Re: BJD_TDM revisited re ACP
Posted by
karenacollins on
Apr 07, 2019; 11:54am
URL: http://astroimagej.170.s1.nabble.com/BJD-TDM-revisited-re-ACP-tp1077p1125.html
Hi Dick,
Welcome to the AIJ forum. The AIJ internally calculated BJD_TDB values are consistent with the Eastman website values within ~0.25 seconds, or for the ultimate precision, AIJ can optionally extract the BJD_TDB values directly from the Eastman website.
Karen
On 4/6/2019 8:58 PM, richardaberg [via AstroImageJ] wrote:
Hi Gavin, Karen,
New to AIJ, new to this forum, but a veteran of the ACP forum.
Gavin, I couldn't find a recent post of yours about BJD-TDB on the ACP Forum, but there is a thread about this subject beginning in August 2017 (ref here if you are still able to access the Forum:
http://forums.dc3.com/showthread.php?10909-Clarification-re-new-BJD-time-stamp-in-FITS&p=65993#post65993.)
Bob Denny's last post on that thread was on September 11th. I'll just paste in the whole quote in case you can't access the forum now. His message I think is verifying that it is the TDB-corrected BJD [despite its FITS Header name BJD-OBS]. Bob refers to Eastman's
online calculator and by implication the other documents referenced there.
"I found two online BJD_TDB calculators, the easier one being this one at Ohio State University by Jason Eastman, who is the principal author of this paper "Achieving Better Than 1 Minute Accuracy in the Heliocentric and Barycentric Julian Dates." Trust me
I have not read this paper but I have applied the methods that are included in the USNO Vector Astrometry (NOVAS) library to achieve something that is within a couple of seconds of "exact" and it is geocentric not for a specific latitude/longitude (which is
another second-ish). The BJD-TDB to BJD-UTC difference is around 70 seconds at present, so I can say that I am producing BJD-TDB plus or minus a couple of seconds. To get more accurate would require a load more code, a dynamic lookup for the current "delta-T"
value, and have a marginal benefit."
As he mentions, he did not write the code for the DeltaT correction, but relied on the Naval Observatory NOVAS code library for that value. If you are looking for more accuracy than a few seconds, you should contact Bob either directly or through the ACP Forum
to advocate for additional changes.
The other day I heard a compelling presentation by Jerry Hubbell at NEAIC 2019 about achieving higher photometric accuracy in exoplanet observations - typically 2-5 mmag using, say, a twelve-inch SCT or 6-inch refractor and exposures of a couple-minutes each.
Intriguing. You have probably seen this earlier reference paper, too:
https://arxiv.org/pdf/1710.01790.pdf. He introduced me to AIJ in this presentation. I have a modest 12-inch SCT in a dark-sky site in rural New York - just perfect for variable star and exoplanet observing.
Regards,
Dick
(virgilobservatory.us)
Dick Berg
Virgil Observatory