2" shaft mounting. Pt 10: Adding disc bearings?

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The discussion on cloudy nights has raised the specter of flexure at the PA flange to shaft junction. The flange does not have enough depth to rule out flexure in the joint between them.

Rather than dump the original flange bearings idea I may add disk/plate bearings to this highly stressed area. The Fullerscopes MkIV cleverly used a combination of both 6" diameter disk as well as normal shaft bearings. This allowed it to carry far larger and heavier instruments than its [mere] 1.25" shafts might suggest. The disk bearings surfaces obviated the usual bottlenecks at the cantilevered junctions between axes. There being no bare-shafted overhangs. A disk of paper-thin PTFE/Teflon was used to reduce friction between mating surfaces. The wormwheels were sandwiched between the 6" diameter disks but deeply recessed to reduce overhang.

A disk [or plate] bearing uses reversed leverage to avoid flexure. Taken to extremes, it need only be two disk rubbing together with a central pivot pin for alignment. The disks are pressed firmly together by the loads placed upon them. In this case the weight of the Declination assembly, telescope and counterweights. Any tendency to tip or separate the disks is greatly resisted by the very poor mechanical advantage of the system. It would be akin to trying to lift something very heavy by pressing on the short end of a normal crowbar. 

The larger the bearing disks the more stable the arrangement. Downside is the increased friction with increasing disk radius/diameter. It follows that low friction combinations of materials become essential. Brass against steel or aluminium would work. As would a sandwich filler of Teflon/PTFE sheet between the disks. 

The Dobsonian altazimuth mounting is an example of a disk bearing being used between the rocker box and the ground board. To achieve low, but not zero, friction usually involves Formica and Teflon pads. There being no need for a full disk of the expensive Teflon material. Slight friction is highly desirable. A disk bearing can have its friction lowered by having only the rims in contact. This may also increase the stiffness of the arrangement.

I have some 180mm / 7" aluminium bar stock which could be used to make bearing disks to reinforce the flange/shaft bearing. The image alongside shows one way of doing this. Rather than reverse the PA/Dec neck flange a disk could be "wrapped around" it. This disk should be fixed to the neck flange by the same fixing bolts which hold the flange to the Declination housing. 

A second disk would be attached to the top PA flange bearing. I have shown this bearing reversed in the image but it be best to leave it flat side down to ensure proper seating on the PA housing. The lower bearing disk would then be bolted to the top flange bearings casting by the same fixing studs which reinforce the PA housing. It is not enough to have loose [bearing] disks. As they would not contribute as much stiffening as those which are firmly fixed to the bearing components themselves.   

A further complication is the addition of the large [and 16mm, 3/4" thick] wormwheel to this multi-layer sandwich. A ring or cup form of packing disk might be essential to increase the diameter of the wormwheel bosses to ensure the full diameter of the disk bearing is utilized. Where should the disk bearing, rubbing surfaces be situated relative to the wormwheel?

The wormwheel's clutch system must be carefully considered as it relates to the vital freedom to point or slew the telescope by hand while the drive motors and slow motions are simultaneously in use. Loading the wormwheel faces with the entire weight of the telescope, counterweights and mounting parts might make the wormwheel essentially integral with the Declination housing by friction alone. This would be highly undesirable as it would then be impossible to point the telescope except by using the wormwheel's drives. A very slow and impractical arrangement indeed! 

If the friction between the disks should prove too high in practice then an adjustable thrust, ball bearing could be employed. This could be placed at the base of the PA shaft or in a counter-bore between the disks themselves. The neck flange could be utilized to support a linear thrust bearing. Washer-like shims could be employed to adjust the friction levels. It might be worth investing in some PTFE/Teflon bearing material first to see if this overcomes the friction problem.

The disk arrangement illustrated above could be thinned by reducing the disks near their hubs to about 3mm or 1/8". Further aided by inverting the neck flange and sinking it into the Declination housing. Or, minimizing overhang at a stroke simply by moving the RA wormwheel to the bottom of the PA shaft as in the upper drawing.

