Error bars for points flagged as negative

On encountering a negative flux, the uncertainty is normally set to the modulus of the flux, converted into magnitudes. Older versions of the code then multiply this by minus one, but newer versions will not (which has happened is obvious, as these stars are faint and should have positive magnitudes). The reason we suggest you normally ignore this number, is that thereafter it will be treated as the uncertainty. So, for example, as soon as a colour is made, it becomes meaningless.