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Portable Dob

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Part 1 Mirror Making

Mirror Grinding

Telescope 2s one big failing is the time it takes to set up.  What I wanted in addition to scope 2 was a portable Dobson.  To make building a third scope more of a challenge and partly to keep costs down I ground the mirror myself.

For guidance I relied upon "How to make a Telescope" by Texereau and advice given in the ATM FAQ files.

The 10" Pyrex blank I bought mail order was distinctly convex.  Its probably worth specifying a flat blank when ordering to prevent a manufacturer sending out a blank they don't fancy grinding themselves.  Anyway a week later and a lot of course grit the blank was starting to go concave.  A lot of ATMers recommend plaster and tile tools especially dental plaster.  I found casting tools from 100% portland cement really effective.  After 4-5 days drying the tools can be varnished and 1"  ceramic tile squares epoxyed to them.

I had decided to go for as faster mirror as I felt I could figure, f4.5.  However before the mirror than been ground deep enough I had run out of course grit.  Rather than buy more I continued by using sandblasting grit.  This doesn't last long before turning to sludge but as its really cheap you can just add more fresh.

When the mirror gets to the required depth its worth casting 2 more tools using the blank as the bottom of the mould.  One can be tiled and used for the medium grits ( with no danger on contamination with course grit ) while the other is ready for the pitch.

The grinding using the medium and fine grits went very smoothly, perhaps because of the sandblasting grit used in the previous stage.

The pitch was cast in 3/4" strips in a mould lined with kitchen foil.  These were cut into squares slightly melted and stuck to the tool.  If I do this again I will also make a subdiameter pitch on plywood lap at the same time to help with figuring.

During polishing the channels in the lap closed up twice and were re-opened by using an old soldering iron.

A Foucault tester was made to assess the progress of figuring.  This was made according to the instructions in Texereau's book down to the use of a 12V car stop lamp as a light source.  With hindsight the wattage's of these bulbs must of gone up in the 80 years since the book was written as this bulb was really to powerful.  On the negative side I ended up with a burnt forehead but more positively the test could easily be performed without plunging the room into total darkness.

Above are the final graphs from TEX.  The focal ratio ended up 4.4
 

Silvering


Rather than send the mirror off for aluminising I decided to silver the mirror myself.  It worth mentioning here that the chemicals used are both toxic and extremely corrosive so this will not be for everyone.

The recipe for the silvering solution is given in Texereau's book.  Before silvering the mirror was cleaned with lots of conc nitric acid followed by distilled water.  I found silvering with the mirror suspended upside down in the solution to be the most straight forward way.  A strip of wood was glued to the back of the mirror using silicone adhesive and used to support the mirror.  Prior to the silvering solution the mirror was first left in a tin chloride solution.

After 15 minutes in the silvering bath the mirror was removed to reveal a good if somewhat bloomed silver coating.  Further polishing removed the bloom to leave a brilliant finish.

Silver coatings are assumed only to have a life of 6-12 months before tarnishing.  I hope by designing an air tight mirror cell for this telescope tarnishing can be delayed.  The mirror is now 6 months old and shows no sign of ageing ( I'll keep you posted ).

Part 2 Making the Telescope

The major criteria for the design of the telescope was that it should be easily transportable car.  A truss tube design seem the obvious choice as these can easily be broken down into their parts.

To keep the truss tubes linked when dissembled the brackets remain on the tubes.


 
 

The spider features a drilled ball bearing in a cadge give the necessary adjustment. 
 
 
 

As in scope 2 a ring of resistors is used to keep the secondary condensation free.
 
 

Red dot LED finders make ideal companions for rich field scopes as you never lose track of where in the sky the scope is pointing. Instruction on how to make one are on Simon Tomkins site.
 
 

Power for the LED and heater comes from a 9V battery beneath the mirror box.  1/2" ply was used for the rocker box  and mirror box to keep the weight down.

As mentioned before the mirror cell is design to be air tight in the hope of postponing the silver on the mirror tarnishing.  The mirror cell is actually an old cake tin whose lid is replaced when the mirror is completely dry.


 
 

As it was designed to, the telescope packs up so small it will even fit into the boot of a Honda Prelude.
 

 

 

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Copyright 2006 Steve Chambers. All Rights  Steve@pmdo.com

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