The lower and final drawing shows the wormwheel used as a disk/plate bearing sandwiched by Teflon disks to reduce its friction independent of the shaft components. The neck flange is inverted and has a disk attached to its face to help to increase the socket depth [against rocking] and provides the top pressure disk for the wormwheel.

The wormwheel rests on top of a lower disk supported by thick, tubular spacers bolted to the bearing flange. This is a much stiffer arrangement since the wormwheel can no longer rock on its shaft. It effectively becomes a plate bearing. The wormwheel boss must rotate freely inside the lower disk and rests on a Teflon washer between itself and the extended, inner bearing race. No other arrangement offers the minimum of overhang beyond the top bearing. Moving the wormwheel to the bottom of the PA [Polar Axis] will reduce the overhang but only by the thickness of the wormwheel and its boss. Which will probably amount to 40mm or about 1.5".

This latest arrangement will help to considerably increase the cross sectional area between the top bearing and the wormwheel. Which greatly reduces the risk of the flange rocking on its shaft. The top disk accepts the heads of the through studs/bolts holding the Declination housing firmly to the 15cm, 6" diameter, inverted flange. While the lower disk accepts the nuts for the through studs compressing and stiffening the Polar Axis housing between the two flange bearings. These fasteners must obviously be sunk below the surfaces of the plate bearing disks.

Click on any image for an enlargement. 

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2" shaft mounting Pt 9: Progress update.

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My neck flanges have been updated as arriving tomorrow. So I can practice boring to a close fit while retaining squareness to the shaft. [Orthogonality. ] I may need to turn a stub mandrel to spin the flanges by their bores to check flange run-out. If the flange faces have any "wobble" when spun in the lathe then the shaft will effectively describe a circle on the sky instead of a point when driven at a more normal sidereal rate [A speed of 1 rev per 23h 56m 04s.]

After considerable online research I have decided to use Loctite 638 bearing retainer to ensure the flanges stay put on their shafts. So I have ordered some 638 online from eBay UK and can expect it just after the weekend. 

YouTube videos show Loctite 638 being used for some seriously heavy duty retention tasks on machinery in a massively different scale to my far more humble needs. Loctite 638 is also fine on stainless steel components. 

I shall leave the application of this industrial adhesive for as long as possible to avoid having to withdraw the shafts completely from their bearings. This isn't so much difficult as fiddly to get the self-aligning, flange bearings correctly orientated to allow the heavy axis shafts to slide freely. A good bevel on the shoulders of the shafts helps initial insertion into the bearings. The shafts weigh so much that they try to sink at every opportunity.

Pretty, but the wrong size! The 48.3 does not refer to the bore diameter [as I has assumed in error.] Despite being stamped 48.3 these measured 42.4mm across the bore.  I have contacted both my supplier and the major importer for bore measurements of alternative neck flanges sizes. Searching online produces either no information on bore size or numbers which do not even match the pair I received. I do realise that these flanges are intended to be welded to standard pipes with nominal designations. I just wish the bore size was routinely listed alongside all the other dimensions published online! How can the bore dimensions of flanges sold worldwide be a trade secret?

A phone call to a distributor produced some interesting news. The DN50-60.3 has a bore far too large for my 50mm shafts at about 54.5mm. But there is a second type of DN50 neck flange. A DN50-55.8 has a bore very close to 50mm. My supplier is hopefully ordering this slightly larger 165mm Ø flange. The DN50-60.3 is very little different in price compared with the 48.3 so it should not be too much of a shock.

Another update: My thread on CN has raised some serious doubts about the efficacy of Loctite 638 retaining compound. Dropping a heated flange over a cold shaft would produce a good fixing, once it cooled, but still needs locking screws for security. I wonder whether I could turn a split cone in brass? This would wedge itself into a turned tapered hole in the flange face as the flange is forced into contact with the axis housing. Probably difficult to impossible to control the gripping power without leaving a protrusion. If the new fit is too loose I may end up having somebody put a weld bead both sides of the flange to fix it securely without resulting flexure on the shaft.

