8 inch f4 newton 1.0



On holiday to Greece in 2005 I realized that it would be nice to have telescope you can take with you on flights abroad. When designing the telescope I had a couple of criteria;

– The telescope needed to be carried as hand luggage when traveling by airplane and had possibly fit in a backpack;
– It had to have as little spare parts as possible;
– The telescope had to have its own suitcase in which all the parts would fit;

– The suitcase would have to conform to airline regulations;
– Of course a wanted a little aperture;
– Setup and breakdown of the telescope had to be possible in under 10 minutes.

The choice for a Newton with a short focal length was obvious because of my previous construction experience and because of the simplicity of the design. Trusses are also a major component and are sometimes a problem (unless they are retractable). Since I already had previous experience with making strings, the travel telescope would be a stringtelescope. I bought a 8 inch f 4 mirror, matching secondary and secondary holder and started building.


Secondary ring

The secondary ring consists of 3 U-profile sections and pieces of plywood that form an asymmetric hexagon. This is a very thin secondary ring which performs perfectly despite my initial reservations with regards to rigidity. The vanes of the spider are a little thicker than usual (3mm) which adds some extra stiffness. The focusing device is a simple plastic thing that is permanently attached to the plywood focuserboard with epoxy.

Strings

The strings are made BCY 450 + bowthread with a “servingjig and are wound 4 times. Because the threads are zero-stretch vibration is practically nonexistent. If, while observing at a magnification of 200x one tapes the telescope, theres hardly any vibration in the image. The strings are semi-permanently attached to the secondary ring. They are adjustable in height, but collimation is done the traditional way through adjusting the secondary. The poles are shopped up Nordic walking poles and are retractable for transport. They have a threaded rod that’s epoxied in the tube. Unscrewing a nut lifts the pole and tensions a pair of strings.


Mirrorbox

The mirror box is made of 6 mm and 9 mm (top) birch plywood. I chose a box because this gives some protection to the mirror and leaves room for the components in the box. A more open design also has little weight advantage in a telescope of this size. The inside of the box is lined with thin type of foam that I also use as a light shield behind the secondary mirror. The primary mirror is glued straight to the holder with three drops of silicon glue, the pressurepoints where calculated with Plop.

Mount

The mount of the telescope is perhaps best described as an asymmetric Dobson mount. With smaller light Dobsons there is a risk of lifting the OTA when pushing against the telescope. In commercial models this is solved by adding a spring. In my telescope, the axis on the observers side is a fixed point. This keeps the telescope stable but he still preserves the qualities of a Dobson. The side bearing has a ebony star glued to the edge and rides on Teflon. The slip on the other alt-axis is adjustable by a rotating a knob to create more friction between a piece of a Teflon plate and Formica. This is especially useful because the weight difference between the eyepieces that I use. In addition, a can shift a laser, that’s attached to one of the poles, to balance the telescope.


Rockerbox

The rest of the rockerbox consists of a cross with wheels attached to the end. It rests on a central circular plate which has slip resistant material which keeps the telescoop from moving around on the ground. The central part of this is connected to the cross via a Telfon disk. The circular plate also serves as a cover for the mirror box. The wheels are just touching the floor and do not bear the whole weight of the telescope. The rocker works very well, upon arrival at a location one just needs to find a flat surface.

Using the telescope

The case, a standard hand luggage case, was reinforced with foam and plywood to make it more rigid. There is room in there for all parts of the telescope including eyepieces, charts and a small laptop. First light was in May 2006, in my backyard, the heart of Amsterdam. The optics by GSO where much better than I expected. After several nights in the field I did however come to the conclusion that I need to improve some aspects of the telescope in the future. The setup of the telescope takes too long, mainly because of the amount of parts needed to construct the secondary ring. Once thats done, the telescope is up and running in under a minute, but to get it back in the case takes too long. The mount works just fine and tolerates high magnification well. Collimation is a matter of seconds with a Cheshire eyepiece. My first real trip (to Italy) with the telescope was a blast.