Founded by Clifford W. Holmes and the Riverside Astronomical Society, the RTMC Astronomy Expo began in 1969 as the Riverside Telescope Makers Conference at Riverside City College in Riverside, California. The annual Riverside Telescope Makers Astronomy Expo consisted of a star party event with a series of lectures about astronomy with telescope making with a competition for quality and innovative telescopes. In 1973 it was moved to a dark-sky site near Idyllwild and in 1975 it was moved to its current home near Big Bear City.
Clifford Holmes introduces famed astronomer Walter Scott Houston at the 1983 RTMC
The organization annually presents several recognition awards including the Clifford W. Holmes Award for "Major Contribution to Popularizing Astronomy, the Warren Estes Memorial Award for "Best Use of Simple Materials and Technique" in making telescopes, and the Clyde Tombaugh Award "Creative Innovation in Telescope Design".
To better reflect the evolution of amateur astronomy, the Riverside Telescope Makers Conference was renamed the RTMC Astronomy Expo in 2003. Each year the scope of activities continues to grow and evolve and has lectures, workshops, extensive beginners activities, a swap meet, astronomy clubs and other organizations, and vendors demonstrating and selling astronomical products. The Expo continues to promote family activities with swimming, canoeing, horseback riding, archery, wall climbing. Extensive hiking and mountain biking trails are also available.