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2" shaft mouting: Pt 8: The pier?

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The pier for our ridiculously heavy mounting really ought to be a massive concrete construction cast off a ground slab the size of a modest bungalow.  Concrete piers are solid but, in controlled tests,  have repeatedly proved to be immovable. 

In desperation we must look around at our well-heeled [fellow] long focus imagers. Those who regularly nip out into the desert with their costly Ritchie-Chrétien instruments. What do they use under their posh AP mountings? Usually an apparently simple, three-legged pier. Note the emphasis on apparently.

Note how the pier legs are large, non-distorting, isosceles triangles in both plan and elevation for stiffness. Not your usual, bolt-on, cast clown's feet attached only at the very bottom of a spindly pier pipe.

 Portable Telescope Piers from Advanced Telescope Systems

The triangular legs reach well up the sides of the pier pipe where they are hinged very firmly into place. Pier pipe diameter is not your 'common or garden' 4" water pipe. These pier pipes are deliberately made large to avoid flexure. So large that they could be used for people smuggling on the return trip from the desert. I jest, of course, but you get the message. Where pier pipes are concerned then BIG is BEAUTIFUL.

What can we learn from this? That bigger is even better if weight must be kept manageable. Steel ventilation ducting comes in various sizes in 2 meter lengths. A 12" diameter tube makes good sense when wall thickness [and greatly increased weight] is denied to us. Our thin steel pier pipe is not going to be the stiffest "factory chimney" around. So it will need careful reinforcing at stress points to avoid ovality and other distortions.

Thick plywood can be laminated into thick, internal circles and fixed where it really matters. Studding can be used in compression between these baffles to tie the thick top flange to the upper leg hinge points. Multiple screws can be added to lock the thick disks into place through the walls of the tube. Because we have a free choice, we can make the pier pipe just clear the ground to greatly increase the area of the vertical leg triangles. The more even-sided a triangle, the stiffer it becomes. The desert imagers demand portability and mobility for the shortest possible pipe length. We cannot afford the luxury of lightness.

The image shows the potential for a fully triangulated four legged pier. Tension cables, metal strips or pipes can be stretched from the upper cross down to the feet. Providing excellent resistance to torsion around the pier relative to the feet on the ground.

We fully intend to balance our heavy mounting and telescope centrally over the pier pipe. So no cantilevered loads are expected. Any tendency to tip will be the result of pointing the telescope to different parts of the sky and [quite probably] wind gusts.

Moving the entire instrument on wheels might be very risky with so high a center of gravity for a large refractor. All the mass is perched up on high. Though the fully triangulated legs become quite rigid, when done properly, they will not be modest in size. Which will make them rather 'bulky' in the dark. But we must totally avoid our Saturn 5 'rocket' from toppling.

The answer may be to ensure a flat running surface [or rails?] without bumps to cause sudden braking lower down. I speak from experience in rolling my heavy refractor pier and mounting around the garden on sack truck wheels. My observing space is not flat and is a mixture of lawn and bumpy gravel with a slope to boot. One hand has to constantly support the top of the pier to avoid it tipping right over on top of me during maneuvers. As I drag the pier in reverse I wear sensible, steel toe-capped work shoes. Just dropping a heavy counterweight on your foot could easily hospitalize you. What damage do you think a massive mounting and heavy telescope can do? Be safe.

If the pier has to be moved then the telescope really should be removed first. The heavy mounting is still up there in the clouds which will need great care. The counterweight will throw the balance off too and greatly increase the mounting's moment in the event of the pier tipping even slightly. Perhaps it would be safer to be less ambitious regarding true mobility? Though four legs/feet will go a very long way to making any significant movements safer. 

I prefer four-legged 'tripods' [quadropods?] to three as they offer a further plane of stiffness and greatly increased resistance to catastrophic tipping. See the associated images for the doubling of the support base between any 3 and 4 feet arrangement at exactly the same radius from the pier center. On non-rigid ground, four legs will easily adapt to the ground by sharing the pressure equally on their foot pads. Local foot or leg adjustment will be needed for leveling anyway.

