People often ask us where we get the Greek and Latin texts we translate from. Fortunately, nearly all the primary Hellenistic source material in Greek and Latin is available in the form of modern critical editions, so we do not have to translate from handwritten manuscripts. We can obtain most of these editions on interlibrary loan. Let me explain what a critical edition is.
A critical edition is prepared by some scholar, usually a classicist, who collates together all the handwritten manuscripts of a given work that have been identified and catalogued to date. Almost all of these manuscripts date from the 9th century or later, so they are already copies of copies. By a comparison of the different versions of the same text, and by a careful attempt to date the individual manuscripts, the editor seeks to establish the oldest and/or most authentic version of this work to serve as his base text. Since this version will no doubt contain numerous obvious scribal errors, lacunae, and instances of illegible or damaged text, he must rectify the base text by comparison with the other surviving manuscripts. He must then carefully note all the textual variations in all the other manuscripts and record this in footnotes, so that the reader does not have to depend solely on the judgment of the text editor. There are actually protocols for doing all this. The result is what is called a "critical edition" of the original text, usually published without translation.
The preparation of a critical edition is often times a huge task. There may be many manuscripts to collate. Occasionally the editor must consult translations of the text into other ancient languages (such as Arabic or Latin) to confirm key passages or supply text missing in the original. Manuscripts from different eras may employ radically different handwriting styles, so the editor must be skilled in paleography. The spellings of words may drift over time, reflecting different changing pronunciations. And particularly in a field such as astrology, there may be a fair number of otherwise undocumented words, or words used in an unconventional sense. We owe a debt of gratitude to those scholars who undertake this largely thankless chore.