240 mm f/4,5 travel Dobsonians: The Quintuplets – Mechanical Components

Design Parameters:

- 240 mm (9.5") mirror size, thickness 15 mm or 18 mm
- mono ring upper part with eccentric spider
- 1.25" Helical Crayford focuser (self built, similar to HC-1)
- 6 pole truss design in 3 pairs, 2 longest poles can be taken apart
- 6 point mirror cell with lateral support by two ball bearings
- mirror box and rocker box made of multi layer birch plywood
- Assembly and disassembly possible without tools
- Total weight 4.4 kg (~ 10 lbs) with the 18 mm thick mirror

View from the Back Side:

The 2 truss poles on the back side are made of 15x1mm aluminum tubes. Threaded plugs allow to take them apart for smaller transport dimensions. The other 4 truss poles are made of 12x1mm tubes. They are fixed at the the top end of the altitude bearings

To add more stiffness to the delicate structure, a 4mm Dyneema rope was added that can be tightened at the upper ring with a knurled nut. The lower ends are spliced loops, that are attached to the lower back holding screws for the altitude bearings. The loops are made in splice technique known from sailing ropes.

Upper Ring:

The ring is made of 20x10x1.5mm rectangular aluminum tube. It was bent with a machine. The truss poles are attached in pairs through short pieces of L-profiles. and fixed from the upper side with M4 size knurled screws.

The 1.25" Helical Crayford focuser was made at our workshop. To further reduce the transport dimensions, the focuser with the holder plate can be disassembled from the ring.

The light shield opposite to the eyepiece is made of 2 mm conductive foam material used in the electronics industry for protecting ESD sensitive components. It fits into the rocker box for transport.

An effective stray light protection is essential at this extreme open truss tube structure. This is especially true at higher magnification, where the image brightness is low and even low stray light outshines delicate deep sky details.

Additionally I am using light baffles baffles screwed into the filter thread of the shorter focal length eyepieces. Each eyepiece has its own size of baffle, which stays at the eyepiece permanently. This type of baffle blocks stray light very effectively particularly in combination with low profile focusers and at eyepieces, that have a lens group in front of the focal plane.

Secondary Holder and Spider:

The 54mm secondary mirror was glued to the holder plate with 3 silicone blobs using 1.5 mm spacers. The holder plate is attached to the spider with one central M4 screw. It can be adjusted with 4 M3 screws that press against it. The eccentric spider of type Ross-Sackett is made of 1 mm aluminum.

The secondary mirror is disassembled for transport to achieve a small packing volume. Small depressions at the screw positions of the mirror holder plate help to get a reproducible mounting position, so the collimation will be preserved.

Lower Tube:

The 260x260x54 mm mirror box (approx. 10.5" x 10.5" x 2.15") is made of aircraft grade multi layer birch plywood. The side panels are 6.5 mm thick and the front is 3 mm. On the middle of the back side two truss poles are attached by plastic balls and clamping plates.

The curved parts of the altitude bearings are made of 15x15x1 mm square aluminum tube bent with a machine. The straight parts are made of 10x10x1.5 mm square tubing attached with small pieces of aluminum sheet and pop rivets We chose a large diameter of 500 mm to get the rotation axis close to the centre of gravity, and to generate enough friction, so the telescope won't move when exchanging the eyepiece. The front ends are connected with a short 12x1 mm horizontal tube and tightened against the mirror box with two crossed bicycle gear shift wires. Instead of wires, Stefan decided, to take 2 crossed aluminum trusses.

Mirror Cell:

The 6 point mirror cell consists of a triangle made of 10 x 10 x 1.5 mm square tube riveted together with shaped sections of aluminum L profile, that also serve as holders for the lateral bearings, the mirror clamps, and the mounting screw on the backside. The mirror support points are on three rockers made of 15x10x1.5mm aluminum U profiles. The rockers are attached to the mirror cell with axes made of 2 mm bicycle spokes, creating a floating support. The mirror rests on self adhesive teflon pads. The lateral support is achieved by 2 ball bearings at +-45° close to the lateral centre of gravity of the mirror, ensuring low friction forces.

Collimation is achieved from the front with 2 knurled M4 screws against springs that hold down the mirror cell. The third support point is fixed to the backside of the mirror box with an M4 hex screw.

Lower Light Shield:

Due to the low profile mirror box the secondary mirror "sees" light besides the mirror, that is coming from the ground around the telescope, which reduces the contrast, when observing above bright surfaces like snow. The light shield will efficiently reduce this stray light.

Rocker Box:

The rocker is made of multi layer birch plywood like the mirror box. The sides are 6,5 mm. The front and the small back are 3 mm. The bottom is a 3 layer sandwich made of 2 mm + 6,5 mm + 2 mm, resulting in a total thickness of approx. 10 mm. There is a large hole in the centre, that reduces the weight and serves for centring the rocker on the base with 3 ball bearings. A rim on the hole holds the base and the rocker together. The azimuth bearing exists of Ebony Star that slides on 3 teflon pads.

The base triangle is made of 15 x 15 x 1 mm square aluminum tube. It is slightly larger than the rocker to give a more stable support. It can be disassembled into 3 single bars for transport.

On soft ground there is a possibility to replace the 3 short M5 screws, which act as feet, by longer screws or add other parts that can be pushed into the ground.

Collimation with a Film Roll Container:

What weighs only 5 g and works better than a collimation laser on a telescope for visual observation? A transparent film roll container with a 4 mm hole in the bottom centre!

It works similar to a Cheshire eyepiece. By looking trough the hole the centre marking on the primary mirror and the mirrored image of the film roll container can be seen. By adjusting the two primary mirror adjustment screws, the images can be centred on each other.

In the dark, the film roll container can be illuminated from the side using a flash light.

Black film roll containers won't work.

Balance:

Originally, Ebony Star was used as sliding surface on the the altitude bearings. The movement was very smooth, but the retaining moment was too small, to hold the 240 g eyepieces in place. So, the the Ebony Star was removed again and the anodised aluminum slides directly on the teflon pads. The overall weight of the tube is so low, that the movement is still smooth enough.

Transport:

The telescope in collapsed condition. If needed, the focuser plate can be dismounted as well.

All parts fit into a daypack.
Firstlight on the roof plattform of the public observatory Munich "Volkssternwarte München".

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