When you check an entry of the Gaia DR2 stellar catalogue in Vizier, for example this one, you can see that there is a value for the G magnitude (in our case 18.0733 mag) and a value for the uncertainty on that (in our case 0.0023 mag). The thing is that the G value is part of Gaia DR2 but the uncertainty is not, it was calculated by the CDS for Vizier (using the values presented for the flux as far as I can understand). I have two small related questions:
How does the CDS calculate the value from the uncertainty exactly? at least so that the one given by the CDS and the calculated one agree to the fourth decimal.
Is there a way in astropy or astroquery to retrieve the photometric uncertainty value from the catalog? Or do I have to calculate it as in the first question?