# GAMA-12: Validation Report

GAMA-12: Validation Report

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Master catalogue used: __master_catalogue_gama-12_20171210.fits__<br>
Number of rows: 12,369,415
<br>
Surveys included:<br>
| Survey | Telescope / Instrument  | Filters (detection band in bold)  | Location        |
|--------|-------------------------|:---------------------------------:|-----------------|
| DECaLS        | DEC              | grz                               | dmu0_DECaLS     |
| HSC-SSP       | Hyper Suprime    | grizy                             | dmu0_HSC        | 
| KIDS          | VLT OmegaCAM     | ugri                              | dmu0_KIDS       |  
| PanSTARRS-3SS | GPC1             | grizy                             | dmu0_PanSTARRS-3SS |     
| UKIDSS-LAS    | UKIRT WFCAM      | YJHK                              | dmu0_UKIDSS-LAS |     
| VIKING        | VISTA VIRCAM     | ZYHJKs                            | dmu0_VISTA-VIKING |

Master catalogue used: master_catalogue_gama-12_20171210.fits
Number of rows: 12,369,415
Surveys included:

Survey Telescope / Instrument Filters (detection band in bold) Location
DECaLS DEC grz dmu0_DECaLS
HSC-SSP Hyper Suprime grizy dmu0_HSC
KIDS VLT OmegaCAM ugri dmu0_KIDS
PanSTARRS-3SS GPC1 grizy dmu0_PanSTARRS-3SS
UKIDSS-LAS UKIRT WFCAM YJHK dmu0_UKIDSS-LAS
VIKING VISTA VIRCAM ZYHJKs dmu0_VISTA-VIKING
 
## I. Caveats

I. Caveats

 
### I.a. Magnitude errors 

I.a. Magnitude errors

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At faint magnitudes (mag > 24), some surveys have very large errors on the magnitude. These objects may be unreliable for science puposes.<br>
This in includes __KIDS aperture and total__ magnitudes (at mag >24),  __DECaLS aperture and total__ magnitudes (at m > 23), __PanSTARRS aperture and total__ magnitudes (at mag > 24), __VIKING total__ magnitudes (at mag > 22) and __UKIDSS total__ magnitudes (at mag > 22).<br>
<img src="help_plots/GAMA-12_magVSmagerr_KIDS_r_mag_total.png" />
In particular, __HSC-SSP aperture and total__ magnitudes can go extremely faint (up to 60 mag!!) with errors larger than $10^{13}$!
<img src="help_plots/GAMA-12_magVSmagerr_SUPRIME_r_mag_total.png" />

At faint magnitudes (mag > 24), some surveys have very large errors on the magnitude. These objects may be unreliable for science puposes.
This in includes KIDS aperture and total magnitudes (at mag >24), DECaLS aperture and total magnitudes (at m > 23), PanSTARRS aperture and total magnitudes (at mag > 24), VIKING total magnitudes (at mag > 22) and UKIDSS total magnitudes (at mag > 22).

In particular, HSC-SSP aperture and total magnitudes can go extremely faint (up to 60 mag!!) with errors larger than 1013!

 
### I.b. Aperture corrections

I.b. Aperture corrections

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In most of the case when comparing the aperture magnitudes between surveys, we observed a two peak distribution in the difference between the magnitudes ($\Delta_{mag} = mag_{survey1} - mag_{survey2}$). We have one peak around 0 for point-source objects, with a small spread. And a second peak at higher $\Delta_{mag}$ with a larger spread for extended objects; implying a different aperture correction between surveys for these objects.<br>
That means that galaxies will not have the same aperture magnitude in different surveys. <br>
In the griz bands, for bright sources, there is a two peaks distribution when comparing Pan-STARRS, DECaLS, HSC-SSP and KIDS aperture magnitues. Except when comparing KIDS and HSC-SSP magnitudes, $\Delta_{mag}$ is similar for point-sources and extended objects.<br>
Also, in the z- and y-bands, only the magnitude of extended objects between HSC-SSP and VISTA, HSC-SSP and UKIDSS, and VISTA and UKIDSS agreed; while with the other surveys we get again a two peak distribution on $\Delta_{mag}$.
<img src="help_plots/GAMA-12_apcorrIssues_KIDS_g_aperture_-_DECam_g_aperture.png" />

In most of the case when comparing the aperture magnitudes between surveys, we observed a two peak distribution in the difference between the magnitudes (Δmag=magsurvey1magsurvey2). We have one peak around 0 for point-source objects, with a small spread. And a second peak at higher Δmag with a larger spread for extended objects; implying a different aperture correction between surveys for these objects.
That means that galaxies will not have the same aperture magnitude in different surveys.

