7" f/12 iStar folded refractor 29: Mounting "The Thing."

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 March continues to be cold but reached 42F in a rare bout of sunshine this afternoon. I could even take off my heavy down jacket while I worked in perfect comfort out of doors.

The crescent moon had been taunting me so I decided to see how I could mount the strange OTA. It quickly became clear that I could not reach the eyepiece with the OTA on the big, Fullerscopes MkIV mounting on its tall pier.

So I moved swiftly on to the smaller MkIII mounting sitting on its massive, all welded, stainless steel tripod/pier. The absence of anywhere to fit conventional tube rings required an alternative method of quickly mounting the framework on the saddle.

Rather than use solid timber battens I decided to use 18mm, 3/4" BWP birch plywood for its longevity and strength. I started with overlong crossbars bolted to the saddle. The extra length allows easier location even if I missed the exact spot during the 'big lift' in the dark. At about 24lbs, without the objective fitted, the OTA's I find the weight is just manageable for the lift. Though it is quite easy to carry around while resting one bar over my shoulder. Recent back problems have demanded I respect my increasing age. Hence the removable [bayonet mounted] objective.

Now I needed rails to allow the OTA to sit absolutely securely while it was being fitted. I also needed it to slide up and down for longitudinal balance. Hence the next set of rails bolted perpendicular to the crossbars to fit comfortably between the bottom main tubes of the OTA framework. The plywood should be quite kind in the long term to the black finish of the Porsa framework. I certainly did not want any metal to metal contact scraping steadily away at the paint finish. Nor any unnecessary holes drilled in the Porsa tubing. There is no worse telescope than one which needs to be bolted into place while being mounted!

Clamping the OTA safely without risking cosmetic damage was the next step. So I made another crossbar and radiused both ends so that the bar could be swung on one bolt and then clamped without removal of any nuts. Nor needing to thread fiddly wing nuts onto bolts in the dark! The clamping system must be completely automatic and safe to use when all the OTA's weight is still being taken by the user.

Note the curved, open slot at the 'free' end of the clamping batten drawn with a beam compass and then sawn out. [Top image] The clamping bar is pointed north during OTA fitting and is then dropped to the horizontal for easy clamping. Had it pointed south it might have fallen with gravity during the tense moments of fitting the rather heavy OTA to the mounting.

I really need another clamping crossbar lower down but became involved in how much extra weight I needed for counter-balancing the new OTA. A stack of 1.5kg barbell weights was not nearly enough. So I had to scavenge the larger counterweights from the MkIV. Finally adding my largest barbel weight brought the OTA to safely balance at probably close to 50 lbs or 25kg.

The image above shows the OTA balanced [but still riding high] without the heavy objective in place. Now I needed to test the overall balance, with the objective in place, but without risking the precious object glass itself. Luckily I had a small, soft-jawed vice which weighed exactly 10lbs on its little board.

In this image, the vice can be seen clamped securely but without damage to the OTA front plate. Fortunately the OTA was now perfectly balanced by the counterweights I had already fitted. With the axis clamps released I could now move the OTA smoothly around the sky without any unwanted tendencies or bias. A fortunate result which saved me hunting for, and then fitting, even heavier counterweights.

Now the moon is hiding behind the clouds so all my efforts were in vain for today. The lack of a bayonet lock put me off risking the real objective when the moon began to tease again later on. Though it was always very hazy at the best of times and really not worth going out again.

Note how much lower the OTA has moved relative to the mounting to achieve longitudinal balance with the objective substitute [vice] in place. Passing a line through the declination axis shows my guess as to the balance point was remarkably precise. Adding a finder will move the C of G only slightly further back as the moment arm is actually quite short. I cannot even imagine how the little MkIII would have coped with the straight tubed 7" refractor! Nor could I imagine myself lying on the wet ground to observe overhead.

The strange appearance of the optically-folded OTA is a matter of taste.  I quite like its purposeful look in its smart, black Porsa finish. Though in retrospect I may not have needed the flanged option for some of the tubes it works well where they were needed for plate location and appearance. I keep looking at the lightweight, stumpy dewshield and wondering whether I should fit it permanently to the objective.

Tomorrow I need to make another OTA clamping bar and a really secure and foolproof bayonet lock. No loose parts to drop on the ground in the dark! Nor any fiddly screws or tools. Mounting the OTA equatorially makes a bayonet lock essential because the OTA rotates well over 180 degrees as it moves around the sky. Whereas an altazimuth would not have been much of a risk as it only points up and down. The eyepiece height [with a star diagonal] is now quite comfortable while standing. The rotating focuser and 2" star diagonal avoids the eyepiece assuming odd angles. The star diagonal can be removed for low altitude observing.

Much as I'd like to have used the MkIV for its hugely increased stability and much better [adjustable] drives this would require the pier to be shortened considerably. Which I really don't want to do since it offers a much wider range of uses as it is. Besides that there is a massive [shaped] flange welded on top!

The trailer jockey wheels lift the entire pier about 15cm or 6" off the ground even in their fully lowered position. So I thought I might clamp the trailer jockey wheel jacks to solid pieces of timber. These would be sized to slide easily into the hollow, horizontal, pier legs and be long enough for guaranteed stability and retention.  Though removable cross pins could be used for retention if they should prove necessary in practice.

This would allow the bare pier 'feet' to rest flat on the ground if I levered the pier legs high enough to slide the [jockey wheel bearing] timbers out, one at a time. This would allow the pier to be 'parked' out of the way until needed. Then wheeled out to a suitable observing spot. Then the pier would be literally lowered to rest directly on the ground without the added height of the wheels interfering. The trailer wheels on their screw jacks were a response to the straight-tubed OTA needing much more headroom to 'get under the eyepiece' at high pointing altitudes.

Or, I could buy some more 'scrap' 7" [flanged and galvanized] pipe from a local, architectural steelwork company. Then make a shorter, wheeled pier for the MkIV for [normally] seated use for objects at modest altitudes. Or, I could finally complete my threat to build a raised observing platform. That would make much more sense for seated observing without losing lower lying objects to hedges and trees.

Click on any image for an enlargement.

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