computer;
e.g. to move to Landolt standard SA 110 229, the TO simply types:
GOCAT 110-229. See La Palma Technical Note No. 95:
Landolt Faint Photometric Standards for more information.
Users should be aware that some of these standards have only been observed
2 or 3 times by Landolt, and may not be suitable if high-precision
photometry is required.
At Prime Focus the fainter standards should be used, and exposure times
will range from around two minutes at U to a few seconds at R and I.
To make this more quantitative, a star with U 
11.5 will
saturate (ie. counts 
60,000 ADU) in 30 seconds. Saturation times
for B=15.5, V=13.5, and R=13.5 are 120, 30, and 5 seconds respectively,
for 1 
seeing.
There
are several fields which would contain several standards within the 7.2
arcminute square field at prime focus. At the auxiliary port longer exposures
are possible, but individual stars must be observed.
A useful set of fainter standards are the cluster sequences of L. Davis
(unpublished), with typically 15 < V < 20 and
0.5 < (B-V) < 1.5. These data were taken at
the 0.9m Kitt Peak telescope, and the field size is
approximately 7 x 4.6 arcmin
in size
(thus fitting within the 7.2 arcmin field
of the TEK2 chip at prime focus).
Data and finder charts for these sequences may be found in
the above folder in the control room, and also in Appendix A in this manual.
Observers are reminded that these are unpublished data, and are available
for use at your own risk. 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 rather use
them in conjunction with other standard star data (e.g. the Landolt
standards described above). As well, observers should limit themselves
to stars brighter than V 
18.5, as the errors increase rapidly
for fainter stars.
Standard colour terms for the Tektronix chip at prime focus are given in Table 8 below; this information can also be found in the information browsers at La Palma and Cambridge and in the LPINFO system on the Cambridge VAX GXVG. See also Appendix B.
Table 8: Colour
terms for the TEK2 chip at Prime Focus. The
Colour terms are given in the sense of
(Observed - Landolt). These data were obtained with the
Harris BVRI filters and the RGO glass U filter.