In the griz bands, for bright sources, there is a two peaks distribution when comparing Pan-STARRS, DECaLS, HSC-SSP and KIDS aperture magnitues. Except when comparing KIDS and HSC-SSP magnitudes, Δmag is similar for point-sources and extended objects.
Also, in the z- and y-bands, only the magnitude of extended objects between HSC-SSP and VISTA, HSC-SSP and UKIDSS, and VISTA and UKIDSS agreed; while with the other surveys we get again a two peak distribution on Δmag.

 
## II. Flags

II. Flags

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### II.a. Pan-STARRS aperture and total magnitudes

II.a. Pan-STARRS aperture and total magnitudes

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Few Pan-STARRS sources have exactly the same error (of <font color='blue'>0.0010860000038519502</font>) on the __aperture and total__ magnitudes in all the grizy bands. The corresponding aperture magnitude should not be trusted for these objects.<br>
<img src="help_plots/GAMA-12_gpc1Issues_GPC1_g_mag_aperture.png" />

Few Pan-STARRS sources have exactly the same error (of 0.0010860000038519502) on the aperture and total magnitudes in all the grizy bands. The corresponding aperture magnitude should not be trusted for these objects.

 
### II.b.DECaLS total magnitudes

II.b.DECaLS total magnitudes

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Few DECaLS sources have excatly the same total magnitude (of <font color='blue'>14.999935</font>) in the r and i bands, but not in the y-band (no sources have g- or z-band magnitudes). These magnitudes should not be trusted. <br>
Also, in the r- and i-band, few sources have very bright total magnitude but no aperture magnitude (<font color='green'>$mag_{tot} < 7$</font>). Even some have negative total magnitudes in the i-band. <br>
<img src="help_plots/GAMA-12_decamIssues_DECam_r_mag_total.png" />

Few DECaLS sources have excatly the same total magnitude (of 14.999935) in the r and i bands, but not in the y-band (no sources have g- or z-band magnitudes). These magnitudes should not be trusted.
Also, in the r- and i-band, few sources have very bright total magnitude but no aperture magnitude (magtot<7). Even some have negative total magnitudes in the i-band.

 
### II.c. Outliers

II.c. Outliers

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By comparing magnitude in the same band between different surveys, we can see that some magnitudes are significanlty different could not be trusted. <br>
The outliers are identified to have a large weighted magnitude difference (equivalent of the $chi^2$).
$$chi^2 = \frac{(mag_{1}-mag_{2})^2}{magerr_{1}^2 + magerr_{2}^2}$$ 
<br>
We used the 75th and 25th percentile to flagged the objects 5$\sigma$ away on the large values tail of the $chi^2$ ditribution. (__NB:__ bright sources tend to have their errors underestimated with values as low as $10^{-6}$, which is unrealistic. So to avoid high $chi^2$ due to unrealistic small errors, we clip the error to get a minimum value of 0.1% (i.e. all errors smaller then $10^{-3}$ are set to $10^{-3}$).)
<br><br>
$$outliers == [chi^2 >  (75th \;percentile + 3.2\times (75th \;percentile - 25th \;percentile))]$$
<img src="help_plots/GAMA-12_outliers_SUPRIME_i_aperture_-_KIDS_i_aperture.png" />

By comparing magnitude in the same band between different surveys, we can see that some magnitudes are significanlty different could not be trusted.
The outliers are identified to have a large weighted magnitude difference (equivalent of the chi2).

chi2=(mag1mag2)2magerr12+magerr22

We used the 75th and 25th percentile to flagged the objects 5σ away on the large values tail of the chi2 ditribution. (NB: bright sources tend to have their errors underestimated with values as low as 106, which is unrealistic. So to avoid high chi2 due to unrealistic small errors, we clip the error to get a minimum value of 0.1% (i.e. all errors smaller then 103 are set to 103).)

outliers==[chi2>(75thpercentile+3.2×(75thpercentile25thpercentile))]