# Convert velocity from barycentric to LSRK

**URL:** <https://community.openastronomy.org/t/convert-velocity-from-barycentric-to-lsrk/1266>\
**Category:** OpenAstronomy\
**Tags:** astropy\
**Created:** [July 16, 2025, 3:02pm UTC](https://community.openastronomy.org/t/convert-velocity-from-barycentric-to-lsrk/1266 "2025-07-16T15:02:59Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Firestar](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/firestar/32/401_2.png) [@Firestar](https://community.openastronomy.org/u/Firestar)\
**Post date:** [July 16, 2025, 3:02pm UTC](https://community.openastronomy.org/t/convert-velocity-from-barycentric-to-lsrk/1266/1 "2025-07-16T15:02:59Z")

</div>

I am a student studying HI HVC clouds, and I need to compare results from several HI surveys.

I ran into a problem of “How to convert HIPASS barycentric velocities to LSRK velocities”.

HIPASS uses ‘VELO-HEL’ /Barycentric velocity; radio convention, while more recent surveys like HI4PI and CRAFTS use LSRK velocities.

These two systems have a slight difference of about 17 km/s but I do not know how to convert them.

I found [Working with Velocities in Astropy Coordinates: Radial Velocity Corrections](https://docs.astropy.org/en/stable/coordinates/velocities.html#radial-velocity-corrections), but it does not give me good methods.

CARTA can do velocity conventions, but I don’t know how they calculate it

This table is what I found on CARTA:

```auto
# V_bary ~ V_lsrk | Delta_V (m/s)
# 600101.679 ~ 583017.524 | 17084.155
# 309892.611 ~ 292791.884 | 17100.727
# 6492.221 ~ -10625.830 | 17118.051
# -6699.100 ~ -23817.905 | 17118.905
# -283716.847 ~ -300851.470 | 17134.623
# -600308.558 ~ -617461.258 | 17152.700

```

I found that Delta\_V is not a constant, so I cannot simply subtract 17 km/s from V\_bary.

It does depend on RA-Dec, when I checked for several different images.

Any help is welcome! If you could provide a simple Python code snippet, I would be very grateful.