The Classical Cassegrain: Proven Excellence

Most telescope users are familiar with the Schmidt-Cassegrain design, a short-tube catadioptric that is that is quite popular among astronomy and astrophotography buffs. But while most people are familiar with the name Schmidt, owing to the Schmidt camera, invented by Bernhard Schmidt, the origins of the Cassegrain part of the name eludes many. The Cassegrain reflector’s design is generally attributed to Laurent Cassegrain, a French priest, who developed it sometime in the early 1670’s. It’s interesting to note that it wasn’t Cassegrain himself who published about this design, but a certain M. de Bercé, who described the design in a scientific journal published by the famous French doctor, Jean-Baptiste Denys. This is probably why it wasn’t until the mid-1700’s that anyone actually attempted to build one. The reluctance to implement the Cassegrain design perhaps stems from the fact that it wasn’t a proven design yet, compared to the well-published and readily-available reflecting telescope designs made by Gregory and Newton, the latter eventually becoming the most popular style of reflector.

Furthering the obscurity of the Cassegrain design is the fact that other notable telescope builders criticized the design heavily, for one reason or the other. The most famous person to have cited its supposed flaws is the Dutch astronomer Christiaan Huygens, who perhaps did this in support of the more popular Newtonian reflector design (Huygens himself was only familiar with open-tube refractors). While we may never know the reason why some astronomers of the period rejected Cassegrains design, the impact of this negativity was sufficient to muffle its popularity among observers.

It wasn’t until the 1940’s when Cassegrain’s design was brought to light again, but only as a component to a new design that incorporated Barnard Schmidt’s corrector plate. This new design, developed by James Gilbert Baker and his team at the Mount Wilson Observatory in the early 1940’s, eventually became known as the Schmidt-Cassegrain reflector, a long-focal length design but with a short optical tube, which made it very popular due to allowing large apertures yet having the portability of a short-tube reflector. It is this design that enabled companies such as Meade and Celestron to establish themselves as premiere telescope retailers, considering the fact that the design empowers even casual astronomy enthusiasts with a compact yet high-fidelity instrument with which to view and photograph celestial bodies.

Despite its popularity, Schmidt-Cassegrains have several issues that observers have to constantly contend with, and can be a headache especially when using the telescope as a pure astrograph. One significant example is the fact that if the temperature difference is significant inside the sealed tube and the exterior, dew and moisture can accumulate on the corrector plate. This usually forces the SC user to invest on dew shields or plate heaters, which only adds to the overall weight of the telescope. One also has to provide battery power to some of these devices, and when observing in a remote area, this would tax into the limited power budget you may have prepared for that evening’s observation session.

Another issue with Schmidt-Cassegrains is the fact that the mechanism for focusing involves the primary mirror having to move back and forth on a chassis linked to the focuser. This can lead to situations wherein the act of focusing causes the mirror to wobble or flop, due to the gear threads shifting position on the chassis while you focus, impacting on collimation and optical precision. Yet another issue connected with this focuser design is the fact that mounting a rather heavy diagonal + eyepiece, or camera or other imaging devices on the draw tube can also have an impact on the collimation, considering the connection of the tube with the chassis that cradles the primary mirror. This further aggravates the mirror flop or shift while doing focusing.

Many Schmidt-Cassegrain users have grown accustomed to finding solutions or hacks to deal with the issues and problems associated with the design. However, there are some who sought out to find ways to eliminate this problem by perhaps developing a yet new reflector design. Some telescope designers however, simply came up with a more pragmatic solution—why not rebuild the Cassegrain the way it was designed by the inventor? The result of this pursuit is the Classical Cassegrain. This modern take on the classic Cassegrain design not only mimics the original concept for this device, but incorporates critical improvements and modern techniques which further enhances its precision and usability. We now take a look at the upcoming 8” OTA of the Classic Cassegrain design soon to be available in the ASC inventory.

