# Question about base\_SdssShape\_instFlux\_yy\_Cov in the pipeline source catalog

**URL:** <https://www.rubin.community/t/question-about-base-sdssshape-instflux-yy-cov-in-the-pipeline-source-catalog/8403>\
**Category:** Support\
**Created:** [March 26, 2024, 8:55pm UTC](https://www.rubin.community/t/question-about-base-sdssshape-instflux-yy-cov-in-the-pipeline-source-catalog/8403 "2024-03-26T20:55:29Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![sfu](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/sfu/32/3060_2.png) [@sfu](https://www.rubin.community/u/sfu)\
**Post date:** [March 26, 2024, 8:55pm UTC](https://www.rubin.community/t/question-about-base-sdssshape-instflux-yy-cov-in-the-pipeline-source-catalog/8403/1 "2024-03-26T20:55:29Z")

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Hi, I’m working on real/bogus classification, and I’m testing different quantities in the LSST pipeline source catalog. I find that the performance of a quantity called “base\_SdssShape\_instFlux\_yy\_Cov” seems to be quite difference from “base\_SdssShape\_instFlux\_xx\_Cov” and “base\_SdssShape\_instFlux\_xy\_Cov”. It is a quantity in both the src catalog of a direct image, or the diaSrc catalog of a difference image.  
The information in the FITS table header is like this.

```auto
TTYPE54 = 'base_SdssShape_instFlux_yy_Cov' / uncertainty covariance between base                                               
TFORM54 = '1E ' / format of field    
TDOC54 = 'uncertainty covariance between base_SdssShape_instFlux and base_Sds&'
CONTINUE 'sShape_yy'    
TUNIT54 = 'count*pixel^2'    
TCCLS54 = 'Scalar ' / Field template used by lsst.afw.table 

```

I notice that bright transients usually have large values at “base\_SdssShape\_instFlux\_xx\_Cov” and “base\_SdssShape\_instFlux\_xy\_Cov” but small values at “base\_SdssShape\_instFlux\_yy\_Cov”. However, in theory covariances should not be strongly related to direction.  
How does the pipeline compute those SDSS shape flux - 2nd moment covariances? I was not able to find much information about that.  
Any suggestions will be appreciated.

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**Author:** ![parejkoj](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/parejkoj/32/138_2.png) [@parejkoj](https://www.rubin.community/u/parejkoj)\
**Post date:** [March 26, 2024, 11:20pm UTC](https://www.rubin.community/t/question-about-base-sdssshape-instflux-yy-cov-in-the-pipeline-source-catalog/8403/2 "2024-03-26T23:20:37Z")

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All of the documentation about SdssShape is here. Unforunately, there’s not much to go on.

[http://doxygen.lsst.codes/stack/doxygen/x\_mainDoxyDoc/classlsst\_1\_1meas\_1\_1base\_1\_1\_sdss\_shape\_algorithm.html#details](http://doxygen.lsst.codes/stack/doxygen/x_mainDoxyDoc/classlsst_1_1meas_1_1base_1_1_sdss_shape_algorithm.html#details)

This is the C++ code for the algorithm, most of which goes back to ~2014.

> <https://github.com/lsst/meas_base/blob/main/src/SdssShape.cc>

Note that we are moving away from SdssShape to HsmShape. I’ve asked about the documentation for that (it was just rewritten in python). That package is here:

> **[GitHub - lsst/meas\_extensions\_shapeHSM: LSST Data Management: HSM shape...](https://github.com/lsst/meas_extensions_shapeHSM)**
>
> LSST Data Management: HSM shape measurement. Contribute to lsst/meas\_extensions\_shapeHSM development by creating an account on GitHub.

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<div class="post-metadata">

**Author:** ![parejkoj](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/parejkoj/32/138_2.png) [@parejkoj](https://www.rubin.community/u/parejkoj)\
**Post date:** [March 26, 2024, 11:34pm UTC](https://www.rubin.community/t/question-about-base-sdssshape-instflux-yy-cov-in-the-pipeline-source-catalog/8403/3 "2024-03-26T23:34:12Z")

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Thinking about this some more: what do you mean by “the performance of”, in your question? Because I could believe that the xx and yy shape covariances do encode information about which sources are bogus. `y` is the column direction, so bad columns and bleed trails would probably have quite different `yy_Cov` than less pathological sources.

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**Author:** ![sfu](https://sea2.discourse-cdn.com/flex002/user_avatar/www.rubin.community/sfu/32/3060_2.png) [@sfu](https://www.rubin.community/u/sfu)\
**Post date:** [March 27, 2024, 12:48am UTC](https://www.rubin.community/t/question-about-base-sdssshape-instflux-yy-cov-in-the-pipeline-source-catalog/8403/4 "2024-03-27T00:48:00Z")

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Thank you for pointing me to those webpages. The SdssShape.cc page shows those flux - moments covariances are derived from a 4D Fisher matrix. In my question, “behavior” could be a better word than “performance”. What I found is that bright sources usually had large Flux\_xx\_Cov and Flux\_xy\_Cov values, but small/random Flux\_yy\_Cov values, and I’m trying to understand why. Thanks for pointing out the issues of bad columns and bleed trails – I think that’s probably the reason.