7" f/12 iSTar fiolded refractor 23: Cell division.

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My fellow, folded refractor ATM, has suggested that the folding mirror cells need very fine adjustment. Two possible options immediately present themselves:

Finer adjustment threads on the mirror collimation screws. Or greatly increased size of each mirror cell's backboard. Tripling the size, of the mirror's initial triangular footprint, would be the equivalent of making the screw pitch 1/3 finer. M6 = 1mm/3 = 0.3mm. Which is finer than most commonly available fine pitch screws. [0.5mm] While the larger triangular form still allows a wide range of normal fasteners.

The greatly increased size should provide much better collimation stability since the loads on the screws and springs will also be reduced to one third. On the original backing disk there was only 7cm between screw centers. On the new triangle the distance between centers is 22cm. Though the screw spacing is slightly narrower across the triangle's base where sideways tilt is adjusted. Still more than adequate to the task.

The top screw will compress a soft, rubber tap washer as before. Providing further stability [compared with a long spring and loose bolt] and a degree of built-in tilt due to the springs on the lower screws having much greater length. Packing on the adjustment screws can easily be provided if need be. The length of the springs themselves may not be enough to achieve the necessary mirror tilt for optical folding.

Finer screws and considerably enlarged cell back plates would provide micro-adjustment of mirror tilt. The problem might be finding the fine screws and matching butterfly nuts. I was unable to find any wing nuts in fine threads online nor even coach bolts. The expense of buying new taps and dies with very doubtful qualities does not suggest I follow this route.

The image of the plywood mock-up shows a rough idea of the potential for a greatly increased size of cell backplate for the 1st mirror. The dimensions of the screw adjustment triangles is now three times greater as should be self evident.  I used a beam compass to ensure the collimation screw spacing was correct.

The increased weight of an alloy collimation backplate could be much reduced by perforation with hole saws or skeletonizing. A triangular form is assumed for minimum starting weight.

The 2nd mirror is rather less accommodating in its position to allow a greatly increased adjustment triangle. The lower pivot point may only be extended so far before the back plate obscures the light cone from the back of the objective lens. Though the triangle could be pivoted from the light baffle for maximum extension. Arranging the initial 2nd mirror tilt would be a matter of chosen screw length.

Both mirrors could be mounted on wedges to provide initial tilt. The triangular, collimation adjustment plates would then lie almost flat with the framework. Allowing the use of much shorter bolts and springs. This arrangement would also have removed the need for skewed screws in their holes. Though alloy plates could be easily bent at both ends to make the screws perpendicular to their plates and the framework plate.

By a happy coincidence the tilt on the 1st mirror backplate was close to that required. I held the camera roughly in the center of the objective aperture and saw the target reflected in the 1st mirror. The red cross marks the center of the 2nd mirror as seen in the shaving/make-up mirror used as a 1st mirror mock-up.

The hairy twine is just to mark a line from the top center of the objective aperture and runs parallel with the framework. This helped to indicate the lowest point allowed for the 2nd mirror backplate without causing obscuration of the conical light beam. The camera is obviously slightly off center but easily done when the lens is not remotely symmetrical to the camera body.

Further measurements and trials suggested I needed to lower the 1st mirror. Particularly because of its slope and raised mounting of the reflecting surface which caused some upwards displacement.  The center of the mirror surface must lie on the axis of the objective and that axis must be parallel with the framework. Having tried long springs on 100mm coach bolts I am now even more in favour of mirror support wedges and flat collimation plates.

Click on any image for an enlargement.

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