7" f/12 iStar folded refractor 2: More Optical Folding.

 

Here I have superimposed the folded light path on an image of Dave Trott's 6" f/15 compact refractor. This design utilizes the smallest possible main tube diameter relative to aperture. 

There could be difficulties with shielding [or baffling] from stray external light with such close coincidence of the folded light beams at the second folding [flat] mirror. A slightly wider spacing of the parallel sections of the light beam will provide easier baffling at the expense of a larger round tube. Mr Trott suggests his more compact design fits into a 10" diameter, round tube, instead of a 12" one, had he used 'standard' parallel folding. The larger tube would have been heavier, much more bulky and more difficult to "manhandle" when off the mounting.

An alternative would have been to use an oval OTA. The main tube really needs to be no wider than that necessary for the objective's cell and/or counter-cell. Plus room for a thin baffle to avoid grazing incidence causing reflections inside the main tube. The downside is an inability to rotate [or even mount] the oval tube in standard mounting rings. Though rotation is rarely required for a refractor since a star diagonal can be simply rotated in the focuser. An oval tube does present some peculiar mounting issues compared with round tube rings. A dovetail on the bottom of the oval probably makes most sense. With the objective nearest the dovetail to avoid flexure of the oval's flat sides due to the overhanging mass. Plywood boxes are commonly used for folded refractors of all designs. Its stiffness and ease of use is handy for building prototypes which can be rebuilt in metal, if desired, once any bugs are ironed out. Metal tends to be less forgiving of later design changes.

Numerous folded refractor designs have been tried over a very long period. Some use smaller tubes joined to the larger main tube at an angle to carry the last "leg" of the light cone to the focuser. Others have used an open truss arrangement. Or even had an exposed, first folding flat, mounted on the far end of a simple spar. A protective sleeve or sock can be slipped over these arrangements to preclude stray light. It must be remembered that the design must hold the focuser, folding mirrors and objective in a fixed relationship to each other. Flexibility and sagging of any of the multiple component must be carefully avoided. Otherwise collimation problems could easily arise along with loss of aperture and astigmatism. If nothing else it could lead to the eyepiece being off axis to the center of the focal plane and no longer perpendicular to the objective. Regular recollimation is time-wasting if required before and even during an observing session. 

Folded paper light cone with the second flat further away from  objective. This requires a larger second flat and remote adjustment of the flat's collimation. [Or an access door in the side of the OTA] This arrangement uses 5" and 4" flats conservatively with plenty of allowance by using them over-sized. The objective is 180mm or 7" diameter @ f/12 and the circle of illumination at the focal plane is now set at 20mm for visual use. This is the arrangement for the very compact design used by Mr Trott.

It is sometimes asked why folding flat mirrors have to be more accurate than [say] Newtonian elliptical diagonals. The reason is quite simple. A 45 degree diagonal magnifies any surface error [from flat] by 1.4. Whereas a folding flat mirror returning the light cone by almost 180 degrees effectively doubles the effect of any surface error on the flat mirror. The much greater distance from the focal plane further exaggerates any surface zonal errors on the flat by optical leverage. It obviously pays to invest in the best possible first folding flat mirror from a recognized and respected source. 1/20th wave accuracy is listed by several US manufacturers.

Objective lens quality varies enormously but ought to be at least 1/10th wave in monochromatic light for a high quality, astronomical telescope lens. Using [only] a typical 1/10th optical flat will effectively halve the quality of your objective lens to only 1/5th correction. This strongly suggests that a guaranteed 1/20th wave first folding flat is chosen. However, it is extremely unfortunate that this first, flat mirror must be so large in so many folded refractor designs. This places a far greater demand on the optical accuracy during manufacture as well as enormously increasing the price.

It should be noted that I am referring to zonal errors here rather than curvature. A perfect optical flat is essentially a mirror of infinite radius of curvature. Flats in the real world will have very long radii of curvature but probably not infinite. Weak curvature should have very little effect on a folded optical system because the mirrors are not [usually] tilted by very many degrees from perpendicular. All that very mild curvature would do is lengthen or shorten the effective focal length slightly.

A slightly different parallel folding arrangement with the second flat mirror cell mounted on the back of the objective support plate. This also uses a 5" diameter 1st optical flat but needs only a 3" diameter for the second folding flat mirror. My original paper light cone was made for a much larger 50mm illuminated circle at the focal plane intended for SLR photography. Today I cut the paper cone again but to a 20mm final circle of illumination. Had I done this earlier I could have chosen to use a smaller second mirror. Albeit one requiring careful placement to avoid vignetting or using the full diameter of the mirror. Which might have impacted on the edge quality. The height of this folded optical layout is greater. Demanding a larger, round OTA or an oval tube.  For a real telescope design the likelihood of using a star diagonal requires that the focus falls well beyond the back plate. This will require a larger, second folding mirror. Which is where the generously over-sized 4" second flat size wins. Allowing far greater design flexibility than with a very tightly constrained, 3"optical flat.  

 There is some doubt as to the claimed accuracy of 1/30th wave commercial flats. This is because it is so difficult to measure even with vastly expensive Zygo interferometers. The huge jumps in price as diameter and accuracy increase recognizes the difficulty of reliable manufacture to such a high standard. Even the substrate, mirror [blank] material, can vary in stiffness, annealing quality and thermal stability. Most flats are polished in bulk while fixed to a supporting medium or tool. Simply removing the polished flats from this medium, after polishing, can produce unwanted curvature as the blanks are released. The quality of annealing and resulting stresses within the mirror blank itself must be carefully considered. Surface polish quality is also dependent on the substrate material. With fused quartz competing with Zerodur, Pyrex and even plate and crown glass each has their own properties and claims. Not least for the amateur telescope builder is their usually, very high cost!

A more realistic folding of the light cone with allowance for the focuser length and star diagonal. The objective lens may be moved forwards if the 1st flat size proves too small. The simple, tapered piece of paper provides immediate insight into the required size of the optical flats and where the folds will not exceed their preferred diameter. The paper will also give instant confirmation of where and what size any baffles may be placed along the optical axis.

 

Click on any image for an enlargement.

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