MAKING THE TWO 16inch TELESCOPES


The 16 inch mirrors have been made of 1 inch plate glass from Specialised Glass Cutting Co in St Helens. This is the same material used to make the 30inch TRO. The blanks where purchased with a nipped edge at a very reasonable price. The edge required a bevel which Geoff took on as 'his' job !

These telescopes would be Twin's. made the same way with the same focal length (almost) and same tube assembly. Both would have 2inch Crayford focus mounts as standard. The tube design would be similar to the truss designs of Kriege and Berry (The Dobsonian Telescope). The idea being also to 'turn around' these Telescopes quickly using efficient techniques.

Images of my completed 16 inch telescope is given below. Details of the making of the telescopes is given after the below images.

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Image below is my 16 inch F6 Dobsonian telescope with John Dobson himslef back in September 2002. The Dome is now where the telescope used to be.

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MAKING THE TWO 16inch TELESCOPES


The 16 inch mirrors have been made of 1 inch plate glass from Specialised Glass Cutting Co in St Helens. This is the same material used to make the 30inch TRO. The blanks where purchased with a nipped edge at a very reasonable price. The edge required a bevel which Geoff took on as 'his' job !
These telescopes would be Twin's. made the same way with the same focal length (almost) and same tube assembly. Both would have 2inch Crayford focus mounts as standard. The tube design would be similar to the truss designs of Kriege and Berry (The Dobsonian Telescope). The idea being also to 'turn around' these Telescopes quickly using efficient techniques.

PREPARING THE BLANKS AND TOOLS

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While Dave Owen, myself (Dave Thomson) and Steve King fine ground the 8 inch mirrors, Geoff Regan was putting the bevel on the 16 inch mirror blanks. Using a downward stroke only with a tool sharpening stone. A water spray is handy for all sorts of mirror making activities!

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Above is myself (Dave Thomson) with Dave Owen and Gromit (my dog). I am holding the new grinding tool for the 16inch telescopes. This tool is constructed the same way as the tool used for the 30inch TRO.
The tool uses 4layers of 18mm ply cut to 16inch diameter and 1 layer of 18mm ply cut a little smaller in diameter to allow for a 'gap' near the top to enable the tool to be lifted onto and off the mirrors safely.
The tool is then covered with car body filler and pressed into the hogged out blank (with a bin bag between) to take the curve of the mirror. The tool is then given a good coat of PVA.
When set the tool face is then covered with fast curing epoxy and a matrix of thick metal washers (pressed out face up). When set the tool is given thick coats of exterior paint.
This tool has been used on both mirrors successfully and may be used for others of the same focal length


CONSTRUCTION BEGINS


While we where making the mirrors we began designing and making the tube assemblies. Both telescopes would be made exactly the same. The only difference may be the length of the the truss poles due to very slightly different focal lengths. This turned out to a little more than anticipated due to a major problem caused by a mistake made by myself !!

Below images show the beginnings of the tube assemblies. The left image shows both the 16 inch telescopes in the early stages of construction. The right image shows the teflon bearing positions on the base disc. The support feet are placed directly below these feet to give extra rigidity as described in Kriege and Berry's book "The Dobsonian Telescope".

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The image above shows a close up of the secondary cage.
The upper and lower discs are cut from 3/4 inch ply. This was done with a jig saw mounted in a jig to cut circles (not by line of sight).
The four connecting poles are 1inch pine. Neat holes have been cut through the upper and lower disc's for accepting the poles. The focus board is 1/2inch ply fixed between the upper and lower disc's.
The inside of the cage is lined with hardboard (cheap and easy to cut and bend).
Four aluminium angle brackets are used for connecting the poles. These are bolted to the cage and tapped for fixing the poles.


POLISHING AND FIGURING LAPS


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The image above shows the laps used for polishing and figuring. The image is difficult to see detail, but it shows a 2inch, 4inch(starlap), 8inch and full size 16inch laps.
The 1inch lap uses a metal engine plug from a car. The 4inch star lap and 8inch laps are based on glass. The 16inch full sized lap is built up of layers of 3/4inch ply. The face was pre-generated into the curve of the mirror using car body filler before pitch was added. PVA and white exterior gloss was used to seal the tool before the pitch was added.

GRINDING AND POLISHING MACHINE

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The image above shows one of the 16inch mirrors on the new grinding/polishing machine.
This machine was made in a few days for around £20 !!
It works great and has been used to make mirrors from 4 to 16inch in size. The 16 inch mirrors are are little too large for it but we are making a BIGGER machine for re-making the 30inch mirror

The machine uses the bearings and back plate from an old washing machine. The table drive uses the drive pulley connected to the wiper motor direct without any other reduction. This gives a table speed of around 40-100 seconds per revolution.
The motors are 12V wiper motors from an on old Jaguar for both the table and the arm. The arm uses connecting rods from the wiper mechanism to ease construction. The arm speed is around 2-6 seconds per revolution.
This is possible due to the supply to the being indipendant and supplied from a car battery charger with four different supply settings.
We have found the machine takes longer than by hand but is easy to use, more productive and gives MUCH smoother figure under ALL tests.


FIGURING AND TESTING

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Above images show the Couder screen to the left and some figuring work with a 2inch lap. The Couder screen has 8 zones. 25mm, 50mm, 75mm, 100mm, 125mm, 150mm, 175mm and 200mm zones

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Pictured above is my daughter Kayleigh helping out in correcting the 16inch mirror using a 4inch sub diameter lap.
You can see that the outer zones are being worked to bring the figure more spherical before reforming the main lap

ALMOST COMPLETE !
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Image above shows myself next to the tube assembly for the first time.
Gromit (our mascot) is looking fed up as usual in the foreground.
The truss tubes are one inch square aluminium. These are cheaper and easier to connect than tubes. I also think they are stringer also.
Fixings are Hand wheels from Maplin. These make setting up of the scope very easy and no tools are needed.
The side bearings are two layers of 3/4inch ply. The use of ply has to be carefully planned out. This way you will make best use of materials and keep the cost down.

Focus mounts are made locally by an Engineering firm. At present they are inch and a quarter, but we are having two inch focus mounts made for £20 each !
Finders will be home made from second hand binoculars obtained from the market for £10 a pair.
Total cost of the projects EACH complete with secondary, aluminising, finder etc will be £300....!!


FIRST LIGHT !!
This was in OCTOBER 2000. I had to lay off making the scope for a while but the team is back together and are now 98% complete for both instruments. The star test shows a little turned edge on one mirror and a slight central hole. These did not seem to have too drastic effect on the image quality or resolution. Epsilon Lyra was EASY at low power 80x and well separated at 400x and easy to move with the mounting operating flawlessly.

16 inch telescope at Pickerings Pasture ( LASSA Sidewalk event) in 2001. Dave Owen's 16 inch telescope has been used a lot at sidewalk events since and continues to be in use today.

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Image below shows both of the 16 inch F6 telescopes together for the first time at the LASSA event at Leasowe Lighthouse.

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