Is the slightest visibility of the true dawn sufficient for the validity of the Fajr prayer, and is the prayer valid if the opening Takbir (Takbīrat al-Iḥrām) occurs at this weak visibility?

If the true dawn, whose light spreads left and right, has appeared, then the time for the Fajr prayer has begun, and the prayer is valid at that point. It is not important to specify a time period between the adhan (definition) and the iqamah (definition). However, if the dawn has not appeared, the prayer is not valid. It is recommended by the majority of scholars to pray Fajr early in its time (taghlees).