Upon first glance at the design, the most obvious difference between a Classic Cassegrain (CC) and a Schmidt-Cassegrain (SC) is the lack of a corrector plate in front of the tube. Most SCs hold the secondary mirror in place via the corrector plate, but in the case of a CC, a traditional 4-vane precision-mounted spider reminiscent of the Newtonian design is utilized. The lack of the corrector plate immediately eliminates the perennial problem of moisture build-up on the plate itself. As for the issue of the primary mirror dewing up, the OTA is slightly longer (f/12) than that of a standard 8” SC, and thus dewing on the primary isn’t a big issue—the tube itself acts as its dew shield. The spider holds a secondary mirror mount that is 68mm across, which may seem large, but obscuration of the primary mirror in terms of diameter is 34%, but in terms of mirror area this only corresponds to only 12%. The long focal length of 2400mm (f/12) seems quite extended, but is in fact “folded” into an optical tube just 25″ long, still within the range of compactness as other SCs or Maks.

Another feature that’s immediately noticeable once you peer into the tube is the addition of internal baffling or ribs along the length of the OTA. Most telescope tubes rely on either a matte black finish, or perhaps some black felt material to reduce light scattering. However, it has been observed that baffles are one of the most efficient ways to subdue light scatter and suppressing stray light that would otherwise wash out images and produce less-than-optimal contrast. The upcoming ASC 8” CC has 11 baffles running the entire length of the OTA, effectively minimizing the light scatter. There have been reports of people attempting or successfully removing these baffles from CC’s, in the hopes of increasing the light-gathering properties of their tubes. An owner is indeed free to do this as a choice, but it must be stressed that these baffles were installed in the tube for a reason, and its removal might actually degrade image quality rather than improve it. More light does not necessarily equate to better image.

Moving on to the focuser, the CC features a very robust linear bearing Crayford focuser that can accommodate up to 2” diameter attachments. The focusing knob has a welcome two-dial dual speed setup, allowing coarse and fine focus adjustment. The fine focus dial is calibrated to move in 1/10th finer scales of the coarse focus knob, which is a necessity during high-magnification imaging and observation. The robust linear bearing design allows for a bit of extra weight for such things as draw tube extensions to hold monochrome cameras and/or filter wheels, with no worry of slippage. The CC comes with one 2″-length extension ring and two 1″-length extension ring, and a 1.25” adapter. The threads of these accessories are blackened to prevent  glare. It’s important to note that since the mechanism doesn’t involve any movement of the primary mirror, as such in an SC, then the stationary nature of the primary and secondary mirrors only lends more to ensuring a quality image. Speaking of the mirrors themselves, both the 200mm primary mirror and the secondary are made from an enhanced aluminium coating with an overcoat layer of thermally stable quartz. This assures that the mirrors won’t exhibit image shift or mirror flop like the Pyrex ones in a typical SC or Maksutov, plus it provides for a shorter cooling time. These mirrors offer 96% reflectivity.

The total weight of the OTA is at 18.2 lbs (8.25 kg), which means that you still have around 11 pounds worth of accessories that you can still attach on the OTA to support 30-40 lb mounts such as an iOptron iEQ45 Pro. For attachments, you are provided with two dovetail finder scope bases, giving you the freedom to mount an optional Orion finder scope on either side of the focuser, or to mount a small guide scope and finder scope simultaneously, a setup ideal if you’re constantly switching between Alt-Az and EQ orientation and require attaching two different finder setups.

In summary, what you get is a telescope with good optics, quartz mirrors, added light baffles for improved contrast, shorter cooling time and all this for a great price. If you’d like to dive into high-magnification sessions, using an 8mm eyepiece for the CC will yield 300x of useful magnification, which makes this an excellent instrument for such a compact setup, yet it is also perfect for wider-field sessions with very minimal distortions up to the edge of the viewing field. All in all, truly an exciting telescope to have and use, a true classic, as designed and conceptualized by Laurent Cassegrain over 300 years ago.

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