A classical cassegrain, as defined by its inventor/French Catholic priest Laurent Cassegrain in 1672, consists of a primary parabolic mirror and a secondary convex mirror. The secondary is placed well short of the focal point of the primary making for a very compact instrument. Cassegrain never made an example of his telescope because grinding the secondary is very difficult. It would take two more centuries to pass before tests were developed which could measure the accuracy of the secondary figure. Cassegrain is also often recognized as inventing the accompanying design of having two mirrors placed on the same axis with the image coming to focus behind the primary through a central perforation of the primary mirror. Except that design was first proposed by Scottish mathematician James Gregorynearly a decade earlier in 1663. The Gregory telescope has a concave secondary placed well in front of the primary’s focal point but able to create a non inverted image.All these telescope designs are referred to as Cassegrain variants. Yet none of them have strictly a parabolic primary or a convex secondary mirror. They should be more correctly referred to as Gregory variants or simply as telescope designs of their own.It is entirely possible that Cassegrain arrived at his design independent of Gregory’s published work. Information did travel slowly in the 17th century. Shown here is British telescope maker Horace Dall in 1947 with his 3.5″ folding pocket reflector. He started making his namesake telescope as early as 1928 but did not publish. In 1938, Oregon native Allan Kirkham published the same telescope design in Scientific American, an ellipsoid parabolic primary with a spherical secondary. Both men acknowledged each other’s independent work and the scope is known everywhere as a Dall Kirkham. However, Cassegrain only published the one time and it took French astronomers considerable research to confirm his authorship. Meanwhile Gregory was a well known contemporary of Issac Newton, widely published, and his scope was first built by none other than Robert Hooke in 1673. Yet somehow, we continue to refer to these scopes as Cassegrain scopes!In 1968, a pair of brand new 12″ and 24″ classical cassegrain scopes were being installed in the brand new observatory domes on the brand new campus of York University, located in the NW corner of Toronto. The scopes were made by a company called Competition Associates which started life as a mechanic’s garage servicing high end sports cars owned by well heeled Harvard University students. The owner had acquaintances with people at the Smithsonian Institute in Washington, DC who commissioned him to build a tracking camera platform. A telescope making subsidiary was born. Ealing Corporation of UK with a history of making microscopes bought the company and marketed the scopes as the Ealing Educator/Research series. The entry level 8″ scope was priced just under $3000, apparently competitive for the era when that amount of money could easily buy a new car. The scope featured a large fiberglass tube, a cast iron tracking base with Byers gears, Timkin tapered roller bearings driven by synchronous AC motors. The 12″ scope at York University served with distinction until 1999 when it was replaced with a Meade 16″ SCT. It passed through several hands, shedding components until only the optics remained when they found me. I was happy because that gave me the freedom to re-imagine what this scope would look like in the 21st century.The optics for all Ealing Educator/Research telescopes were ground by The Optical Craftsmen and confirmed by owner Dick Nelson that the optics were ground to a precision of 1/20th wave for all institutional telescopes up to 24″ in size. That’s him in front of his 16″ Dall Kirkham on the front of his 1964 catalog offering finished telescopes and everything to build your own. All the great US telescope companies of the 1960s, namely The Optical Craftsman, Cave, Jaegers, Criterion, Unitron, Coulter and Novak, are all gone, victims of globalization and capitalism. Followed now by Meade and Celestron.The rebuild begin with a pair of 18″ diameter 1/4″ thick aluminum disks with ventilation and strut pole holes cut by CNC waterjet. The baffle tube and primary mirror cells were independently mounted.Equations make determining the length of the baffle tube and the size of baffles easy and precise. The baffle tube in Cassegrain/Gregory type scopes are crucial and should be well designed since they are prone to having stray light wash out the contrast of the image at the eyepiece.The scope features a 4 vane spider suspending the secondary and attached to the primary via three carbon fiber strut poles, 1 1/8th” diameter.The finished scope in action at my front door steps in 2015.
Afternote: (added Jan 15, 2023).
This appeared in the newsletter of Starfest 2022 where I gave a talk about this scope.
There are many reasons why amateur astronomers choose to build their own telescopes. Sometimes a scope to suit a specific need is not commercially available. Other times it’s for economic reasons. But there are other reasons not generally considered which I would like to visit.
In the 21st century, many men (and unfortunately amateur astronomy and especially ATM is still a male domain) are ensconced in careers with tenuous connections to the tangible world. What they do might be important but it leaves little physical trace. Middle class parents insist their children attend university with the mistaken belief that an academic education prepares its graduates for employment. Working with one’s hands is frowned upon as is a career in the trades since that’s something parents can’t boast about with friends over dinner.
So for some men making a telescope is a chance to be physically productive, to build something of value whose function can be shared. The value of a man amongst his peers is not judged by hunting prowess or how many heads he has crushed, but he is still judged by his skills. The primal ability to build and repair anything is an indicator of self sufficiency. In an increasingly harsh and dangerous world, the worth of a man is not his net worth but his ability to get it done.
For others it is a chance to indulge in role playing. To be a Michael Faraday performing real experiments and collecting real data on self built Victorian steam punk contrivances. To entertain that inner Walter Mitty that anyone who has ever worked in a cubicle surely develops.
And finally building a telescope is a chance to tell a story. Every scientific advancement is borne upon the shoulders of those who came before and very often a successful new telescope project is the result of a failed and abandoned project taken on by new eyes. There is great value in that story to be told because it emphasizes the spirit of collaboration and that no person toils in isolation. That all accomplishments are made by a group and help and assistance is just one member away.
My 12” f/13 Classical Cassegrain was one of two scopes I brought to my first meeting of local telescope makers and the first time I met its congenial host, Norm Folkers. These groups are notoriously hard to organize because everybody has busy lives so there has to be star attraction and that of course was Norm. Norm had done it all. Norm was the mentor that everyone wanted an audience with and in the best tradition of mentorship his wisdom was passed on to us and continues.
My Classical Cassegrain has its origins so deep in the past that it nearly predates me. The optics came from the smaller of two telescopes purchased and installed in the nascent York University in 1968 and then replaced in 1999 by the current Meade 16” SCT. The scope exchanged hands several times, its equatorial mount was lost and finally only the optics and the original OTA were all that remained.
Rather than be dismayed, I was delighted. This gave me the freedom to reconfigure the optics in a modern open truss structure, save valuable resources and bring a historic instrument back to life.
And in my own story, as a I transition from a house to a 2 bedroom condo as I downsize in the last quarter of my life, this will remain as the largest aperture scope I have and for which I will use for mainly planetary imaging and observation.
A couple of restored ‘60s era telescopes. (left) 12” Ealing f/13 Classical Cassegrain, (right) ATCO 80mm f/15 refractor
I have degrees in Biochemistry and Dentistry and practice clinically 2 day a week. The rest of the week I devote to photography and bringing you the best writing in this blog.
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