# ELAIS-S1: Validation Report (FULL)

Master catalogue used: __master_catalogue_elais-s1_20180122.fits__<br>

Number of rows: 1,655,564

<br>

Surveys included:<br>

​

| Survey     | Telescope / Instrument      |      Filters (detection band in bold)      | Location                    |

|------------|-----------------------------|:------------------------------------------:|-----------------------------|

| ESIS       | ESO 2.2/WFI                 | BVR                                        | dmu0_ESIS-VOICE             |

| DES-DEEP   | Blanco/DECAM                | grizy                                      | dmu0_DES        |

| VIDEO      | VISTA/VIRCAM                | Y,J,H,Ks                                   | dmu0_VISTA-VIDEO            | 

| VHS        | VISTA/VIRCAM                | Y,J,H,Ks                                   | dmu0_VISTA-VHS              |

| SWIRE      | Spitzer/IRAC MIPS           | IRAC1234                                   | dmu0_DataFusion-Spitzer     | 

| SERVS      | Spitzer/IRAC                | IRAC12                                     | dmu0_DataFusion-Spitzer     | 

​

​

## I. Caveats

### I.a. Magnitude errors 

At faint magnitudes (mag > 24), some surveys have very large errors on the magnitude. These objects may be unreliable for science puposes.<br>

This includes __DES aperture and total__ (at mag > 24) and __VISTA aperture and total__ magnitudes (at mag > 20) and __IRAC1, IRAC2 aperture and total__ magnitudes (at mag > 20). <br>

​

​

<img src="help_plots/ELAIS-S1_magVSmagerr_DECAM_g_mag_total.png" />

​

### I.b. Aperture corrections

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 z and y bands, for bright sources, there is a two peaks distribution when comparing DES and VISTA aperture magnitues.<br>

​

<img src="help_plots/ELAIS-S1_apcorrIssues_VISTA_Y_aperture_-_DECAM_y_aperture.png" />

## II. Flags

### II.c IRAC aperture magnitude

​

Few IRAC sources have exactly the same aperture magnitude (of <font color='blue'>3.9000000001085695</font>) in the IRAC1 and IRAC2 bands. These magnitudes also have extremely small errors (around 10$^{-8}$-10$^{-9}$). The corresponding magnitudes should not be trusted. <br>

​

<img src="help_plots/ELAIS-S1_iracIssues_i1_i2.png" />

### II.b. Outliers

​

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>

$$outliers == [chi^2 >  (75th \;percentile + 3.2\times (75th \;percentile - 25th \;percentile))]$$

​

__NB__: Only z and y bands of VISTA and DES are compared in this field, as DES is the only optical survey and VISTA the only NIR survey.

​

<img src="help_plots/ELAIS-S1_outliers_VISTA_Z_total_-_DECAM_z_total.png"/>