Tempo2 DM polynomial is not a Taylor expansion
Issue #27
resolved
TEMPO, PINT, and the Tempo2 paper (Eqn 39 of Edwards et al.) treat DM, DM1, DM2, ... as a Taylor expansion around DMEPOCH.
Tempo2 instead uses a polynomial that is not a Taylor expansion. This is noted on line 122 of dm_delays.C. For consistency with other codes, and the paper that defines the Tempo2 timing models, this should be changed to a Taylor series.
Comments (9)
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Here is the par file
PSR 1748-2021E RAJ 17:48:52.75 DECJ -20:21:29.0 F0 61.485476554 F1 -1.181E-15 PEPOCH 53750.000000 POSEPOCH 53750.000000 DMEPOCH 56000.0 DM 223.9 DM1 0.1 DM2 -0.2 DM3 0.03 SOLARN0 0.00 EPHEM DE421 CLK UTC(NIST) UNITS TDB TIMEEPH FB90 T2CMETHOD TEMPO CORRECT_TROPOSPHERE N PLANET_SHAPIRO N DILATEFREQ N TZRMJD 53801.38605120074849 TZRFRQ 1949.609 TZRSITE 1
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And here is the .tim file:
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reporter
Just to clarify, this only starts to be a problem at DM2 and above. For DM1, the polynomial coefficient is the same as the Taylor expansion.
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Also, part of the ugly residuals plot above is due to a bad CLK line. Changing it to CLK TT(TAI) gives this (corrected) plot. Sorry about that! The overloading of the CLK parameter between the different timing codes can cause trouble.
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I am pretty sure I’ve discussed this issue before and the issue is that we will break previously published parameters. However, having recently run into other issues were people left “wrong” things in the code to avoid breaking backwards compatibility I’m inclined to agree. I’ll have a think about it, and maybe email George.
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reporter
Any update on this? What does George think?
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I did discuss this with George and he generally agreed. I’ve started to implement a fix that adds a parameter DM_SERIES which can be POLY or TAYLOR. Default is TAYLOR, POLY is only for backwards compatibility with old par files. I’m not 100% sure this is nessicary, but I’m also kind of interested in trying legendre polynomials at some point, so this adds potential for new possibilities.
My intent is that tempo2 will print a warning if you have DM2 +, but don’t specify DM_SERIES, pointing to this bug report. I’m not sure if this is nescisary or not either, but at least for now it’s good to make clear if there is a serious change.
I’ll send you a link to a branch to test once I’ve got it working.
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reporter
- changed status to resolved
This was resolved in a recent PR
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This issue is serious for anyone that uses multiple packages. I just made a sample par and tim file and computed residuals with TEMPO, Tempo2 and PINT. The TOAs were initially computed assuming that DMn represents a Taylor expansion. Since PINT and TEMPO both treat it that way, they have flat residuals. Tempo2 then uses a different definition where the DMn represent polynomial coefficients, not a Taylor series, so it has large residuals.