<p>I am trying to cross match a catalog derived from SDSS-MaNGA(self) with a catalog derived from DESI (Yang+2021). I am using TOPCATs Sky Algorithm, based on RA and Dec similarities to perform the crossmatch. However, the cross match yields far fewer galaxies than expected (lesser by a factor of 4).</p>
<p>On closer inspection, the reported RA and Dec value from the DESI and SDSS catalogs are different for both the surveys for identical galaxies, mostly at the 4th - 5th decimal place. This might again be a selection bias as I had forced the values to be quite close by giving a small margin of error in the cross match, but anything larger seems to translate into quite abnormally large differences in RA and Dec, which seems to be strange for an instrument of this precision. One would expect the RA/Dec doesn't change much between the two surveys.</p>
<p>I wish to know if there can be large differences in reported RA and Dec for objects found in two different surveys, and by how much it could possibly be.</p>

CC BY-SA 4.0
2022-09-15T22:28:24.533Z
Assuming both catalogues are on the same ICRS system 1" (=0.00027degrees) or slightly larger would not be an unreasonable matching tolerance given the differences in determining the centroids of extended objects such as galaxies and the different depths of the SDSS-MaNGA from the SDSS 2.5m telescope and DESI from the 4m Mayall telescopes. Comparing the point-like quasar samples from both would probably be a fairer assessment of what offsets exist between the two
astrosnapper
CC BY-SA 4.0
2022-09-16T09:27:01.757Z
I tried using multiple tolerance values - from 1" to 5", and all of them returned only ~2000 matches, whereas my original MaNGA sample had upwards of 8000 galaxies. The DESI Catalog was derived from the DR9 Legacy Survey, and had a total of 128 million galaxies. I am absolutely stumped as to how such a large tolerance is still unable to give a reliable cross match. I will try calculating the quasar offsets and get back.
haricash
CC BY-SA 4.0
2022-09-17T23:14:26.870Z
@haricash from [FAQ](https://meta.stackexchange.com/q/7931/303080) you can find [How do comment `@replies` work?](https://meta.stackexchange.com/q/43019/303080) I'm not sure astrosnapper received a notification that you replied to them.
uhoh
CC BY-SA 4.0
2022-09-17T23:15:03.637Z
@astrosnapper there was an informative reply, I'm not sure it generated a notification.
uhoh
CC BY-SA 4.0
2022-09-18T03:25:13.107Z
I got the notification, but am mystified by what's going wrong in the crossmatch (unless there is a ~2 mag difference in depth or a small area of overlap)
astrosnapper
<observational-astronomy><data-analysis><sky-survey><raw-data>
SDSS and DESI - How much variation in sky location?
haricash
# SDSS and DESI - How much variation in sky location?

(Asked by: username_0 on 2022-09-15)

I am trying to cross match a catalog derived from SDSS-MaNGA(self) with a catalog derived from DESI (Yang+2021). I am using TOPCATs Sky Algorithm, based on RA and Dec similarities to perform the crossmatch. However, the cross match yields far fewer galaxies than expected (lesser by a factor of 4).

On closer inspection, the reported RA and Dec value from the DESI and SDSS catalogs are different for both the surveys for identical galaxies, mostly at the 4th - 5th decimal place. This might again be a selection bias as I had forced the values to be quite close by giving a small margin of error in the cross match, but anything larger seems to translate into quite abnormally large differences in RA and Dec, which seems to be strange for an instrument of this precision. One would expect the RA/Dec doesn't change much between the two surveys.

I wish to know if there can be large differences in reported RA and Dec for objects found in two different surveys, and by how much it could possibly be.

**Tags:** observational-astronomy, data-analysis, sky-survey, raw-data
> **Comments:**
> **username_1**: Assuming both catalogues are on the same ICRS system 1" (=0.00027degrees) or slightly larger would not be an unreasonable matching tolerance given the differences in determining the centroids of extended objects such as galaxies and the different depths of the SDSS-MaNGA from the SDSS 2.5m telescope and DESI from the 4m Mayall telescopes. Comparing the point-like quasar samples from both would probably be a fairer assessment of what offsets exist between the two  
> **username_0**: I tried using multiple tolerance values - from 1" to 5", and all of them returned only ~2000 matches, whereas my original MaNGA sample had upwards of 8000 galaxies. The DESI Catalog was derived from the DR9 Legacy Survey, and had a total of 128 million galaxies. I am absolutely stumped as to how such a large tolerance is still unable to give a reliable cross match. I will try calculating the quasar offsets and get back.  
> **username_2**: @haricash from [FAQ](https://meta.stackexchange.com/q/7931/303080) you can find [How do comment `@replies` work?](https://meta.stackexchange.com/q/43019/303080) I'm not sure astrosnapper received a notification that you replied to them.  
> **username_2**: @astrosnapper there was an informative reply, I'm not sure it generated a notification.  
> **username_1**: I got the notification, but am mystified by what's going wrong in the crossmatch (unless there is a ~2 mag difference in depth or a small area of overlap)  

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## 0 answers

