Anomalies in DP1 right ascension errors?

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#1

Hello,
In DP1 schema for the DiaSource table raErr is defined as: “Error in right ascension”. It is not explicitly specified whether raErr is the uncertainty at the source position (i.e. if it includes the factor cos(dec) or not).
If raErr is the uncertainty at the source position, I expect it to be comparable to decErr. If raErr is to be scaled with declination, I expect it to be generally greater than decErr.

Io order to check if raErr already includes the factor cos(dec), I compared raErr and decErr for a DP1 field at high declination (47 Tuc Globular Cluster RA:6.02, Decl:-72.08) to better see the effect of declination. I checked first DiaSource coordinates error and then the Object table errors. The results are shown below.

47 Tuc

47Tuc_raErr_diaSource_check
47Tuc_raErr_diaSource_check1400×700 44 KB

47Tuc_raErr_object_check
47Tuc_raErr_object_check1400×700 43.8 KB

Surprisingly to me raErr seems significantly smaller than decErr (left panels) and appears to be comparable to decErr after being divided by cos(dec) (right panels). This is true both for DiaSource data (upper panels) and Object data (lower panels).

The same effect id visible in the Extended Chandra Deep Field South (ECDFS), but the difference between raErr and decErr is much smaller because of the lower declination (-28.1 deg).

ECDFS

ECDFS_raErr_diaSource_check
ECDFS_raErr_diaSource_check1400×700 65.4 KB

ECDFS_raErr_object_check
ECDFS_raErr_object_check1400×700 74.8 KB

Here the notebook I used to produce the attached plots:
testCoordErrButler.ipynb (675.0 KB)

I do not expect raErr and decErr to be identical, but the difference increasing with declination and the fact that
dividing by cos(dec) the two errors are similar seems strange. Are data ok and these trends are expected? Am I doing something wrong?
Thank you.

1 Like
#2

Dear Giuseppe,
Thank you for identifying this issue, for providing a well-documented report, and alerting us to this issue!
Indeed, it is appears that raErr does not include a correction for cos(DEC).
The Rubin team will either update the documentation or update the code accordingly.
Once again, many thanks for bringing this to our attention!
Best regards,
Douglas

#3

Dear Douglas, thanks a lot for your reply! I’m not sure how to apply the missing correction: the *cos(DEC) correction is expected to reduce the error, but raErr is already significantly smaller than decErr and I expect the two errors to be comparable. If I compute decErr*cos(DEC), the two error distributions are very similar, as shown below (I used the RSP Aspect to retrieve and plot the data for 47Tuc), as if the two columns were reversed.

chart_raErr-green-decErr-blue
chart_raErr-green-decErr-blue529×634 20.5 KB

chart_raErr-green-decErrcos(dec)-blue
chart_raErr-green-decErrcos(dec)-blue529×634 18.8 KB
.

If it were raErr to be divided by cos(DEC), (i.e. if raErr is the uncertainty at the source declination) I would expect it to be comparable with decErr and not smaller, but maybe I’m missing something.
Thanks again!
Giuseppe

#4

Any update on this? I’m interested too. Thank you!