DELTAT

                      function which returns the expected time resolution of a
                      periodogram or similar for a given period as
                      derived from the total length of the
                      observation ("timebase")

                      helpers: estimate for fourier transform

                      result = deltat( period_of_interest, total_timebase [,/rel] )

                      p (period_of_interest): period at which time
                                              resolution of
                                              periodogram is to be
                                              evaluated
                      t (total_timebase):     total length of run
                                              ("timebase")

                      rel: if set, return relative resolution instead
                           of absolute one

                      time resolution if periodogram is given over
                      period instead of over frequency (which would
                      be the more natural way)
                      beware that this is NOT the time resolution of
                      the original lightcurve, i.e. the sampling
                      rate (which would give us something like the
                      shortest measurable period), but rather a
                      measure of how exactly you can determine a
                      given period!

                      relative value instead of absolute one if requested

                      none

                      none that I can think of

                      none that I can think of

                      determine frequency resolution from timebase:
                         deltanu = 1/t
                      first point:
                         p = 1 / (1/p) = p
                      second point (separated by what corresponds to
                      deltanu):
                         p-deltat = 1 / (1/p + deltanu)
                                  = 1 / (1/p + 1/t)
                                  = p*t / (t + p)
                      then separation is given by
                           deltat = p - (p-deltat)
                                  = p - p*t / (t + p)
                                  = p * (1-t/(t+p))

                      t=findgen(864000)
                      plot,t/86400.,deltat(12116,t)
           shows how delta t for a period of 12116 sec gets smaller
           the more days the observations has (from 1 to ten days)
           similarly,
                      plot,t/86400.,deltat(12116,t,/rel)
           shows how the relative error decreases (to below 3% after ten days)

                      Version 1.0 2001/07/25 Sonja L. Schuh

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