# Help regarding cross-matching catalogs! Separation and match\_catalog\_sky

**URL:** <https://community.openastronomy.org/t/help-regarding-cross-matching-catalogs-separation-and-match-catalog-sky/1014>\
**Category:** Astropy\
**Created:** [June 13, 2024, 1:16pm UTC](https://community.openastronomy.org/t/help-regarding-cross-matching-catalogs-separation-and-match-catalog-sky/1014 "2024-06-13T13:16:41Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![lifemannequin](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/lifemannequin/32/546_2.png) [@lifemannequin](https://community.openastronomy.org/u/lifemannequin)\
**Post date:** [June 13, 2024, 1:16pm UTC](https://community.openastronomy.org/t/help-regarding-cross-matching-catalogs-separation-and-match-catalog-sky/1014/1 "2024-06-13T13:16:41Z")

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I am trying to crossmatch the latest Fermi LAT catalog to an sdss catalog. I found tsuji et al. 2021 where they create a distance profile and use it to decide the cutoff. I did it by using a separation function in astropy but the histogram shows a linear increase where I was expecting a bump near 0 and then a linear increase.

Then I used the match\_to\_catalog\_sky function and in the output there is the bump but not the linear increase… WHY!!!

I created a random catalog to check number of false matches and whether the distribution was a result random matches. I did a KS test and I got KS Statistic: 0.7842946490618485, p-value: 0.0 . Is it correct to just use the output from the match\_to\_catalog\_sky function and setting the cutoff from CDF of the d2d of random position catalog? What is the significance of the distribution close to 0 and no linear increase?  
Thank you!  
 ![Histogram_separation_original_randomised](https://europe1.discourse-cdn.com/flex005/uploads/openastronomy/original/1X/37e9cd78f2599a4678853fb2a006867e26636e26.png)