Rich Field 250mm Newton Telescope

The travel dob I started my ATM carrier with just did not cut it. Main reason is that the mirror is too light (<2kg) and the focal length to large. The only functional dob has a very high elevation axis, which makes the design too complicated to carry along.
So this project shortens the focal length of the mirror and turns it into a rich field F/4 telescope. The telescope is going to be used for photography, and will be a closed tube design that rides on a German Equatorial Mounting (NEQ6).

The design

Building log

Secondary holder and spider

The secondary holder is built as a simplified variant of this design by Conrad Hoffman.It partly uses kinematic principles, at least to determine the tilt of the mirror. A lot has been said about the need for a full kinematic design of a secondary holder and spider, but in my opinion there are other more serious mechanical problems to solve to ensure that the scope keeps its collimation. I think a little tweaking (if needed at all) before the observation is not too bad as an alternative for a complicated design that requires advanced tooling. The spider is centered in the telescope tube by means of shims under the vane mounting points. Since you can only adjust a mm or so, it requires some care when glueing the mirror to the holder, in order to get its position right. The axial position as well as rotational orientation can then be adjusted by means of the M6 center bolt. When right this position is locked by means of a locking nut. Finally mirror tilt is adjusted with the single setscrew in the secondary holder. The spider vanes are made of curved brass strips, along the way it is proposed by RF Royce. The vane curve radius is taken approximately equal to the tube radius. The center nut in fact consists of three nuts, since a coupling nut was not available in brass. The brass strips are 25mm wide, 0.8mm thick. They will be sanded and sprayed flat black to minimize reflections.

Primary mirror cell

The primary mirror cell and bottom of the telescope tube are constructed as a single module that is attached to the tube wall with three M4 screws.



The cell is a six point flotation type, which is just sufficient for this mirror. A six point cell is fairly easy to construct well. The higher performance theoretically achieved with a more refined cell could easily be cancelled out by construction errors.
Since the telescope will be on an equatorial mounting, the side support is a bit of a compromise. I have chosen for six setscrews around the edge of the mirror, so always two will be touching in any orientation. To hold these setscrews, as well as the retaining clips, the cell is constructed as a ring.
The increased thermal insulation of the edge by the ring might even help to achieve a flatter lateral temperature curve (see mirror cooling article). To further improve thermal behaviour, a fan has been mounted in the bottom to suck away air from the mirror surface. To guide the flow a baffle is mounted a few centimeters in front of the mirror a-la Mauro da Lio. When running at full capacity, the fan sustains an airflow through the telescope tube of about 10cm/sec.



Alas, mirror broken, had to make a new one.

 

The cell had to be rebuilt for the new mirror, since it was a few mm larger and did no longer fit. The same principle was used and the collimation screws had to be retrofit. The M4 threaded rod that functions as the fulcrum for the lever is glued into a V-groove. The use of a V-groove is actually quite good, since it gives you very good control over the position and orientation of the rod. The levers were made to be flush with the cellbottom top surface, so the glueing of the rods could be done with the levers as guide.



For positioning accuracy and smoothness of movement the M4 rods fit into a piece of 1mm wall aluminium tube 6mm external diameter. This small tube is just longer that the lever U-beam width, to avoid clamping. The levers are fixed with two rings and a locknut. The mirror rests on felt pads, maybe not the best material but it will do as a start.

Result

The mirror was startested in a preliminary version of the telescope on 27th of March 2015. The stars seem to have a very slightly better defined edge inside than outside focus. Otherwise there is no difference. The moon views are tack sharp at 160x, lots of detail. Unfortunately the 6.3 mm eyepiece is the shortest I have...

Unfortunately the mirror has become irreversibly damaged during the coating process (see mirror log). A new mirror has been made, a bit larger (255x25mm) and of borosilicate rather than soda lime glass. This means also that the mirror cell also has been rebuilt.

The result: