# Herschel-Stripe-82: Validation Report (SUBSET)

Master catalogue used: __master_catalogue_herschel-stripe-82_RANDOM10PCSAMPLE_20180113.fits (10% of master catalogue)__ <br>

Number of rows: 4,924,329

<br>

Surveys included:<br>

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

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

| HSC SSC PDR1 | Subaru/HSC                |   grizy,N921,N816                          | dmu0_HSC                    |

| PS1 3PSS   | Pan-STARRS1/Pan-STARRS1     |   grizy                                    | dmu0_PanSTARRS1-3SS         |

| DECaLS     | Blanco Telescope/Dark Energy Camera| ugrizY                              | dmu0_DECaLS                 |

| RCSLenS    | CFHT/MegaPrime/MegaCam      |   grz                                      | dmu0_RCSLenS                |

| SDSS S82   | SDSS                        |   ugriz                                    | dmu0_SDSS-S82               |

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

| VICS82     | VISTA+CFHT/VIRCAM+WIRCA     |   J,K                                      | dmu0_VICS82                 |

| UKIDSS LAS | UKIRT/WFCAM                 |   Y,J,H,K                                  | dmu0_UKIDSS-LAS             | 

| SHELA      | Spitzer/IRAC                |   IRAC12                                   | dmu0_SHELA                  |

| SpIES      | Spitzer/IRAC                |   IRAC12                                   | dmu0_SpIES                  |

​

__NB__: No error is available for VICS82 J and K magnitudes (aperture and total)

### I.a. Magnitude errors 

At faint magnitudes (mag > 24), some surveys have very large errors (> 10) on the magnitude. These objects may be unreliable for science puposes.<br>

This in includes __SDSS aperture and total__ magnitudes (at mag > 25), __DECaLS aperture and total__ magnitudes (at m >  20), __PanSTARRS aperture and total__ magnitudes (at mag > 21). Also few sources have large errors in __VHS total__ magnitudes (at mag > 20) and __UKIDSS total__ magnitudes (at mag > 23).<br>

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<img src="help_plots/Herschel-Stripe-82_magVSmagerr_SDSS_g_mag_total.png" />

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In particular, __HSC-SSP aperture and total__ magnitudes can go extremely faint (up to 60 mag!!) with errors larger than $10^{13}$!

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<img src="help_plots/Herschel-Stripe-82_magVSmagerr_Suprime_r_mag_total.png" />

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### 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>

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In the griz bands, for bright sources, there is a two peaks distribution when comparing Pan-STARRS, DECaLS, HSC-SSP and SDSS aperture magnitues. Except when comparing Pan-STARRS and DECaLS magnitudes, $\Delta_{mag}$ is similar for point-sources and extended objects.<br>

Also, in the near-IR bands, only the magnitude of extended objects between UKIDSS and VHS agreed; while with the other surveys we get again a two peak distribution on $\Delta_{mag}$.

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<img src="help_plots/Herschel-Stripe-82_apcorrIssues_SDSS_r_aperture_-_GPC1_r_aperture.png" />

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## II. Flags

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

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>

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<img src="help_plots/Herschel-Stripe-82_gpc1Issues_GPC1_r_mag_aperture.png" />

### II.b.DECaLS total magnitudes

Few DECaLS sources have excatly the same total magnitude (of <font color='blue'>14.999935</font>) in the g- and r-bands. These magnitudes should not be trusted. <br>

Also, in the grz bands, few sources have very bright or negative total magnitude but no aperture magnitude (<font color='green'>$mag_{tot} < 7$</font>).

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<img src="help_plots/Herschel-Stripe-82_decamIssues_DECam_z_mag_total.png" />

### II.c. 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 $chi^2$).

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$$chi^2 = \frac{(mag_{1}-mag_{2})^2}{magerr_{1}^2 + magerr_{2}^2}$$ 

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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>

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

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<img src="help_plots/Herschel-Stripe-82_outliers_GPC1_r_aperture_-_DECam_r_aperture.png" />