All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no specific time for reciting the Sharee‘ah-approved Ruqyah (protective and healing supplication); it is prescribed to recite it whenever it is needed.
As for explaining the beginning of the Fajr prayer time with astronomical terms and specifying the correct opinion in this regard, this matter should be referred to specialists rather than seeking a Fatwa about it. It should be noted though that determining the time of the true dawn is a controversial issue among contemporary jurists.
Anyway, some researchers mentioned that the true dawn is what is known in astronomical terminology as the “astronomical twilight”.
It is sufficient for a non-specialized lay Muslim to rely on one of the approved famous Islamic calendars, such as Umm Al-Qura calendar or others, to identify the times of the prayers.
Allah Knows best.