...
Furthermore -- if we understand correctly, gtpphase finds the best ephemeris for the whole file being analysed, and then applies it to all gammas in that file. If we're stacking photons downlink by downlink, that's fine. But in Bordeaux our tendancy has been to create a single large FT1 file for a given pulsar for a long integration time (e.g. 1 year Service Challenge simulation, et cetera) in which case you have the worst of both worlds, i.e. only 3 rotation parameters to cover a very long exposure time with a single ephemeris. We're unclear about what Standard Recommended Procedure is (sorry -- we didn't read your recent update of the Workbook, tell us if we should).
Feedback
Here are interesting reactions from Dave Thompson and Roger Romani:
Date: Tue, 04 Dec 2007 13:45:09 -0500
From: Dave Thompson <David.J.Thompson@nasa.gov>
To: D.A. Smith <smith@cenbg.in2p3.fr> Cc: Alice Harding <ahardingx@yahoo.com>, Roger W. Romani <rwr@astro.Stanford.EDU>
Subject: Re: limitations of pulsar science tools (fwd)
Hi David,
This issue was a lot easier on EGRET, when we had viewing periods.
The danger with a multi-year ephemeris, even with many fitting parameters, is that it may still have residuals that go unnoticed (and there is that pesky problem of possible changes in DM). One of the reasons that 1951+32 was the last pulsar to be found in the EGRET data was that we were trying for quite a while to use a single long-term ephemeris, and it just didn't do the job. Breaking up the data into pieces that match simple timing solutions may seem harder operationally, but it adds some confidence that the phase assigned to each photon is good. Once that is done, adding up the results is pretty simple. I'll go for a high-confidence result over an elegant one every time.
Dave Thompson
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Date: Tue, 04 Dec 2007 16:02:21 -0800
From: Roger W. Romani <rwr@astro.stanford.edu>
To: D.A. Smith <smith@cenbg.in2p3.fr> Cc: rwr@astro.stanford.edu, ahardingx@yahoo.com, David.J.Thompson@nasa.gov, rwr@astro.stanford.edu
Subject: Re: limitations of pulsar science tools (fwd)
Hmmm -- I think that if we are going to have a robust science tool PSR environment, we need at least a bit of a wrapper code that takes each selected photon and grabs a default ephemeris from the D4 data base before computing the phase. If we use a simple critereon (e.g. TOA MJD furthest from the validity boundary of the chosen ephemeris, or chose ephemeris with smallest rms among those covering the TOA MJD), then we can make the existing tool general enough to be usable. Certainly all we need is a decent set of phases for any given photon set. Forcing high polynomial approximations to fit timing noise is NOT the way to go, though...
What do you think? If we want this, who writes it? -- Roger
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Dave Smith comments: "wrapper code" assumes that nothing changes in the ST's, but probably the best technical solution is a modification of how the "best" ephemeris from the D4 for a given photon date is chosen, given that file start & stop times in general won't match ephemeris begin & end times. Before getting into "who" writes the code, let's get clear with the SSC about "what" is the right solution.
In any case, we have a consensus from the users' side that piecewise ephemerides is the way to go. So it seems that a change to gtephcomp's ephemeris selection algorithm may be what we need.