Another tripod-pier with similar characteristics:

 http://schickworld.com/astronomy/Resized/Monolith.png

Some commercial piers are using turnbuckles in cables to steady the pier pipe but using conventional clown's shoes for the feet. The problem with this arrangement is the poor resistance to torque around the head of the pier. The piers with triangulated legs offer far greater torque control. I built a tripod for my ATM 5" f/15 refractor in my youth from 2" scaffolding poles with heavy angle alloy bolted to a thick plywood disk. Even with the light 1lb force required to move the telescope over most of the sky the tripod still 'gave' slightly to azimuth movements. Requiring I overshoot and let the tripod "unwind" itself to its relaxed condition.

I have a heavy Bogen video tripod with duplicated legs but it falls down on stiffness when the lower legs are extended. The leg extensions are of quite large diameter for a photo tripod but undo the triangulation offered by the stiffer upper legs.  Here it is shown beside the road at the top of a local hill with the Vixen 90/11 aboard for dawn solar viewing. It worked for my afocal snaps but is not something to try in the slightest breeze!

Click on any image for an enlargement.

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2" shaft mounting: Pt7: PA- mounting base considerations.

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The entire and very heavy mounting must be supported on the pier in a manner which is as rigid as possible. Yet still allow the polar axis to be tilted to match the latitude. I have had good success with a heavy set-up using a humble turnbuckle. These are readily available at very low cost for tensioning wire fences. Better quality turnbuckles are available in polished stainless steel at several times the price of the galvanized variety.

The turnbuckle applies tension in very fine increments by rotating the body. The combined left and right hand threads turn against matching L&R threaded hooks or eyes. Naturally it will need the polar axis housing to be hinged or pivoted somehow. The studs and shafts of my [skeleton] bearing housings allow at least a 20mm bar, pipe, screw or stud to pass easily between them. So no silly ideas, like the MkIV Fullerscopes mounting, thank you. Which required that two small and short pivot screws must be tightened in putty soft, aluminium castings to prevent flexure of the polar axis casting in the thin supporting, cast fork.

A nice, big, fat, threaded crossbar can be tightened with a nice big spanner.[US.wrench] This has two advantages. First it ensures security and longevity of the crossbar threads. More importantly it also allows the cheeks [tines] of  the fork to be solidly locked to the polar axis housing by friction alone applied over a wide area. Once the nuts are tightened the polar axis housing becomes an integral part of its supporting fork. The fork is in turn supported by the polar axis as one whole unit. No silly and spindly little altitude tabs supporting the whole 'kit and caboodle for' us!



The image of the SWEQ2 has been overlaid with potential flexure modes. These are only shown in elevation and there will also be 'cross' flexure modes. All of these must be eliminated in a serious, heavy duty mounting. It takes only one mistake to introduce flexure at one point in a mounting. That will be enough to make the image wobble in a breeze or when focusing at high powers. Even a dovetail fitting can be a disaster waiting to happen. It fits in a very short casting and relies on a side screw to hold it fast.

The local pressures on the relatively soft castings must be enormous. A long saddle firmly attached to the declination shaft via a solid, steel flange and numerous bolts should avoid this commonplace flexure bottleneck.

Imagine a small Dobsonian rocker box. Usually the sides are separated by the telescope. So no clamping between the supporting side boards is possible. Turning the telescope in azimuth causes the side boards to splay and distort slightly. Our solid polar axis housing, with its massive threaded cross bar, can lock up the whole arrangement preventing any twisting or flexure at all.

It may be highly convenient to have a dovetail but it has no place carrying a large refractor. You simply cannot ignore the enormous leverage of a long telescope applying massive pressure so close to the fulcrum. The width of the head which accepts the dovetail is the distance from the fulcrum to the applied force.  A long telescope applies effort at up to 48" from the fulcrum. A typical dovetail housing might be only four inches across. 1lb of force applied 48" from the fulcrum = 48 lb/inches. The pressure on the far side of the dovetail is 48 x 4 = 192 lb/inches. That is only a 1lb push at the eyepiece. You could lever a large rock out of the ground with a 4' crowbar! Or even lift a car!  

