BJD to HJD and UTC conversion

[chrismlt]

Hello Karen,

The photometry goes well now from TESS TICAs, since you have upgraded the software about the faulty BJD time function. Thanks for that !

Now, as most of the epochs/observations/ephemeris in the AAVSO VSX server are in HJD, then converted in geocentric time JD_UTC, I'm trying to convert the BJD timings from TESS observations.

But I'm not sure to use the correct procedure. Maybe I'm missing something.

Here is my process :
-photometry extraction in the TESS TICAs,
-update of the MP coordinate converter with a query upon the observed star designation,
-add astronomical data to table, using BJD_TDB as input and active column, and asking HJD_UTC and JD_UTC new columns.

The correction applied from BJD to JD_UTC I've got after that process, is – 8 minutes 7 sec, which seems right in the expected value.
Now, the BJD to HJD conversion is about -66 sec, which, it seems, may be a bit large from the expected correction, as I suppose the barycenter to be inside the sun, no more than a few light secs from the heliocenter.

Could you be nice to take a moment, if you don't mind, to tell me if the value I've got for JD_UTC are correct, and explain the exact procedure to apply to get a correct HJD, please ?

Thank you sooo muuuch in advance,

Happy holidays, and Happy New year,

Christophe

[karenacollins]

Hi Christophe,

The add astronomical data to table process you described is correct.

The roughly ~66 second offset between BJD_TDB and HJD_UTC is about right (depending on the date of the TESS observations). The offset is a combination of 32.184 seconds plus N leap seconds since 1961 (which is currently 37 - you may need to click "Update" in MP Coordinate Converter panel to get the most recent values), plus up to the ~+/-8 seconds of offset of the heliocenter to the barycenter. See Jason Eastman's converter page and the reference to his paper there-in.

However, note that for TESS data time stamp conversions within AIJ, there will be an inaccuracy of up to +/-1.3 seconds because BJD calculations in AIJ assume the observer is at an observatory on the surface of the Earth, so we not account for the position of the TESS spacecraft in it's orbit. Since HJD is not a very accurate way to report astronomical timings, I doubt the residual error from the AIJ calculations will be important.

Karen

[chrismlt]

Thank you Karen for the help.
Indeed, I didn't take into account of the leap seconds to add during the BJD to HJD conversion.
This validated process will greatly simplify the task to submit data to the VSX.

Have a good day,
Christophe