While not as popular as the Schmidt-Cassegrain or the Maksutov-Cassegrain, the Classical Cassegrain design (employing a parabolic primary and a hyperbolic secondary mirror), which is often thought of as an expensive telescope, is making a minor comeback thanks to low-cost overseas manufacturing. This optical tube assembly is available under many different nameplates. Doing a quick search turned up the “Orion” and “Astro-Tech” branded variants, along with nearly half a dozen others being sold in various places around the world. The price varies, depending on where you live but you can expect to pay anywhere from $600 to $800.
I borrowed this GSO-branded sample from a local club member. The scope comes with a Crayford-style two speed 2” focuser, finder bracket, and a Vixen-compatible dovetail plate running along the entire length of the tube. In a move sure to please those who dislike moving mirror-based optical systems, the scope’s primary mirror is fixed, and can even be collimated. The disadvantage of this design is that all the focus travel must be taken up by the focuser. To aid the user, three extension tubes (two 1” and one 2”) are provided. For most of my use, I found either 2” or 3” was enough to accommodate my visual and photographic needs.
Beginners, take note! A 14 – 15 lbs. optical tube with an 1800+mm focal length needs a sturdy mount, and you shouldn’t compromise here. I used the scope with my Celestron AVX ($900 USD). While it was adequate for most of my needs, at medium to high power it was happier aboard my larger CGE and EM200 mounts. Luckily, I believe most purchasers of this optical tube will be experienced observers who already own a mount or two.
Classical Cassegrains usually cause seasoned astronomers’ ears to perk up, but they may be disappointed here – but in a good way. The optical tube is a perfectly competent 6” telescope, and a decent value for the money. The star test showed minor overcorrection and a minor zone about 2/3 of the way from the center. While this may sound alarming, I didn’t notice anything untoward while viewing images in focus. Keep in mind that we are talking about an optical tube selling near the “budget” category.
I caught all six members of the Trapezium with a 13mm Nagler Type 6 eyepiece (141X). With a slightly lower power 19mm Panoptic (96X), open clusters like M35 through M38 were nicely framed. If you like to look at clusters or planetary nebulas, this just may be the scope for you.
Cassegrain telescopes excel at lunar and planetary webcam imaging. Some of the most skilled people in the world at this discipline use stock, off-the-shelf Schmidt or Maksutov Cassegrains. On a couple of clear nights in February 2022, the GSO Classical Cassegrain performed very well in lunar imaging with my ZWOASI120MM-mini (see below).
Interested in deep sky imaging? Eh…you might want to look elsewhere. The scope’s long f/12 focal ratio and the lack of dedicated field flatteners or coma correctors make it less than ideal for capturing the dim stuff. As an example, see the image I took of the Horsehead Nebula below. While it’s acceptable, you can see the struggle I had pulling out faint details.
Drawbacks? Not much. It’s a competent scope at an attractive price. The Cassegrain’s major fault may have nothing do with the scope itself. Meade and Celestron have been selling 6” f/10 Schmidt-Cassegrains for decades at similar prices. During that time, they’ve had plenty of opportunities to refine their products, and there’s an immense amount of product support out there. The SCTs are smaller and lighter (7-8 lbs. vs. 14-15 lbs.) which will pose less of a load for the mount. Still, for those who have always been curious about this “classical” design, I can recommend this optical tube. The choice, as always, is yours!
For more on the GSO 6” f/12 Classical Cassegrain, see Ed’s video review at: https://youtu.be/Stlw04tsrc8