The “Foot” ‘Scope Project: Part II

Foot sized powerhouse; the author’s 12″ f/5 Newtonian.

















April 13 2018

In a previous communication, I described my acquisition of a Revelation 12″ f/5 Dobsonian telescope, which delivered fine images of selected double stars and which was used to establish the Taylor hypothesis, namely, that if the seeing conditions are fair to average or better, sub–arc second pairs can be readily observed at appropriately high magnifications using the generous aperture of this telescope.

After conducting this body of work, I have had little time to enjoy the considerable benefits of observing with a big Newtonian system such as this, but in this blog I hope to report still more observations with the same telescope and improve its optical and thermal  performance in the field.

During the course of that previous communication, I described the optics in this telescope as being unreasonably good. Indeed, they far exceeded my expectations given the very modest cost of the telescope! I also described some modifications I made to the instrument, including the re–coating of the secondary mirror (just 70mm semi major diameter) with super–high (97 per cent) reflectivity coatings as well as the procuring of Bob’s Knob’s  to assist precise collimation in the field, but I postponed some other aspects of this project. In particular, I wished to also have the coatings on the large primary mirror similarly upgraded in order that it would increase light throughput to the eye as well as improving the overall contrast of the images so rendered.

Today, I endeavoured to resume work on the telescope and that meant removing the primary mirror from its cell in order that I could despatch it to the mirror coaters located south of the border in England. The mirrors for this telescope were sourced from GSO and seem to have been more or less consistently good, as judged by other experienced amateurs who had taken the time to assess one or more units of the same product. One such assessment is documented here and I would heartily agree with the conclusions of Mr. Stoitsis.

After removing the mirror cell from the rear of the optical tube, I was able to accurately measure its thickness, as well as assess the design of the accompanying cell. The thickness of the mirror was measured to be 36mm (so 1.5 inches), yielding a mirror thickness to aperture ratio of 1:8. This result is consistent with this author’s finding in respect of the ability of the telescope to acclimate adequately to ambient outdoor temperatures, allowing him to make those important observations with regard to resolving sub–arc second pairs.


The 30cm aperture GSO primary has a thickness of 36mm (1.5 inches).

















The mirror cell is housed in an open cell. It appears to be well designed and incorporates a 9–point floatation system; just about perfect for a mirror with these dimensions.

The 9–point floatation system of the 30cm aperture GSO primary mirror cell.

















The mirror was then carefully packed away for safe passage to the mirror coaters.

Packaging up the primary for re–coating.

















I hope to despatch the mirror early next week, so hopefully I will have it back in a few weeks; just in time to explore the deep sky glories of spring!


April 14 2018

Time: 23:20UT

Almost forgot to tell you: several weeks after exchanging resources for the foot ‘scope, I received an envelope in the post. Curiously, I prized it open, and there it was! A small, battery powered fan; the same one that originally came with the telescope! As I explained in the opening blog, I wasn’t too bothered about not having it, nor did I really need it. But it was a warm gesture from the original owner of the telescope to send it on; something I appreciated!

The little fan.

















Time: 4:20 pm

As described in previous blogs, I also wish to line the inside of the 1450mm long tube with cork and overlay this with flocking material. Materials were ordered yesterday and should keep me busy until the primary returns home.

April 21 2018

Time: 5:45pm

Lining the 12 inch optical tube assembly with cork.

















With a decent spell of settled weather now upon us, I spent the afternoon lining the inside of the optical tube with a thin layer of cork. The product I chose has an adhesive layer on the underside of the cork substrate and adheres to the rolled aluminium tube fairly easily. The cork itself is very delicate though, and so some care must be taken not to tear it while preparing the strips. 3 full 100 x 45 cm rolls were used up today, but I needed to order up a couple of extra rolls to complete the job. These arrive on Monday so I can complete the task then. The flocking material has already arrived so that will be overlaid on the cork. This afternoon, I only flocked the inside of the focuser draw tube.

The mirror arrived safely at the coaters and on the invoice they noted its diameter to be 303mm. Hoping to have it back in a week or so.

Date: April 24 2018

Time: 3:10pm

The tube now fully insulated and flocked.

















Well, I finally managed to complete the cork lining as well as covering it over with flocking material. Since the rolls are the same size ( i.e. 45 x 100cm), I can report that it will take 4 complete rolls of each material to insulate and flock the optical tube assembly of the 30.3cm F/5 Newtonian. I’m pleased with the result. All I need do now is wait for the arrival of the primary mirror and I’m back in business.


To be continued……


De Fideli.

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