7" f/12 iStar refractor 6: Focuser backplate.

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My attempts to line the new tube rings with strips of foam proved slightly more demanding than expected. Cutting the foam went rather badly despite the foam being rested on a smooth cutting board. Even a brand new hobby knife blade snagged repeatedly in the "sticky" plastic foam.

Cutting the foam with scissors produced rather ugly scallops. The foam strips had to be clamped level in the workbench vice jaws and "sanded" smooth. Using finer grades eventually produced an acceptable surface. Then the contact adhesive I intended to use had dried up in the bottle. The remaining liquid lying on top had zero gripping power even after waiting overnight for it to dry.

Then I tried thin, double-sided tape. This worked well but I'm unsure of its long term holding power when repeatedly dampened with dew. The grip of the foam on the tube is so high that it probably wont slide easily through the rings when needed for re-balancing. Not a serious problem since the clamps will just need to be opened with the declination axis locked in the horizontal position. The tube can then be lifted free and moved along as desired.

Another tour of the village recycling shops produced some cheap saucepans and baking utensils. I needed two items of exactly 8" OD in aluminium. One to house the lens cell [sometimes referred to as a counter-cell] and the other to act as a fairly solid tailpiece face-plate for the focuser.

Without having seen the objective lens cell I am a little unsure exactly what I need. The solid saucepan will be very stiff once inserted into the main tube rim first. The heavy aluminium base will provide a good, solid surface on which to mount the focuser. A little extra weight here won't do any harm as it will help to balance the OTA.  The walls of the deep pan in the foreground have only a small amount of taper.[~1mm overall] This should allow the pan base to be brought flush with the main tube if pressed in rim first. The pan walls would be cut down to only a couple of inches high to provide a shallow end cap to the main tube. This cap could then be fixed securely with small screws through the walls of the main tube and the pan.

The lighter shell [fitted into the main tube in the picture] would be cut away to leave a large hole the size of the rear of the objective lens cell. Its depth may help to sink the vulnerable lens into the protective end of the main tube. Two identical saucepans could have served at each end of the main tube. However, the other one I brought home was a little larger by a millimeter or so. I am unsure whether it will safely press into the end of the main tube without causing damage.


The last thing I want is to force the folded seam of the main tube open and weaken the tube. This other pan also has more taper on its heavier sides. So might serve better as a counter-cell if the sides are cut down to match the full depth of the lens cell. The heavier base would allow cell fixing holes to be threaded. There are still other potential sources of old saucepans left to scour and 8" Ø seems to be a fairly common size. We shall see what tomorrow brings.

Yet another cycle ride produced a vintage steamer of about 8" diameter from another recycling shop. Measuring with a vernier caliper, back at home, made the useful outer components about 202mm external Ø. Even allowing for some remaining, slight ovality of the main tube, that extra millimeter may be just a millimeter too much. Though I could make some longitudinal saw cut to make one of the pans fit. The cuts would be invisible once the shortened pan base is inserted to bring it flush with the tube end. The nicely flat surface would be ideal to mount the focuser and look well once polished or lightly grained.

The slightly tapered pan, which I bought yesterday, fits perfectly when an external diameter of 101mm is reached. That is just a hand pressed fit of 1" depth without the use of a weighted plastic hammer.

The heavy, steamer, base pan set up in the 4 jaw chuck. I really need the depth of the objective cell to know how deep to make the counter-cell. I'd like to sink the lens below the end of the main tube for extra protection. It may be impossible to fit the objective cell into an 8" pipe anyway. The reason for using cooking pans again is lightness. It also saves on the cost and difficulty of obtaining an 8"+ aluminium bar! The largest bar of aluminium in my metal stock is [only] 7" in diameter.

My old S&B lathe can turn items up to 9" diameter and the 6" 4 jaw chuck can easily span that size from the inside. I can turn the pans to make the central holes in the counter-cell and focuser base, perfectly concentric. Thinning and adding an external taper to the oversize steamer pan will ease insertion. The base pan of the steamer is over 4mm thick at the rim. With an OD of 202mm it can be safely turned to fit the main tube. I have just noticed that the pressed seam of the main tube is welded at each end to avoid the ends opening out where they are most vulnerable. Another worry over.

A couple of hours on the lathe fighting with the soft aluminium pan base had it shortened to 2" and a hole made for the large, Vixen focuser base. This is the original thinner pan I bought. It made an awful racket while I was cutting because the pan rang like a bell. The image shows the pan now inserted the correct way around but not yet hammered fully home.  I have yet to discover the objective's focus point with a 2" diagonal in place. The pan only owes me pocket change so I don't care it it gets scrapped in the light of later developments. I may need to extend the tailpiece to match the focal plane with low power eyepieces. A length of 4" alloy tube with large flanges may be pressed into service if it proves necessary.

BTW: The double sided tape fell off the fibrous ring liners. No adhesion.

Click on any image for an enlargement.

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