  Notions and limits from standard time series analysis must be modified when
treating series which are measured irregularly and contain long gaps. Classical
Nyquist criterion to estimate frequency range which is potentially recoverable
must be modified to handle this more complex situation. When basic exposition
of the modified criterion is given in earlier papers, some minor problems and
caveats are treated here. Using simple combinatorial arguments we show that for
small sample sizes the modified Nyquist limit may overestimate the obtainable
frequency range. On the other hand we will demonstrate that very high Nyquist
limit values which are typical to irregularly sampled data can often be taken
seriously and using proper observational techniques the frequency ranges for
"time spectroscopy" can be significantly widened.
