7" f/12 iStar folded refractor 22: Focuser & 1st folding mirror cell.

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Yet another rainy afternoon so I cut the 155mm hole for the Vixen 2" focuser base. I chose the highest position which would not impact on the framework tubing.  Having used a large pair of dividers to scribe two lines, with slightly different radii, I sawed out the hole slightly undersized with an electric jigsaw. Oiling the blade and plate made cutting very much easier. I then filed to the line and the focuser dropped in with no play.

I have re-used the plywood clamping ring which I had employed on the earlier, straight tubed refractor, to allow focuser rotation. I may dump the plywood ring and turn an alloy ring instead. A quick "sand" with an electric orbital sander cleaned up all the marking on the backplate.

I discovered that there was no room for straight handles on the front plate with the dewshield in place. It would take cranked handles to clear the very large dewshield at the base. I shall have to see what is available or rethink the dewshield support method.

The next morning was sunny and reached 42F by lunchtime. So I was able to work out of doors in good light and perfect comfort.

Firstly I fitted the objective and tipped up the OTA to bring the sun's conical beam onto the back plate. This provided a 100mm 4" circle of bright light. I then measure down to the center of the objective and marked the backplate to match. This would be the center of the 1st mirror. The objective was then removed and returned to safety indoors.

A simple hinge was bolted to the 1st mirror's, plywood backing disk. I then sighted through the backing disk's central hole for the pencil marks and then marked the top of the hinge onto the backplate. A spare hinge was offered up to the line and the central screw hole marked and drilled. A single screw fixed the hinge and backing disk in place. The mirror shell was slid over the backing disk and a flat shaving mirror laid into the shell.

This mirror represented the real one without risking damage to the high precision surface. I then tipped up the OTA to shine the sunlight through the objective aperture to check the required tilt angle on the 1st mirror. No great precision was necessary at this stage as I centered the 1st mirror cell in the circle of light.

I was merely checking that I hadn't overlooked anything or that the hinge wasn't being jammed at the required tilt. All seemed to be well. I shall need long bolts passing through the backplate with compression springs inside the OTA to allow collimation by adjusting the 1st mirror's tilt. The single bolt through the hinge and backplate will provide rotation if necessary.

I then measured down to the center of the focuser and checked where that point lay inside the framework's shoulder.

By a happy coincidence the center of the 2nd folding mirror lay midway on the Porsa tubing flange. The 2nd mirror's support hinge could be applied to the face of the tube or even underneath for greater neatness.

Whoops! As you were! A helpful contact has pointed out that hinges would not allow for sideways tilting. Attempting to tilt the cell/mirror sideways, via the other sprung collimation screws, would have little effect. The hinge would need to bend midway along its length. Rotation on the single screw would not be in the correct plane.

I have therefore returned to conventional three [sprung] screw collimation. With the exception of using a soft, rubber tap washer under the top screw instead of a traditional spring. The two springs on the other two screws will supply the necessary pre-tilt. While the tap washer will flex enough to allow lateral and vertical tilt when the other screws are tightened individually against their firm springs. The thin tap washer also avoids adding unnecessary depth to the mirror support. Having now dumped the hinge idea I shall have to add a plate inside the OTA shoulder [arm pit?] for the 2nd mirror cell collimation screws. 

With reasonable luck my measurements should ensure that the light path follows parallel lines as it is folded back and forth between the objective and the focuser. It does seem as if there is a lot of fresh air around the folding mirrors. But the minimum dimensions of the framework are set by the diameter of the objective and its focal length. I also had no desire to compromise the design with stray light shining directly from the objective into the focuser. So I opened out the angles of reflection at the folding mirrors to help avoid this. Baffles will also be applied to prevent stray light spoiling the contrast.

Click on any image for an enlargement.

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