7" f/12 iStar refractor 7: Rotating focuser.

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Latest progress: I have fitted the large Vixen focuser to the back plate. More by serendipity than deliberate intent the three fixing screws provided a perfect chance to make the focuser fully rotatable. I had to turn the screw heads down repeatedly in the lathe until they allowed complete freedom of movement against the inside of the back plate. Just to help matters along I added some thin plastic tubing to the screw threads but could improve these further to kill the slightest bit of "slop".

The back plate is not a finely turned surface so some leeway must be allowed for very slight irregularities. I could fit a liner between the screws and the focuser back plate. Above all, I want to avoid a circular scar in the matt black paint on the inside of the back plate due to turning the focuser repeatedly over time. The particles of scuffed paint might find themselves where they are not wanted as the telescope is constantly moved around.

This image shows the three, overlong, retaining screws in place. At first I thought I was going to have a serious problem getting a fixing on the narrow Vixen flange. When I experimentally fitted the three screws I could hardly believe my luck. It would be so easy to provide a fully rotatable focuser for very little effort. Note the scuff marks from earlier attempts with larger screw head sizes.

It was important to be able to tighten these screws fully home in their threads to ensure they were safely locked in place. I tried using pliers but there was too little room to grip the small screw heads so close to the back plate. In the end I bored a small hole in the rim of the back plate to be able to insert a long, thin screwdriver. The focuser back plate will need to be sprayed flat black inside to kill all reflections. A thin sheet of black plastic proved to be too thick and prevented the focuser from turning. The screw heads cannot be made much smaller without loss of driver function.

The objective cell fitting is another problem altogether. I had originally planned to have a flanged tube inserted into the main tube, probably using the other saucepan for raw materials. However, the difference in diameter between the 200mm main tube bore and the 195mm OD of the cell body made this impossible. Now I have a dimensioned drawing of the cell I can see that I need a much larger flange to take the cell adjustment screws. The flange must be inward facing if a larger tube is used.

Even without the focuser fitted the bare OTA now stands over 7' tall while balancing on its nose!

I have no desire to cripple my OTA with  massive, solid aluminium counter-cell. A counter-cell is really no more than a pipe-size adapter. It need not be any stronger than that required to support the weight of the glass and its cell without flexure. If it should flex then collimation will change with the OTA's attitude. If it is heavy it will force the objective towards the mounting to achieve balance in the tube. A common enough sight with refractors. APOs are particularly vulnerable with heavy triplet objectives.

The increased weight all adds up to a heavier OTA to carry about. Worse still is the increased moment arm of adding unnecessarily  heavy components on the extreme end of the long, main tube. The greater the moment arm the worse the handling and damping times on a typically undersized mounting. We are looking for a revolution in refractor weight reduction here! Weight is the one luxury which most amateur astronomers cannot afford. Yet reviews often mention the "solid construction inspiring confidence" in the quality of the instrument. Short-sightedness or just typical hype and bullshit?

I am not in the market for an AP1200 or even larger mounting as are many amateurs struggling with a heavy refractor. Adding solid "CNC'd" components to already heavy refractor tubes is much like handing out free takeaways at Weight Watchers. Can you imagine rewarding members for gaining even more weight? The whole idea is simply ridiculous in a famine of affordable telescope mountings. See me! See my pot-bellied refractor, Jimmy! 

Needing a larger counter-cell actually aids my cause when fitting a dewshield. Ideally I need another saucepan which fits comfortably over the main tube and provides a larger diameter base for the dewshield and objective cell to sit on. The base of the pan will provide the necessary flange for the "push-pull" adjusting screws of the objective cell. I will need packing between the main tube and the inside of the empty counter-cell shell. Since the main tube is flanged I shall have to use a split shell turned in the lathe for accuracy of fit. Probably made from rings of Birch plywood. Though I should also examine alternatives which would allow an easily retractable dewshield.

I would still like to arrange a solid front edge to the OTA. To allow it to be rested on its nose without risking the precious glass. My planned, self-rolling, black plastic dewshield will not take the weight of the OTA and will be highly vulnerable to damage. The plastic makes a stiff and ultra-lightweight [light and] dew shield. It is deliberately not made a structural part of the OTA. The intention was to slide the dewshield  back over the main tube, when in storage, to help reduce the great length of the OTA.

Click on any image for an enlargement.

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