For bluer wavelengths the original flat field should have a fringe frame added to it and then this new flat field should be divided into the program frame. Take as many sky flat fields at twilight as possible. This option is to keep the telescope at zenith, and switch the upper end flat field lamps using the module by the data acquisition computer. At least two standard fields should be observed per night at different air masses to be able to correct for atmospheric absorption. A selection of photometric standard fields is given in Appendix E .
Some people defocus the telescope in order to get better photometric measurement accuracy.
For the fields in Appendix E, exposures times are typically 30 and 90 sec for B, V, R and I filters.