Our load spreading measures mean that the mounting's base and fork can be built from humble [birch] plywood without a single qualm or restless night. Provided that the crossbar applies pressure through roofing washers then the plywood won't and can't be crushed. It is backed up inside the fork blades by the multiple, birch ply laminations of the polar axis housing. The entire mounting base and polar axis become a rigid monocoque. One unable to flex or twist by mutual reinforcement.

Dob style, altitude bearings will provide the necessary load spreading to avoid damage to the laminated plywood tines of the PA's supporting fork. We certainly aren't amateur enough to [ab]use the crossbar itself as the tilt pivot alone in a bare plywood hole. That might tend to make the heavy mounting head south over time whenever the crossbar nut [or nuts] were loosened to adjust the polar attitude. A suitably large, galvanized coach screw would do just as well as any fancy, threaded crossbar. Though domed brass or stainless steel nuts might look well if you can find them in seriously large sizes.

The supporting mounting fork blades will be firmly attached to a thick, plywood base flange. Once the three components [base/fork/PA housing] are locked together by the compression bar there will be no bottlenecks left to flex.

Click on any image for an enlargement.

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2" shaft equatorial mounting: Pt6: Flanges as an alternative to bushes.

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Finding some means of fixing polished stainless steel shafts rigidly to the declination axis housing [and also the saddle] has proved difficult. Initial euphoria at finding affordable, conical locking bushes at RS was short lived. The price doubled in the time it took to look around the website's list of different bushes! 

So I looked elsewhere using various terms for "bushes" and then found an alternative in very attractive, stainless steel, necked flanges. Which sell at about the same price as locking bushes but are of much larger diameter.  Sadly I never found any QD split bushes outside of the US.

These neck flanges have a smooth machined bore. In standard 1½" water pipe [DN40?] fittings there is a 150mm [6"] diameter flange listed with a 48.3mm bore in a raised neck. The advantage is that it offers enough depth for a rigid joint. The downside is that it needs a tiny smidgen bored out to bring it out to the full 50mm diameter to fit the shaft. However, the top is flat and easily large enough for our purpose without needing extra load spreading plates necessary for the much smaller bushes.

Alternatively, and perhaps easier: the axes shafts could be turned down very slightly to fit the original [and presumably, highly accurate] flange hole. Just putting the flange in a lathe may throw off the accuracy of the nicely finished bore. The resulting, slight shoulder in the shaft might have location advantages for the Polar axis. Though the declination axis might be far too long to fit in many lathes and will not likely benefit from a shoulder when it is pointing downwards. I am not a fan of thinner, declination shaft extensions. The counterweight mass is applied at distance and may find a natural resonance of mass x length x compliance.

Moreover, these pretty flanges have enough length in the "neck" to allow [grub?] security screws to ensure it stays safely in place on a pre-dimpled shaft. Which just means the newly drilled holes in the flange neck are spotted through with a marker pen. Or even the drilling continued into the shaft long enough to make a dimple. Or, the shaft can be removed from the flange and the markings drilled just deep enough to securely locate a grub screw's 'pointy' nose on reassembly. You'll have to remove the shaft from the flange to thread the security screw holes anyway. Always use a weighted rubber or plastic hammer to avoid marring the nice stainless steel finish of these components if one is needed.

Always keep firmly in mind that an unintended/accidental separation of a flange from a shaft might be a hideously expensive and potentially fatal affair! Particularly in the sizes and considerable weights we are discussing here. Imagine dropping a priceless historical achromat or large APO from ten feet up! Absolute security of the shafts in their flanges must be assured 150% or better. Having a shaft drop out of a mounting while it is carrying a huge telescope on this scale is very likely to kill you! Even if it only injures you, but you break an irreplaceable instrument, you may feel like finishing the job off yourself! [Though this is obviously not a serious suggestion.]

