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A useful set of fainter standards are the cluster sequences of L. Davis (unpublished), with typically 15 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 Observers are reminded that these are unpublished data, and are available for use at your own risk.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.