A folding telescope unfolds to observe and folds backup to transport. The telescope bends over at the pivot points so that one part fits on top of or within another part.
Here's my folding 13 inch telescope being unfolded during the 2011 Oregon Star Party Telescope Walkabout.
And here I am observing at night with the scope (image courtesy Craig Stott).
A sliding telescope slides in one direction to observe and slides back to transport. The telescope's parts slide over each other so that they fit within each other.
Here's Bert Bogchelman's scope that unfolds in seconds. Bert says, "The telescope has a very quick setup, within a few seconds you can set up and collapse the tube. I have build a few truss Dobsons (I still have the word record smallest 8 inch) but I was sick and tired to set them up and even worse, to collapse them again after a long night of observations.But a full tube 12.5 inch Dobson has a huge tube and don't fit in a small car. So, I build this telescope and honest, I am very, very happy with it. I have had a star party a few weeks before, and I was the only one with a big telescope who was able to look between the rain showers.
After the launch (3 seconds) you only have to turn the focuser and the finder from inside the tube to outside, and they snap into their places with magnets. The scope needs after setup only some minor collimation, like a truss design."
Collapsed high: 61cm
Weight: 23 kg all-in (incl. the rockerbox), so you can carry the hole telescope in one.
12.5 inch F5 Strehl 0.94 mirror
Secondary mirror: 63 mm (20%)
2 inch Antarsi helical focusser
50 mm finder, right angle, correct view (Amici prism)
Images courtesy Jan van Gastel and Bert Bogschelman.
Any telescope that doesn't fold or come apart or requires assembling or connecting of parts such as truss tubes or constructing some part of the telescope is not a folding or sliding scope.
For example, here's my compact and easy to transport 6 inch. It's small enough to fit into the back seat of a small car with the rocker fitting over one end of the tube assembly. This scope stays fully assembled.
Here's a 6 inch scope that I took to Africa. The truss tubes come apart to fit into a backpack. It uses a single strut slide and pivot mount.
Travel scopes that pack tightly but take assembly time and effort. Typically one travels to a destination, assembles the scope and observes without further disassembly and re-assembly.
Here's Greg Babcock's 12 inch travel scope that he took to Argentina.
A folding scope offers extreme ease of use, is quick to setup, is easy to setup, is lightweight and takes up less room (camping gear occupies a much greater volume). The usability of a scope is not a single factor. A standard truss dob disassembles and stacks into a smaller volume. But the assembly takes effort. The usability is a combination of several factors: lightweight * compact travel size * ease of setup * speed of setup * aperture * absence of ladder. A folding scope attacks several of these usability factors: compact travel size, ease of setup and speed of setup.
The Prime Directive is to connect to the universe, contemplate our place in it and the meaning of it all (spiritual), gather observations in pursuit of a scientific study. A high usability scope can make us feel more connected.
Whatever design we pick needs emotional appeal: it must be alive, not feel cold or dead.
The basic technical challenge is to squeeze air out of the telescope by folding or sliding itself into a much more compact space.
The design and building challenge? Iterate, grow by small changes, favor fewer number of simpler ideas, build on ideas of others, make it beautiful, be a creator and craftsman.
I judge telescope designs by asking, "Is it more useful than before, is it more exciting than before?" In the end, I develop for myself. Perhaps others will find my designs worth looking at. I studied Origami for folding inspirations along with other fields, particularly engineered products like folding bicycles. My goal is to squeeze air out of the telescope.
Here are my concept sketches.
Here is a study of Ken Lulay's folding design, adapted for a 13 inch f/3.0. Google Sketchup athttp://sketchup.google.com/3dwarehouse/details?mid=58efdb0b1da43e3599c9bc4868836a56&ct=mdsa
I also designed and built a folding spider design which takes up very little room. The Google Sketchup can be found herehttp://sketchup.google.com/3dwarehouse/details?mid=18d7ba37a579484899c9bc4868836a56&ct=mdsa
The Google Sketchup of my Zip Dob is athttp://sketchup.google.com/3dwarehouse/details?mid=447e8896bc0fb328b47a4848f4241ae0
For more on my 13 inch folding scope shown at the top of this page, see http://www.bbastrodesigns.com/ZipDob/ZipDob.html
Michael Koch's folding ruler scopehttp://www.astro-electronic.de/reise.htm
Horace Dall's folding Cassegrain. (images by Peter Abrahams). Also check out a video interview with Horace at http://vimeo.com/28160854
Bob Cox's folding Cassegrain
Ken Lulay's folding 8 inch (on the left)
Victor Nikolashin's folding Cassegrain (1970 RTMC) Image from http://www.rtmcastronomyexpo.org/RDS/RTMC70.html
Carl Lancaster's articulated mast folding scope can be seen at http://www.was-ct.org/2013/07/july-2013/, http://stellafane.org/convention/2013/2013-scopes.html, http://stellafane.org/convention/2013/images/scopes/scope27a.jpg, http://stellafane.org/convention/2013/images/scopes/scope27b.jpg, http://stellafane.org/convention/2013/images/scopes/scope27c.jpg, http://stellafane.org/convention/2013/images/scopes/scope27d.jpg
Besides BertBogchelman's shown at the top of this page, here are other sliding scopes that I know of.
Pat Cannon's folding 17 inch. (Swayze's 40 inch is in the right background.)
Mark Yonkers, current owner, unfolding the scope.
The Teleport telescope, a commercial product, see http://www.teleporttelescopes.com/index.html (image from the website).
Dale Sander's telescoping telescope. See http://sites.google.com/site/telescopingtelescopes/ for more images and descriptions.
Oragami kayak, see http://vimeo.com/20047767
Folding kayaks, see http://www.foldingkayaks.org/WP/
Folding bicycles, see http://www.bikefriday.com/ and http://www.designswan.com/archives/12-creative-folding-bike-design.html
Folding structures, see http://www.brianmicklethwait.com/culture/archives/2005/01/bridge_that_rol.html
Folding tables, see http://www.studiotoer.com/postable