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A new contact has raised the specter of folding my 7" f/12 iStar refractor. I had been seriously considering a folded refractor before becoming involved with other telescope projects. The finished 7" OTA is still well over 7' long and much heavier than I had hoped. The weight could be reduced at the considerable expense of rebuilding the whole thing or paying for a carbon fiber tube.
The commercial dome to house my "straight" refractor is right at the limit of those commercially available at remotely affordable prices. [Pulsar UK] Beyond that point the price of observatories rises rapidly. The dome has to be imported from the USA or Australia at vast expense. Not to mention the massive addition of import taxation to every part of the purchase price plus freight. Then the crane hire making the purchase of such a beast as unlikely as my ever owning a Ferrari.
I want [and now have] my dream 7" classical refractor but I want [need] it to be light enough for a septuagenarian to lift onto the mount. Or [much better] have it permanently mounted ready for use under a shelter from the wind and showers. Anything which gets between ease of use and the sky is a reason not to go out under changing cloud or weather conditions. When it is both cold and windy one can wrap up well but watering eyes in a gale, as the telescope moves to every gust, is just another hurdle to enjoyment of the night sky.
I can still have my refractor but I can make it much more compact. [At considerably greater expense!] The weight might not change much but it would be enormously easier to protect the instrument and myself. Both from the wind and the weather under quite a modest dome. Domes are unique in allowing almost instant observation at a whim, without requiring any set-up time. So how can I have my 7" cake and eat it? By folding the light path with flat mirrors.
The commercial dome to house my "straight" refractor is right at the limit of those commercially available at remotely affordable prices. [Pulsar UK] Beyond that point the price of observatories rises rapidly. The dome has to be imported from the USA or Australia at vast expense. Not to mention the massive addition of import taxation to every part of the purchase price plus freight. Then the crane hire making the purchase of such a beast as unlikely as my ever owning a Ferrari.
I want [and now have] my dream 7" classical refractor but I want [need] it to be light enough for a septuagenarian to lift onto the mount. Or [much better] have it permanently mounted ready for use under a shelter from the wind and showers. Anything which gets between ease of use and the sky is a reason not to go out under changing cloud or weather conditions. When it is both cold and windy one can wrap up well but watering eyes in a gale, as the telescope moves to every gust, is just another hurdle to enjoyment of the night sky.
I can still have my refractor but I can make it much more compact. [At considerably greater expense!] The weight might not change much but it would be enormously easier to protect the instrument and myself. Both from the wind and the weather under quite a modest dome. Domes are unique in allowing almost instant observation at a whim, without requiring any set-up time. So how can I have my 7" cake and eat it? By folding the light path with flat mirrors.
Folding a refractor involves employing at least one, or more usually two, flat mirrors to bend the objective's light cone back on itself. The idea being to reduce the moment arm of the typically, very long, straight refractor tube to produce a [hopefully] more compact form of telescope. One not subject to catching the wind nor requiring extensive storage. Not to mention a suitably hefty [and therefore very expensive] commercial mounting. The downside is that it usually leaves a desirably shorter but much fatter OTA. Which may not appeal to the purist who prefers the long, straight tube soaring high into the air. [And the wind!]
If only life were simple there would be far more folded refractors than straight ones. The main problem is the considerable expense of the flat, folding mirrors. Suitably precise optically flat mirrors are not remotely cheap unless sourced secondhand with a very large element of luck. Secondhand purchases hold obvious risks in ensuring the required precision even if the source is known. The first and largest mirror ought to be at least 1/20th wave accuracy according to most informed sources. Anything less accurate will undermine the optical quality of the objective lens. Surface curvature on the folding mirrors will introduce astigmatism. This is due to the tilting required to return the light beam clear of the objective lens.
A single reflection back in the direction of the objective has rather limited usefulness. The observer would have to turn their back on the object being studied. Though a star diagonal might be introduced if the light beam was brought out near to the side of the tube. However, the observer's body heat would be right beside or [worse] right under the objective in normal use. Probably leading to severe thermal problems due to body heat crossing the incoming light path. What you might call the "Herschellian" form. Which the great astronomer and telescope maker pioneered with his reflecting telescopes. He was trying to avoid having to use a [constantly tarnishing] speculum metal, secondary mirror. He turned his back on the sky and looked straight down at the primary mirror through the eyepiece.
One other alternative is to employ a single, rather large, elliptical flat to push the converging light cone out through the declination bearing. Or altitude bearing in the case of an altazimuth mounting. The equatorial design might force very awkward eyepiece positions while the altazimuth would make the EP horizontal. The latter might sound better but would need to be high enough for comfort [while sitting?] and would cover a large circle of ground with changes in azimuth. A heavy counterweight would also be required to balance the heavy objective in the absence of the usual bottom half of the normal straight tube. It could be made to work but has some severe limitations in larger apertures and longer focal lengths.
Two mirror, folded designs, have a greater number of useful options. The simplest bounces the light beam back from the first mirror to a smaller, second one beside the objective. Thence back to to the focuser plate at the "bottom" of a much shortened tube. This design was used by Unitron for their 3" folded refractor but is best used with larger apertures and longer focal lengths. The cost of the two, precision, flat mirrors must be added to the price of the objective lens. Though the flat mirrors can make for a far more compact design than a long, straight tube. It also places the observer well away from the light beam and may feel more natural as it follows conventional refractor practice.
