Is it correct to rely on astronomical timings to determine the time for Fajr prayer, and what is the ruling regarding prayers performed based on that before the appearance of redness in the horizon?

The entry of the prayer time is a condition for its validity. The time for the Fajr (Dawn) prayer begins with the appearance of the true dawn, which is the white light that spreads horizontally across the horizon, and ends with sunrise. This is based on the Almighty's saying: {Indeed, prayer has been decreed upon the believers a time-constrained ordinance} [An-Nisa: 103], and on prophetic hadiths, including: "...and the time for the Fajr prayer is from the appearance of dawn until the sun rises," and "Do not let the Adhan of Bilal nor the horizontal white light of the horizon deceive you regarding your suhur, until it spreads out horizontally like this (meaning across the horizon)." The presence of darkness does not contradict the appearance of dawn, as women used to pray Fajr with the Prophet, peace and blessings be upon him, in the ghalas (darkness before full light). It is disliked to delay the Fajr prayer until redness appears.