6.08.2024 Folding refractors:

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  Tuesday 6th. Folding the 150mm/6"? 

 There is always the option of folding both refractors into more compact shapes. This reduces the size of the housing or observatory required. As well as helping the mounting. Many of which struggle with lengthy telescopes. Not due to the weight but due to their moment. Which is a term used in physics for mass [M] x distance [D] from the pivot.

 There is usually a heavy component at each end of a refractor. The objective in its metal cell and the focuser at the other end. This effectively doubles the moment. Since the majority of the weight is so far from the pivot point. [Mounting axis.] Add the considerable length of the filter stack and things get even worse.

 Classical refractors are normally of much greater focal length than modern, Newtonian reflectors. Though the tendency is towards faster [shorter] APO [apochromatic] refractors. Which often use special glass types to correct the false colours. From which normal achromatic refractors suffer. Which historically meant very long and unwieldy instruments. If only to overcome false colour.

 These older designs have a very high moment. Demanding large and heavy mountings. Often made of heavy cast iron. With very large slow motion gears.[wormwheels.] This made them unsuitable for amateur use. For those not rich enough to own a large observatory. The modern amateur often carries all of his equipment outside for each observing or imaging session. This demands light weight and compactness.

 I had already built a folded version of my 180mm/ 7" f/12. The same could be done for the 6". In this case only folding the tail end at the very beginning of the very long, H-alpha, filtration stack. 

 Two 2" star diagonals and a suitably long extension tube. Used as a spacer between them. Would probably do the job. To bring the filter stack parallel and close to the outside of the main tube. But pointing forwards towards the objective. The star diagonals safely avoid worrying about collimation. It is automatic thanks to the 90º bending of the light. By a mirror in a precision housing.

A main tube ring and connecting link could easily steady the folded stack. Though slight linear movement must be allowed for during focusing. A typical, felt lined tube ring need not be tightened unduly. So that it can slide freely along the main tube.

 The main telescope tube of the 6" might need shortening slightly. Or the FT focuser removed altogether. The distance from the focus to the Lunt 60MT etalon is critical. The primary focuser was useful for making these fine adjustments. Often in the hope of improved images.

 The ridiculously long, cantilevered, filter stack was already an embarrassment! I had to provide support in the form of a rectangular, aluminium tube. Not easy to attach to the stack. Because of the need for linear movement and the rotation of the OTA over time. Sagging of the multitude of push fit elements is almost inevitable!  

 I only have one [no name] 2" star diagonal. So I have ordered a Bresser 2" star diagonal. To experiment with possible folding arrangements. The Bresser has "only" 93% reflectance. Compared with the usual 99% claim for the dielectric diagonals. Which hardly matters with the intense solar energy available in a 6" refractor.

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 My 180mm/7" badly needed remote, screw adjustment for the 2nd folding mirror. Bringing the adjustment back, via rods, to suitable knobs on the telescope back plate. Without which collimation was a nightmare! I went back to the straight tube version instead. Before finally giving up.

 My massive mounting struggled with the weight of two [or sometimes three] OTAs. All down to the weak worm housings. The Ioptron CEM120 could probably manage both 'scopes by weight alone. Though only if they were optically folded. Folding has the advantage of greatly reducing the moment of a telescope.

Moment = mass x distance from the pivot point. If the mass doesn't change. Then the moment is reduced by bringing it nearer the pivot. i.e. By physically shortening the telescope. Though there is usually some small increase in overall mass due to added components. The distance at which the weight lies from the pivot is more important than the weight. The classic big kid, little kid on the seesaw.

 The moment of an equatorial mounting can be similarly reduced. By using more counterweights and sliding them towards the body of the mounting. This may help where the weight limits of a mounting are reached. Particularly with long and heavy refractors. Hanging fewer weights on the very end of the axis shaft may be a poorer choice. Unless you really want to save weight with a mobile set-up.

 

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