[vsnet-chat 240] Re: PDM troubles

[Fraser Farrell <fraserf@dove.net.au>]

G'day all,

On 1997-04-18 tkato@kusastro.kyoto-u.ac.jp said:
   >I look forward to hearing from other "period analysts" on
   >determination of errors of period estimates, since this point is
   >quite often a focus of debate.

I recently read a paper by Fernie (PASP 101, 225-228 (Feb 1989)) which
describes a simple algorithm for determining period uncertainty.

Fernie's method does a least-squares fit of a parabola to the "valley"
(PDM plot) or "peak" (Fourier Transform plot).  For a parabola of the
form:  P = a*f*f + b*f + c   (where P="power", f=frequency), the
relevant PASCAL equations are:

true_frequency := -b/(2*a);    {the min or max of the parabola}
sigma := sqrt( abs( (4*a*c-b*b)/(4*a*a*(N-1)) ) );

where N= number of observations, and "sigma" is the standard deviation
of the true frequency.  It is assumed that the analyst has already
produced a frequency-domain periodogram of some kind, and that
parabolic approximations to the interesting bits are possible.

This seems to be an easy cure to the problem of period errors, so I'm
adding this function to VSSPlot.  Or have I made a bad decision?



cheers,
Fraser Farrell

http://vsnet.dove.net.au/~fraserf/   email: fraserf@dove.net.au
traditional: PO Box 332, Christies Beach, SA  5165, Australia

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