7" f/12 iStar refractor 27: Ringing the changes.

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I woke early this morning and had my first and only sighting [so far] of the three planets perfectly lined up in the Eastern sky. Brilliant Venus low down, then dim Mars and brighter Jupiter were arranged in ascending order of altitude. I tried my 8x42 binoculars but could see little more detail than with the naked eye. A quick 'snap' with my Lumix T27 proved worthless. I captured only Venus and the dimmer Jupiter with Mars unseen.

The Moon was just visible, very low down, through the deciduous hedges thanks to a winter thinning aided by a storm. With the sky lightening rapidly clumps of cloud were further spoiling the contrast. The Pleiades were just visible over the trees to the west with Orion already sinking out of view. I couldn't use the telescope while the 7" is still dismantled as I consider how best to stiffen the counter-cell arrangement.

The 1/4 BSW stainless steel studding arrived in the post today along with the matching 1/4 BSW tap. As did the 202mm rolled rings from Orion[UK.] The finish is difficult to see in the images but has a lightly "hammered" texture.

I had no idea these rings have no "proper" base. It is inevitable that a tangent will lie across the ring's circumference with a small gap each side. [Arrowed in the image] Small screws are supplied to fix each ring, through the rolled band to a mounting dovetail [or saddle] via pre-drilled holes. Some dovetail bars do have a radius on their upper surface and would help to support the ring more evenly at the fixing point.

A considerable length of thread can be taken up by the fitted plastic [?] knob on the clamping screw. I was initially afraid the rings were slightly too large until I allowed the thread to pass right through the knob by a very small amount. There was some resistance from the knob when I tried this but it was free to turn on its thread immediately afterwards. The rings were then nicely snug on the main tube with the folded, main tube seam lying neatly under the gap where the hinges are fitted.

I don't think I'd be happy simply screwing these tube rings straight onto a flat surface. The entire tube would want to rock from side to side with nothing to stop it doing so except for the highly stressed fixing screws. A simple alternative would be to add suitable supports as packing under each ring fixing. The tube ring would then be stopped from any lateral rocking by the packing pieces. Round rod, with the fixing screwed passed through perpendicularly drilled holes would have a stabilizing effect. It might be best to fill the gap between the tangent and the curve until the base of the curve between the screws and their packing strips or rods just touches the flat mounting surface in the middle.

I ordered these rings weeks ago when no other [budget/affordable] ring option in the correct size seemed available anywhere. Why on earth a commercial, cast  8" [200mm] ring is not part of the standard range seems very strange.

Orion listed the 202mm size as if it were standard stock but apparently they are made to measure. I have since obtained a couple of pairs of well-oversized 235mm diameter "Skywatcher" type tube rings. One pair was packed out with 30mm wide birch plywood rings precision cut to size with my router and DIY circle cutting jig. Otherwise I would have had no rings to hang my new refractor project on the MkIV mounting.

I am left wondering whether the Orion rings are actually an improvement on the massive looking "standard" rings with their thick plywood packing. The packing does make the cast rings look the part. As the sort of typically over-engineered castings one might find on a classical refractor. Particularly when they have been painted all over in one colour to make them appear as solid items. Their "massive" appearance is very much a matter of taste. There is absolutely no "give" at all once the clamping screw is tightened on these rings. Even when opened these rings fit so well that sliding the OTA through them to obtain the correct balance is very hard work.

The Orion rings do seem very likely to suffer from local flexure at the fixing points if stressed perpendicular to the telescope tube. Unless, of course, they are seated on suitably thick strips or rods. Otherwise, the wind, or just pushing the focuser sideways to nudge the image back on center would tend to make the tube rock from side to side on the dovetail or saddle. That said, Orion do seem to use these rings on a large range of their own telescopes.

In the images here I have fitted the Orion rings on the tube to see how they look and feel compared with the [plywood] modified "Skywatcher" design. The Orion rings would be too low to hold a carrying handle because there would be no finger clearance between the handle and the main tube unless suitable packing pieces are added. I shall have to ponder on how best to use these rings.

They are certainly very lightweight compared with the [heavily packed] "standard" ring design. One Orion ring could be used as a stop ring so that the OTA cannot slide down through the main rings. As such it would not add much weight to the OTA for carrying out to the mounting. These rings might also be used for supporting a guide scope or larger finder. However, such additions all add to the OTA's overall weight.

I remain convinced that having the tube rings previously fitted on the MkIV's saddle is the best way to mount the heavy OTA. A dovetail bar would only add to the weight since the bar and the rings would already be fitted to the main tube. There is a nice sense of security as the OTA is lifted to the vertical and lowered into the open lower ring. The saddle has already been set and locked pointing at the pole star. The OTA is then tipped up at the focuser end to lower it gently into the upper ring. The fit is now so good that the OTA shows no tendency to slide downwards. I just need to fit a stop of some kind to ensure the OTA is balanced before I climb the ladder to tighten the top ring, clamping screw. The lower ring is just reachable from the ground provided the pier has not been jacked up too far.

Well over a foot [30cm] increase in height is available from the lowest jack setting to the highest. The trailer jockey wheels continue to allow fairly easy movement around the area of activity. It has to be dragged slightly uphill from its normal parking space. But rolls readily back down again after working on the telescope or its mounting. It is not a matter of wheel friction or resistance but overcoming the inertial mass when first getting it moving uphill.

This image shows a third ring being glued to the front of the previous two. The small flange on the main tube is now trapped between the front and second ring. With a relief shoulder routed out to make just enough room for the flange. The difference in internal diameter between the 20cm [8"] main tube and the 195mm of the objective cell is too small to allow the counter-cell to be sunken within the main tube. Keeping the  main tube flange will help to maintain the roundness of the main tube thereby avoiding distortion and sag. The 5mm collimation "pull" screws are being used for exact location of the front ring. The ring also slips nicely onto the flange thanks to careful routing. Thus maintaining the maximum cross section of plywood.

Once the glue was dry, the 36mm thick, birch plywood rings make a solid counter-cell for the objective. The rear section of the objective cell enters the front of the plywood ring to achieve a reasonable seal and some extra support. The three, long 'pull' screws pass right through the objective cell, the stumpy dewshield and then the plywood counter-cell rings where they are tightened into the trimmed T-nuts. The stumpy dewshield is trapped flat against the front of the counter-cell and provides a solid surface against which the collimation 'push' screws press. The push screws are furnished with stainless steel Nyloc nuts to spread the load rather than indenting the stumpy dewshield. The full length dewshield now slides over the stumpy one rather than being a permanent feature. This allows the refractor to be rested vertically on the shorter dewshield with colliding with the ceiling.  A sturdy refractor should maintain collimation in the long term despite being repeatedly moved between mounting and storage. An f/12 isn't particularly sensitive to optical misalignment but accuracy of collimation is still highly desirable.

Click on any image for an enlargement.

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