My Eight Inch Ritchey-Chretien Travel Scope

Without any of my tools or any recent experience I think it would be best to do something small, but I want to do something more interesting and challenging than the three Newtonian reflectors I made as a teenager, so I've decided to make a travel scope, one that can be packed in a carry-on bag to take on an airline trip. I recently married a woman from Newfoundland, and we have friends in Nova Scotia, and my parents are in Washington State, my sister in Idaho, I have friends in California, my aunt's in Long Beach, and wouldn't it be nice to have a scope I can take with me to show all my friends and relatives in my travels?

I had considered at first ordering a six-inch pyrex glass grinding kit from Newport Glass but Dan Cassaro approached me about his plate glass kits after I mentioned this on the amateur telescope maker's mailing list. Pyrex holds its shape better as temperatures fluctuate, but plate is easier to grind. You can make up for the thermal problems somewhat by using a thinner disk, which will work if you use a well designed cell to support the primary mirror.

Cassaro's kits seemed attractive and I could get an eight inch kit from him for the cost of a six inch kit from Newport, and I thought it would be good to do business with a fellow amateur. I ordered his 8 inch kit on May 28, 2001.

The basic design I will use for my scope uses a large concave primary mirror with a smaller convex secondary mirror mounted near the focal plane of the primary. The beam is reflected back towards the primary and passes through a hole where the eyepiece or camera observes the image in the back. Using this basic design allows one to get a long effective focal length from a compact instrument. There are different specific designs that use this basic principle:

The Cassegrainian: This design uses a parabolic primary mirror and a hyperbolic secondary mirror. The chief advantage of the cassegrainian over the other similar designs is that the primary mirror can function on its own to make a properly focused image; one can remove the secondary and put a flat tilted mirror in its place to form a newtonian (or mount the flat in front of the hyperbolic secondary) and get a telescope with a shorter focal length and wider field of view.; The 200 inch telescope on Palomar Mountain is a cassegrainian.
The Dall-Kirkham: The Dall-Kirkham uses a prolate spheroid primary mirror and spherical secondary mirror. Its chief advantage is that it is easier to figure the secondary - for reasons I will go into later it is very hard to figure a convex mirror of any sort, and much harder to accurately figure one that is not a sphere.
The Ritchey-Chretian: This design uses hyperbolas for both mirrors. Its chief advantage is that it is free of coma - that is, off-axis images of stars are round.; The space telescope, and many modern observatory instruments are Ritchey Chretiens.

I'm going to need to spend some time working out some designs before I settle on one, but at the moment I'm thinking of doing a Ritchey-Chretien. My understanding (from my hazy memory) is that a Ritchey-Chretien allows one to have a shorter focal ratio while still maintaining good image quality. I want the scope to be compact, but as the f-number of a non-spherical mirror is made smaller, the coma increases, and dramatically so in the range I'd like to make my mirror. The Ritchey-Chretien, though, is free of coma.

There are other problems to consider, such as curvature of field (that is, the focal "plane" is not flat in a Ritchey-Chretien), so I will need to work with some design software (guess I can't get away from computers after all). Others have urged me to make a cassegrainian or a dall-kirkham.

My 10 inch is also meant to be of this general form, although I've always meant to make it a cassegrainian so I can use the prime focus. Here's a funny anecdote for you - mirrors are perforated by grinding through them with a metal tube and abrasive, going slowly on a drill press. One can either grind all the way through and cement plug back in the hole until polishing is completed, or grind from the back almost all the way through to the front.

My 10 inch is ground through almost all the way, so I'll need to finish the perforation when I'm done. There is the problem of finding another grinding tube of the same size when the time comes - but I know I will have no problem, because I ground most of the way through with a Campbell's soup can! I drilled a hole in the unopened end of the can, tightened a bolt in the hole then chucked it in the drill press at the Chabot Amateur Telescope Maker's Workshop in Oakland, California and spent an evening grinding.

There's a couple other designs that should be noted here - the compact commercial telescopes one often sees that are made by Meade and Celectron are Schmidt-Cassegrains. They are similar to the Cassegrainian design except that they use a "corrector lens" on the front of the scope which appears flat but has a complex curve ground into it to correct spherical abberation. Telescopes with schmidt corrector plates can be made quite short without coma, but they are pretty hard to make. I'd like to make one in the future but I'm not up to it yet.

The other I want to mention is the gregorian. This also uses a secondary mirror that points the beam back through the hole in the primary, but in this case the secondary mirror is a concave prolate spheriod (ellipsoid) and it is on the far side of the focal plane. It has the chief advantages that the image is right side up, so it works well for terrestrial viewing, and it is extremely easy to test the secondary mirror. Its main disadvantage is that it makes the telescope longer than it would have to be for cassegrain and its variations with the convex secondary inside the focus.

My Eight Inch Ritchey-Chretien Travel Scope

Progress Notes