Part 3: 12" OTA restoration.

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This is the third post in this series illustrating the restoration of a 12" Newtonian-Cassegrain, Fullerscopes reflector on a MkIV mounting.

 The adjustable Newt-Cass cell mounted on the backplate with the perforated primary in place.
The long tube is to block stray light from diluting the field of view. The observer is looking straight up at the open end of the tube. So the optical path must be reduced (by the tube) to only the area obscured by the small secondary mirror. Thus the light from the open sky is blocked.

Matt black paint is always used inside the OTA (optical tube assembly) to kill all stray light reflections.

This instrument has F5 or F30 optics depending which optical system is used. The convex Cassegrain secondary mirror amplifies the primary image to produce a large primary image scale for planetary or lunar photography. When the convex mirror is swapped for a conventional diagonal mirror the instrument performs like any other F:5 Newtonian telescope.

 

The main tube has been resprayed and the ring components cleaned, primed and repainted with Hammerite. It takes skill and lots of patience to achieve a fine finish like this. Making haste will never achieve such results.

A view down the main tube towards the primary mirror. Though the backplate assembly and primary mirror are absent in this view. Note the overall use of matt black paint inside the tube to kill stray light.

The secondary mirror cell will be mounted at the centre the four vane spider. The narrow blades of which are tensioned to avoid sag. Which would ruin the focussed image.

Great accuracy is required in optical alignment. Leading to potential difficulties with a convertible Newtonian-Cassegrain.
Simply changing the secondary mirror will require tiny and tedious adjustments to optical alignment.

The OTA in all its glory after restoration. The Hammerite 'Charcoal' paint offers an updated alternative over the original matt black wrinkle paint.

Note the two access doors. The larger one is to allow the primary cover to be removed before use.

The other door is to allow the secondary mirrors to be swapped between the two optical systems.

Another view of the superbly restored main tube. The mounting rings and guide telescope rings have been treated to the Hammerite finish and look fabulous compared with the tired, original paint.

Such restorations are always a difficult choice between total authenticity to the factory gate finish. Or an updated version designed to inspire potential new converts to amateur astronomy.

Stages in the restoration of the classical 3" F/15 refractor, photographic guide telescope. Fullerscopes offered both refractors and 4" reflectors as photographic guide telescopes.          

The gloss white and polished brass dewshield look well together after restoration. The great length of this small instrument can look surprising to our modern eyes. Where refractors have been reduced to a fraction of their former length. Not always with the best results. Chromatic aberration is a unavoidable with increasing aperture and reduced focal length.

An advantage of a guide telescope is that it may be used in its own right. Either for solar observation. (with a suitable solar filter and the main instrument covered) Or when a second observer is present and both wish to see the same object visually. At the time this entire instrument was built a 3" refractor was often the only instrument available to many amateurs. Many amateurs forget that a great deal of real science was done with such small instruments.

Components being painted with primer prior to a Hammerite finish. Hammerite requires its own primer to achieve good adhesion to non-ferrous materials like aluminium, brass and bronze. Warmth is useful when painting. Particularly with Hammerite. Unless warm, well-ventilated conditions can be provided, restoration is best carried out in summer. Or delayed until reasonable warmth can be provided.

The sheer bulk of the mounting rings shows the huge scale of this large instrument. The main tube is probably 15" in diameter. Telescope tubing is usually made made larger than the aperture of the instrument to avoid the light path being affected by heat currents clinging to the tube walls. A larger tube also allows ring baffles or matt black paint to avoid low angle, reflected stray light from interfering with the image. Many supposedly matt surfaces reflect very easily when the light is just glancing off the surface.

Hopefully further images showing the MkIV mounting after restoration will become available.

I am most grateful to my contact in the UK and the skilled restorer in Australia for sharing these unique and fascinating images of the restoration of such an interesting instrument.

Click on any image for an enlargement.

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