I recently read an article by Rabbi Yaacov Schwartz in the Journal of Halacha and Contemporary Society (Fall 1988) titled "Jewish Implications of Astrology." In the article, he quotes the Rambam in the letter he wrote to the rabbis of South France. Rabbi Schwartz also quotes one Alexander Marx, who discusses this responsa in Vol. III of HUCA (1926).
Can I find these letters online?