Hello,
I downloaded recent Rubin alerts via Fink and checked them also in Lasair, and I have a question to clarify midpointMjdTai conversion to UTC.
For example the alert with diaObjectId 170389126969294877 (see the screenshot), the packet field is: midpointMjdTai = 61172.00230775271
When I interpret this as MJD in TAI and convert it to UTC with Astropy, I get:
2026-05-12 00:02:42 UTC
However, both Fink and Lasair display the same alert time as:
2026-05-12 00:03:19 UTC
So it looks like the ~37 second TAI→UTC offset is not being applied in the broker-displayed UTC time.
For visual alert inspection and cutout review this may not matter much, but for MPC/SkyBoT matching with known asteroids and comets, 37 seconds can be a significant offset.
Could someone clarify whether midpointMjdTai should be converted from TAI to UTC with 37 sec adjustment before querying MPC/SkyBoT, or whether the broker-displayed UTC time is the intended time to use?
Thank you!
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Hi @LienaDreams, thank you for your question.
You are correct that the Rubin output field midpointMjdTai is an MJD in TAI and is not converted to UTC. So, if you would like the time in UTC, you will need to do the conversion in your analysis. It looks like you have identified a bug in the Fink and Lasair brokers that assume the MJD provided by Rubin is in UTC when it is in TAI. I will change the category of this post to include support from the Lasair team so that they can be informed. I’ll also put this on our #broker-support Slack channel to inform the Fink team and other brokers as well in case fixes need to be made on their ends as well.
I’m going to mark this post as the solution, but please let us know if it does not fully address your question.
Also, in case it’s helpful, we have documentation on time measurements within Rubin data available here: Time Measurements — DP1.
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@sgreenstreet
Thank you, Sarah, this is very helpful.
I will keep the TAI → UTC conversion in my own pipeline for SkyBoT/MPC matching.
Regarding brokers: I noticed the same UTC display behavior not only in Fink and Lasair, but also in ANTARES and Alerce. So it may be worth checking whether this assumption is shared across multiple brokers.
Best,
Elena
Hello,
Thanks for bringing this important point, but I do not think there is a bug here.
Brokers have discussed in the past about the fact that observing times are given in TAI scale, not UTC (although bugs happened in the past, and there will be!). For Fink, we convert to UTC when necessary internally in our processing (and all our solar system tools are performing conversion internally if need be, especially for ephemerides).
I reckon though the scale does not appear explicitly in the user facing tools like the portal. We use the raw value sent by Rubin, that is TAI scale. I guess all brokers do the same, hence the confusion.
For Fink, i will make the scale explicit in the legend to clarify. Thanks for reporting!
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