[Photo] The MAS Solar Telescope


By Steve Hodgkinson

The telescope was originally designed as an ultra low cost telescope of high optical quality for use by school children.

It was realised that the design lent itself to a solar telescope particularly well.

Four telescopes were made during the winter of 1993-4.

The optics are 5" Newtonian f.11. The mirror and diagonal are unaluminised, matt ground on the rear surface and painted black to stop rear refections.

For ease of construction no optical adjustments have been built in. The mirror is glued to bubblepack on a plywood disk acting as the mirror cell. The diagonal is stuck onto a 45 deg block on a single cross member in the tube.

Collimation is done at assembly by aligning the mirror on its plywood disk correctly and then fixing the disk with screws in the tube. The diagonal is stuck to its block with "Milliput" and when collimated the milliput is left to harden. This works very well. An f.11 system is very easy to collimate by eye and very tolerant of residual errors. No changes in alignment have occurred since assembly.

For cleaning the diagonal is swivelled on its block to face the top of the tube, and the main mirror is removed on its plywood disk. A wipe with acetone cleans the glass, and the mirror will go back in an identical position. The diagonal is swivelled back into position and aligned by eye.

The mirror was made of 1/2" window glass. Four mirrors of identical focal length were made on one tool. They were polished and figured in turn on a single lap.

The two unaluminised surfaces produced an image of the sun that is very bright but cannot damage the eyes. A double polarising filter fitted to the eyepiece barrel and eyepiece tube enables adjustment of the final image brightness to comfortable levels.

The eyepiece used was salvaged from faulty or damages binoculars, and can be fitted to the telescope in a number of ways as long as it can move to adjust for focus, this only needs to be a push fit in a block of plastic of even wood drilled out to suit the eyepiece.

The mount is a standard Dobsonian design.

Materials, source and cost.

Mirrors     Window glass cut by glazier to rough disks - 5.00 each.
Diagonal    Astronomical Suppliers at cost - 10.00 approx each.
Tube        One 6 metre length of 6" PVC ducting - Plumbing merchants
            at 3.00 per length of tube (12.00 for the 6 metre length).
Cell, Diagonal holder, Wood for mount, screws etc: very little cost.
Total cost per finished telescope 25.00

Aluminised this would make a good first lunar and planetary telescope.

Results.

FANTASTIC - Far better than projection. Very fine sunspot detail can be seen. The solar image nearly fills the eyepiece field. The magnification is x70 and the image is very clear and sharp.

Diagram

[Diagram]
By Rob Cummings.

Warning

Under NO circumstances must aluminised mirrors or flats be used, this would be very dangerous and eye damage would result.

How Does it Work ?

When light hits the surface of glass, about 95% passes through and 5% reflected, it is this 5% that is passed onto the flat, 95% of this remaining light will pass through and only 5% reflected again. So the amount of light (and Heat) reaching the eyepiece is 5% of 5% = 0.25% This is still to bright for the eye so a polarising filter is used, but the infrared heat (which is the dangerous part) is now low enough to be harmless. The backs of the mirrors are matted and painted black so that no internal reflections occur which could send more heat to the eye, and also degrade the image.


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Manchester Astronomical Society