Two travelscopes

My long running ideal for a telescope has been a foldable lightweight design which is still small enough to transport to the darker areas in the world, i.e. by airplane. These designs have become a bit outdated, now most foldable scopes seem to boil down to a variant of the Strock design.
These Dobs however all have a relatively heavy primary, and hence balance can be achieved with smaller trunnions. In my case, the main mirror (my first) is thin and light. This imposed some challenges that are detailed below. The first emanation (MkI) was top heavy and requires a large counterweight. The second one (MkII), using the same mirror, balanced better but is quite cumbersome to set up.
When I start a MkIII travelscope it will probably be similar to the very elegant Litescopes.


Travelscope MkI

So the design challenge (back in 2001) was to fit the 10" aperture I wanted into the airline restrictions. The size restrictions applied by most airlines are approximately: 35cm wide x 25cm high x 50cm long. For a 10 incher this means there is only 5cm of room on each side of the mirror, which must cover ventilation space, mirror box and rocker box. If the mirror box is square, and the rocker box is as big as the restriction allows, there is about 15 cm of room inside to contain focuser and eyepieces. The obvious thing to do is make it a truss design, where secondary cage and mirror box can be folded together as a small packet. The trusses can be U-profiles, which may be folded together into a very small package that can (if neccessary) be stacked with the main luggage inside a PVC pipe.

The scope has been built, and recycled. Click here to see the (historic) result.

Click on the drawings below to zoom in on an area:

Optical tube assembly Rocker box

Travelscope MkII

The travelscope MkI indeed turned out just within carry-on luggage size limits (550x350x250mm), but waaaaay over the weight limit of approximately 5kg. Instead of focusing on the outer box sizes, a better approach is to first start with the lightest weight collapsible OTA construction. With the projected lightweight mirror (250mm, 2kg) the key design decision appears to be moving the elevation axis (i.e. the balance point) up, towards the focuser. This also reduces the imbalance effects caused by using different size eyepieces and also results in a lower overall weight.

The OTA consists of three parts: a thin mirror box, a secondary ring and a central- or trunnion ring. A strength of this approach is that the precise telescope dimensions can be determined on the fly:

  • 1: The three OTA parts are finished completely
  • 2: The truss lengths are calculated and the OTA is finished
  • 3: The rockerbox is made, since its sizes depend on the OTA

The resulting telescope can be seen on this page.
Click on the drawing below to zoom in on the various parts:

Optical tube assembly

For a similar design, see Frodo.