BJD_TDM revisited re ACP

[gboyle]

Hi Karen

we touched on this before.  Bob Denny at ACP has added BJD to fits headers presumably in response to requests from science users but the implementation gives a value for BJD_OBS and therefore is UTC based as opposed to TDM .

I have posted a "new feature request" for BJD_TDM and pointed towards the 2010 Eastman paper - but if he were inclined to add the feature do you know where we might find a code or calculation that he can download so that I can point him towards that as otherwise doing TDM from scratch might look like a lot of work...!

best
gavin

[karenacollins]

The main difference for TDB is keeping track of the number of leap-seconds at the time of the observation, which is downloadable from here:
http://maia.usno.navy.mil/ser7/tai-utc.dat

Other than that, it is simple offset.

The code is available on the Eastman website here:
http://astroutils.astronomy.ohio-state.edu/time/index.html

Karen

On 11/19/2018 4:40 AM, gboyle [via AstroImageJ] wrote:

Hi Karen

we touched on this before.  Bob Denny at ACP has added BJD to fits headers presumably in response to requests from science users but the implementation gives a value for BJD_OBS and therefore is UTC based as opposed to TDM .

I have posted a "new feature request" for BJD_TDM and pointed towards the 2010 Eastman paper - but if he were inclined to add the feature do you know where we might find a code or calculation that he can download so that I can point him towards that as otherwise doing TDM from scratch might look like a lot of work...!

best
gavin

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[richardaberg]

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)

[karenacollins]

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)

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