A 400 mm (16 inch) Newtonian

by The Bolton Group


Finished Telescope on the fork mount

Introduction

This telescope, originally known as Stealth, is a team effort. The tube assembly is an aluminium frame constructed by Gerald, the mount by myself and the optics by Brian. The stealth name comes from its attempt to cut out light reflections. Rather than the more usual Surrurier truss tube, which is more suitable for dark skies, a design using diaphragms was adopted. Diaphragms or baffles provide the best weapon against light pollution. The central aperture in each baffle increases away from the mirror and matches the field of view of the telescope. In this design the baffles also act as structural elements along with 6 x 50mm aluminium tubes. Shown below the structure of the telescope is clearly visible but on the finished telescope (left), plastic panels have enclosed the tube masking the construction.


Tube structure prior to being
clad in plastic panels

The Tube Assembly

The tube comprises 6 baffles or diaphragms, hexagonal in shape, and 6 x 50mm diameter aluminium tubes. The tubes are not continuous but are in fact in sections. Through full-length threaded rods pass inside the tubes and clamp the whole structure together with locking nuts at each diaphragm. This creates a strong but very light structure. Diagonal bracing was added to the last section, ie next to the main mirror, just to stop any chance of deflection. Aluminium plates between the diaphragms are used to take the dec. shafts and at the top for the focuser.


Telrad, 80mm Crayford Focuser and Finder

After initial testing of the telescope I decided that sooner or later I would drop something and with the tube being open there would be nothing to stop it hitting the main mirror. The tube has now been totally clad with 3mm thick lightweight plastic, similar to plasticard. The panels were stuck on with "liquid nails" gun adhesive except for three which are removable and held in place with velcro. This has provided the protection of a solid tube but without the weight. It has been painted blue resulting in a name change for the telescope to Blue Streak. I hope it is more successful than its namesake!


Low profile mirror cell

There is an 18-point mirror support which is very low profile keeping the tube as short as possible. It's geometry was designed using the Sky & Telescope basic program. Adjusting collimation is via three big knobs and it is very smooth and easy. The finder is 20x60 and has a illumianted reticule. A telrad is also used. Other features are an 80 mm electric and manual Crayford focuser with a super large knob. The crayford has no trouble holding the weight of the CCD camera even when the filter wheel and 3 times converter are both in line.


Finished telescope on the Fork Mount

The Mount

The mount was built by myself using 150 x 100mm steel rectangular hollow section (RHS) for the support and 100mm square RHS for the fork. The polar axis is 51 mm and rides in two self-aligning ball-bearings with the thrust taken on a 25mm steel ball. It is driven in both axes. The RA drive has a 10 inch Matthis worm and wheel and was originally powered by a synchronous motor with the DEC a sector arm and DC motor. These have now been converted to stepper motors under computer control - see Telescope Making 7.

Brian's grinding machine in action

The Optics

The mirror was made by Brian on his new grinding machine shown left (The smaller mirror on top is for weight). It is made from 40 mm thick pyrex sheet and is f4.67. Pyrex mirror blanks are very much harder to find these days and this had to be cut from a 20 inch square piece - in other words a 20 inch blank would have cost the same! The tool used for grinding to the correct profile was steel. For fine grinding this was covered with glass tiles stuck down with "Liquid Nails", a proprietary gun adhesive.


Brian foucault testing

It was polished with a sub-diameter tool with figuring assessed by means of the Dall Null Test. Final accuracy was calculated by means of the formulae in Texereau. The foucault tester was specially modified to get the pinhole and the knife edge as close together as possible. This was achieved by using a fibre optic for the pinhole, which incidently produces a much brighter image. The wavefront error in the final mirror is around 1/30th ie 1/60th on the glass - it shows text book ronchi patterns before, at and beyond focus. Not bad!!! It was aluminised by Orion Optics. The diagonal mirror is 3.5 inches minor axis and was made by Hinds Optics.

Last updated: January 2007
Return to Telescope Making main menu