It has been shown that PHT-S chopped measurements (especially at low
fluxes) can be reasonably well modelled by Fouks function, including
signal jumps between chopper plateaux. Since the correction may change
the photometry it becomes necessary to check the algorithm by application
to known calibration stars. On the other hand, a spectral response function
can be derived from these measurements, then such function will be consistently 
applied to drift corrected measurements.

We propose to observe around 5 calibration stars (from 1.7 to 6.6 K magnitudes) 
which cover the range of all chopped observations perfomed up to now.