#AGENA ASTRO SCOPE RINGS UPGRADE#

Carbon fiber will not expand and contract with temperature changes like steel, so it should maintain focus much better as the temperature drops over the course of the night. For an additional $500, the telescope can be purchased with a carbon fiber tube. The basic 8-inch RCT comes with a steel tube. These two upgrades combined cost almost as much as the OTA! I also purchased a Technical Innovations Robofocus so I could focus the telescope remotely using my computer.

#AGENA ASTRO SCOPE RINGS UPGRADE#

Because I planned to do a lot of imaging with this telescope, I decided upfront to upgrade to a JMI EV-1 focuser, which I consider one of the top focusers on the market. The 8-inch optical tube assembly (OTA) came with a finderscope mounting shoe, one two-inch focuser extension ring, two one-inch focuser extension rings, a two-speed GSO focuser, a Losmandy-style dovetail plate, and a Vixen-style dovetail plate. The only real difference is the RCT has a much larger secondary mirror, which is required to obtain the faster focal ratio. The both look pretty much the same – flat black interiors, baffled to improve contrast, identical secondary mirror cells and spiders. Image 2 shows the inside of the GSO RCT (left) compared to the inside of the GSO classical Cassegrain (right). You can use an RCT for visual use, but with a 47% secondary obstruction by diameter, the views are not as great as in an 8-inch Newtonian or classical Cassegrain. I purchased my 8-inch RCT ( Image 1) from Astro-Tech solely for the purpose CCD imaging. The most popular size sold is the 8-inch model, which starts at $895 with a steel tube. They are made by Guan Sheng Optical in Taiwan and are sold by Agena Astro, Astro-Tech, iOptron, Orion Telescopes, and Third Planet Optics (TPO). Smaller amateur RCT with sizes down to 6-inch mirrors did not hit the market until this decade. PlaneWave Instruments and RC Optical Instruments still make these large, research-grade RCTs. I ordered the third 20-inch telescope they made. Parallax only made a few RCT, all in the 20-24 inch size.

Back then only three companies offered 20-inch RCTs: Parallax Instruments, PlaneWave Instruments, and RC Optical Instruments. My first experience with an RCT was when I ordered a 20-inch RCT for the Coast Guard Academy’s Astronomical Observatory in the year 2001.

Ritchey-Chrétien telescopes are the telescopes of choice for major observatory instruments. RCTs with the same primary mirror diameters have smaller overall tube lengths than classical Cassegrains and SCTs. Typically you don’t need to use a field flattener with an RCT unless using a large format camera. With smaller secondary mirror magnifications, RCTs also have less field curvature than SCT. Image 2Ĭomparing RCTs to Schmidt-Cassegrains telescopes (SCTs), which usually have secondary mirror magnifications of 5x and overall focal ratios around f/10, the RCTs are also much faster. The overall focal ratios are f/7 to f/9, much faster than classical Cassegrains. RCT usually have f/2 to f/3 primary mirrors with secondary magnifications factors from 2.5x to 3.5x. Both have hyperbolic secondary mirrors.Ĭlassical Cassegrains usually have focal ratio (f-number) f/4 primary mirrors with secondary mirrors that yield 3x-5x magnification factors. Unlike classical Cassegrains, RCTs do not have parabolic primary mirrors, but hyperbolic mirrors. Ritchey-Chrétien telescopes are a type of Cassegrain telescopes designed to eliminate coma. The second RCT was a 40-inch built in 1934 for the Naval Observatory and is currently at their Flagstaff Station. Ritchey actual built the first RCT in 1927. Image 1Īmerican astronomer George Willis Ritchey and French astronomer Henri Chrétien invented the Ritchey–Chrétien telescope (RCT) in the early 20 th Century. The classical Cassegrain would be the better choice for visual use, while the Ritchey–Chrétien is the better instrument for astronomical imaging. In this article, I review the GSO 8-inch GSO Ritchey–Chrétien. I recently reviewed a Guan Sheng Optical (GSO) 8-inch classical Cassegrain telescope for Astronomy Technology Today.

Agena astro scope rings