A 127 mm (5 inch) Refractor

by The Bolton Group

Brian with the 5 inch Refractor

Gerald with the finished Telescope

Introduction

The Refractor is now finished. The optics and tube assembly were made by Brian and the mount by Gerald. One thing about refractors is that they are big for their aperture! The telescope and mount are optimised for planetary observation.

The Optics

The objective is 127 mm clear aperture with a focal ratio of f11.7. It is an air-spaced Fraunhofer doublet designed using Rutten and Van Venrooij's program. The front element is crown (BK7), the second element flint (LF7). The specification is:-

R1 = 880mm
R2 = -396.153mm
Thickness at centre= 14.918mm
Airgap = 0.6mm
R3 = -400.907mm
R4 = -2735.419mm
Thickness at centre= 7.0mm

Because three of the surfaces are convex, matching concave test plates had to be made and tested using a foucault tester. The convex surfaces could then be tested against their opposite surface by inspecting the interference fringes between them. There is no measurable wedge in the finished elements.

The tube is 6 inch diameter, 3mm thick extruded aluminium, with internal baffles. The lens is located in a push-pull cell for precise collimation. The focuser is a crayford located internally with just the draw-tube projecting.


Close-up of the head

The Mount

The aluminium with brass trim mount is an alt-az with offset altitude bearing to enable the telescope to reach the zenith. Ball bearings are 4 inch azimuth, 2 inch altitude. It has slow motions to both axes via flexible cables and worm and wheels, 6 inch azimuth, 5 inch altitude.

The pillar comprises 6 no. 1.5 inch aluminium tubes with circular diaphragms, similar to the telescope tubes made for the binoculars but much sturdier. It is tensioned to the three tripod legs by cables and turnbuckles. The result is a well-damped mount capable of supporting the telescope even under high powers.



Last updated: January 2007
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