function spectrum, ts,tinc
;
; From a time serie of length T=tinc*N
; Return power spectrum and array of periods
;
  N=n_elements(ts)
  N21 = N/2
  ps=ABS(FFT(ts, -1))
  ps=ps(0:N21-1)

  period=tinc*N/(findgen(N21)+0.0000000000001)
  period(0)=99999999999.

  ret=findgen(2,N21)
  ret(1,*)=ps
  ret(0,*)=period

  return, ret
end