7" f/12 iStar folded refractor 11: Layout.

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After further folding of the full scale, paper light cone I found a suitable arrangement. One where the positions of the folding mirrors made best use of their available diameter. The resulting design would fit comfortably into a plywood box 82cm long x 30cm high x 20cm wide. Very different from the 20cm diameter straight tube with a length of 183cm plus lengthy dewshield and focuser. The straight OTA weighed 40lbs with tube balance weight, handles and finder. Well beyond my ability to lift it safely above my head into the mounting rings.

The folded design utilizes 120mm of the 125 mm first flat mirror and 80mm of the 100mm. The tilt and retaining lip do not allow much more than this on the 1st flat. The 2nd fold is much more generous but such odd diameters are unavailable. 

The image gives a rough idea of the optical layout but is not to scale. Space has been provided behind the folding mirror blanks for their collimation cells. The '?' merely suggests that this portion of the box could be cut way without affecting the optical arrangement. A generous length of lightweight, aluminium dewshield, 25cm in diameter, will be borrowed from the straight-tubed original OTA.

Once the plywood OTA is completed and the optics mounted, the longitudinal balance point can be found. Allowing Dobsonian altitude bearings to be fixed. The advantage of greater compactness must be weighed against the addition of the folding flats and their cells. The intention of the plywood construction is to confirm the layout actually works reliably. Hopefully without loss of collimation in normal handling and use. A lighter, more sophisticated construction might, by then, have suggested itself.

 A contact has kindly pointed out that for a little extra length in the OTA "box" I could use rather less of the 125mm flat. Then, if the flat does have any degree of turned down edge, it should not affect the final image.  If I reduce the active diameter to [say] 115mm at the first flat it will be spaced at 80cm from the objective rather than the original 72cm. Which only adds a little over 3" extra to the OTA's length. Another way of looking at it is that the greatest diameter of the folding flats is only used at relatively low powers. The field lens of higher power eyepieces is proportionally smaller so will not accept a wider beam. So any flat mirror TDE is unlikely to affect the image when it matters most.

While searching endlessly for folded refractor inspiration I came across various references to Jim Chung's [D&G] 8.5" f/12.5 skeleton folded refractor. This used click-lock telescoping alloy poles and 1/2" plywood baffles. It looked rather amazing but weighed 50lbs. The objective lens weighed 10lbs so the plywood baffles must have been the major problem. When a similar instrument came up for sale in the UK I contacted D&G to be told that they have never made 8.5" achromats. The seller then admitted the objective had been damaged. Be very wary of distance buying anything online without seeing lots of images to ensure exactly what is on offer!

Planning a folded open 8.5" f/12.5 refractor - ATM, Optics and DIY Forum - Cloudy Nights

A much lighter 6" f/15 skeleton, folded refractor design by Tom Gideon used lightweight aluminium angle sections with HDPE baffles. This weighed only 16lbs and replaced a much heavier, conventional, tubular design. Both of these skeleton designs offer inspiration but Tom Gideon's wins easily on weight. He went on to clad his instrument in thin, transparent plastic to give an even more modern appearance. High density polyethylene is a form of plastic unaffected by moisture.

Stellafane Telescope Gallery: Tom Gideon

Click on any image for an enlargement. 

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