Apparent discrepancy in BJD

[Davidgp]

I am new to AIJ and from answers on this website I have been able to insert into the Measurement Table, BJD times in the format BJD_TDB_MOBS.  Previously I had always used the Iowa State University (ISU) website for converting from JD_UTC to BJD_TDB and so decided to cross check.  In doing so I get a difference of ~1.89s +/-0.03s taken over 61 observations.  I used the same Simbad coordinates for V*NY Vir (RA 13:38:48.15; Dec -02:01:49.1) in both MP Coordinate Converter and the ISU webpage.  Likewise I used the same observatory coordinates (Long -110:23:53.18; Lat +31:39:56.08) for both with altitude set to zero for both.  Exposure time was 15s.  I am unable to find the cause of this difference but wondered if it was related to the definition of BJD_TDB_MOBS.  As an example I get 2458190.882506 and 2458190.882483 for ISU and AIJ respectively.

[karenacollins]

Hi David,

I did a quick search and didn't spot an ISU JD_UTC to BJD_TDB converter, but maybe you are referring to the OSU one here:
http://astroutils.astronomy.ohio-state.edu/time/

If that is the case, the OSU website is more accurate than AIJ. However, the largest difference I have seen is about 0.2 seconds. Before digging deeper, please confirm that you have an up-to-date leap second table to clicking the "Update" button circled below in the MP coordinate converter panel, and then re-running the calculations. If it is still off by more than a few hundredths of a second, let me know.

I will also note that if you want the ultimate precision, AIJ can also extract the times from the OSU website, but that takes longer. To do that check the "OSU/Internal" option circled below.

Karen

On 3/21/2018 12:05 PM, Davidgp [via AstroImageJ] wrote:

I am new to AIJ and from answers on this website I have been able to insert into the Measurement Table, BJD times in the format BJD_TDB_MOBS.  Previously I had always used the Iowa State University (ISU) website for converting from JD_UTC to BJD_TDB and so decided to cross check.  In doing so I get a difference of ~1.89s +/-0.03s taken over 61 observations.  I used the same Simbad coordinates for V*NY Vir (RA 13:38:48.15; Dec -02:01:49.1) in both MP Coordinate Converter and the ISU webpage.  Likewise I used the same observatory coordinates (Long -110:23:53.18; Lat +31:39:56.08) for both with altitude set to zero for both.  Exposure time was 15s.  I am unable to find the cause of this difference but wondered if it was related to the definition of BJD_TDB_MOBS.  As an example I get 2458190.882506 and 2458190.882483 for ISU and AIJ respectively.

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[karenacollins]

Hi again David,

Since you gave the target and observatory locations, I ran a quick check with the current time (JD_UTC = 2458199.595596):
AIJ internal -> BJD_TDB = 2458199.601646
OSU             -> BJD_TDB = 2458199.601645

So, I expect you just need to update the leap second table.

Karen

[Davidgp]

Many thanks.  Yes it was my not hitting the update button.  All fine now to within 0.1s. and I will try the OSU button.  I had thought that having the “Auto” boxed ticked was sufficient but clearly not.  Is there any reason for having the “update” box?  Would much flexibility be lost if this was automatically updated from USNO without the need for user intervention?

[karenacollins]

The update is typically only needed every few years and requires an internet connection. Would cause connection errors if not connected to the internet, and would make AIJ start up slower.

On 3/22/2018 6:02 AM, Davidgp [via AstroImageJ] wrote:

Many thanks.  Yes it was my not hitting the update button.  All fine now to within 0.1s. and I will try the OSU button.  I had thought that having the “Auto” boxed ticked was sufficient but clearly not.  Is there any reason for having the “update” box?  Would much flexibility be lost if this was automatically updated from USNO without the need for user intervention?

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