The mounting requirements shrink to match the new tube length and could even be considered as providing the necessary savings to pay for the folding flats. [Optically flat mirrors are commonly known as 'flats' or 'optical flats.'] For most users a further reflection will be required from a star diagonal to avoid the usual neck wringing. The loss of light at the necessary mirror reflections can be much reduced by using enhanced aluminum or dielectric coatings.
The imaginative builder could come up with an "X" shaped light path to bring the focus out at a far more comfortable viewing angle. The downside being the increased depth required for a very long focus design. I drew an 8" f/18 design for a D&G lens available at that time. This seemed to leave a large hole in the middle where the altitude bearings should normally be on a Dobsonian design. It would have been quite bulky too. Though the straight tube version would have been all but impossible to mount at reasonable expense. 8" x f/18 = 144" or over 12' long without its dewshield! Still suitable for a quite a small dome but only in the folded form.
The "X" or "Figure of 4" form belongs to the "Newtonian" style of folded refractors. Where the eyepiece exits the side of the tube somewhere near the top of the tube. This design could also be employed to throw the light cone out through the altitude or declination axis. It makes much better sense than trying to do this with only a single mirror reflection because the folded light cone is already much reduced in length. A Dobsonian mounted design, similar to a Newtonian reflector in appearance, could provide comfortable viewing with an eyepiece which remains almost static. The observer is still close to the light beam but no more than with a typical Newtonian reflector. A dewshield will provide further thermal shielding.
A single, round, first mirror at the bottom of the tube does the major folding. While an elliptical [Newtonian] secondary mirror can manage the [near] right angle turn. This places the focal plane where it can be examined with an eyepiece in a normal focuser on the side of the tube. With suitable design the eyepiece movement could be reduced to a minimal arc around or even through the Dobsonian altitude bearing itself.
A greater degree of folding would be achieved by bending the light beam across the main tube to use up even more of the [usually] long focal length. Though this requires a larger secondary mirror than the shorter route at necessarily, greater expense. The opinion seems to be that 1/10th wave is good enough for the second [45 degree] reflection. Though higher accuracy does remove all doubt as to its effect on performance.
Baffling the intended folded design, to avoid stray light, is vital to retaining the refractor's highly desirable, image contrast. Seemingly simple, folded designs can easily make it impossible to avoid incoming light flooding the field of view. Complex tubular baffles may easily add internal or external reflections. Or actually block the folded light beam unless great care is taken in the design. Just because the refractor is folded does not mean that the builder can relax their standards on baffling and the use of low reflection lining or painting matt black internally.
I strongly recommend that the potential folded refractor builder draw and cut out a full sized light cone to try various folding ideas. This is highly preferable before committing to great optical expense and materials through over-optimism. It will also give the builder a true sense of scale of the likely finished instrument. Manhandling the long, wedge-shaped piece of paper is a great educator and will quickly indicate whether the "white elephant" is really worth pursuing.
The lower image shows how the focuser can be brought right back to the tail end of the OTA. Given sufficient tilt this will avoid using a star diagonal for observing objects at modest altitudes around 45 degrees. For objects at or near the zenith a star diagonal will provide a comfortable downward, sloping view into the eyepiece much like using a microscope. This angle reduces the incidence of floaters in elderly eyes.
If the OTA is fitted onto an altazimuth mounting then a star diagonal can provide a horizontal eyepiece which moves only over a rather small arc while the observer remains comfortably seated. A 5" first folding flat will be situated about 36" from the objective. A 6" flat is a rather close 21" from the objective and of limited benefit for a two mirror, folded system. A 4" flat will be situated 58" inches from the objective. I have been generous in over-sizing these flats to avoid potential edge problems. A rolled edge from polishing may be the limiting factor on an otherwise excellent flat. Simply by avoiding using the edge of the flat mirror one can [sometimes] enjoy a much higher quality level of accuracy at the same cost as the lower quality. This improvement in quality is not guaranteed!
It is so much quicker to fold and refold a long, wedge-shaped piece of lining paper than to draw and redraw to scale on much smaller sheets of paper. The size and position of potential folding flat mirror sizes can be checked in moments and the realistic size of the OTA either admired or discarded as completely impractical. Strangely folded refactors can be mounted in deep plywood boxes. Or suitable tubes mitered and welded together at the correct angles.
Weight should always be kept in mind even with a compact folded refractor. While the more compact folded designs are almost certainly smaller than a straight tube original the addition of the flats and their mounting cells can easily add to the overall weight.The main advantage is that a compact, folded refractor is highly unlikely to need a tall mounting. So stretching high overhead with a heavy OTA should be completely unnecessary. Consideration should preferably be given to a seated position to maximize ease of use and comfort in observing. This holds true whether the instrument is mounted on an altazimuth or equatorially.
The optical folding mirrors should always be over-sized to avoid any chance of a rolled edge spoiling the performance. Nor allow undersized mirror to reduce the effective aperture when tilted. The mirrors need not even be round except for greater ease of mounting. Elliptical [Newtonian secondary] mirrors are far more readily available than round and could be substituted for round ones provided they were of sufficient accuracy.
Click on any image for an enlargement.
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