Over time, bare stainless steel contact surfaces tend to "freeze" together if they are a nice tight fit. Though this should never be relied on for a fixing. Alternative fixing methods might be industrial adhesives like Loctite. Not some cheapo, water soluble "superglue" from the supermarket! Loctite industrial adhesive would need heat to separate the flange from the shaft if it proved necessary at a later date. It may also need special cleaners and/or primers to achieve their claimed adhesion levels. Check the instructions very carefully and make sure you are using the correct adhesive. There are many Loctites! All flange or bush security screws should be lubricated from the first tightening. Then checked at regular intervals for corrosion and tightness. Opposed screws, or three spaced at 120° intervals are far safer than trying to get away with only one.

Stainless steel is tough to drill unless the drills are brand new. The material easily work hardens with blunt drills. Cutting threads also takes quite some care to avoid breaking the fragile tap. Those without the necessary skill or tools could ask a local engineering firm to do the job properly for you. A through, split [roll] pin might be employed if you can find one long enough and the fit of the shaft in the flange is good enough. You must never rely on a pin [nor screws] in a floppy fitting shaft/flange as the pin or screws will soon wear or even break! Disaster awaits the negligent or "it'll be-alright" type!

I briefly considered screwed flanges and threading the end of the shafts but there are serious pitfalls. Threading the shafts requires that they be very tightly gripped. On water pipe this means nasty toothed jaws gripping down hard on the softer steel/iron pipe to resist the massive torque applied by the threading die. Then there is the very difficult problem of threading tough stainless steel with a cutting die meant for much softer stuff. This job really ought to be done with a properly shaped, thread cutting tool in a large and equally tough lathe. The flange will probably need to be faced off afterwards on the flat side to undo any inaccuracy in the thread cutting process and final assembly. Don't forget a locking screw or pin in case the whole thing unscrews over time!

Black and galvanized flanges are also available as is plain steel shafting. If you don't want to get involved in stainless steel then follow the black "iron" route. Lower cost and much more easily worked and threaded [or welded?] if you want to try that. Just remember to keep the inevitable rust at bay or the mounting may never come apart again! This is no joke [at all] when you can't change the counterweights to balance a new telescope. 

Update: I went ahead and ordered pair of stainless steel, weld-on, neck flanges in the 48.3mm bore size 150mm [6"] OD. Those who want to copy me will need quite a large a lathe. Or know somebody who has, to skim the bore to match the 50mm shaft size. Or turn your shafts to match the flange bore if necessary or even possible. 

Anybody with easy access to QD split bushes can ignore my choice and still have a bolt together, heavy-duty mounting. You just have to match the bore of the QD bushes to your shaft size and slip them on, tighten to the supplied instructions and you should be in business. The flange sizes of QD bushes are likely to be much smaller than those I am using. Those locking bushes I found were 92mm outside diameter but cost 25%  more than the neck flanges.

N.B. I can only suggest how I might overcome the various build problems myself. I can accept NO responsibility whatsoever for unsupervised work carried out by an unskilled, untrained, drunk, drugged, tired, senile and/or slapdash person on the other side of the world. Nothing I say here should ever be considered as a license to cause real or potential injury, or loss, to yourself or others.  Copying any of my suggestions here will NOT make me liable for your unknown inadequacies. I have a lifetime of experience in many constructional and metal working fields. You may not even realise the obvious dangers which an experienced worker would spot from a mile off. For example: I can use a motor mower and power tools/machines without losing any fingers or toes. [So far.] It seems that many others can not. Enough said? 

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2" shaft mouting pt5. Everyman's, large equatorial mounting.

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What I was ideally aiming for when I started this 2" shaft, mounting project was something which could be copied by those without a shed full of tools, welding facilities or any particular manual skills. A sort of "Ikea-style," heavy duty, equatorial mounting which could be scaled up and down to taste and need. All it should require is the ability to assemble the sum of the parts into something useful and practical. A sort of "large equatorial mounting for the common man" if that does not sound too pompous.  

It might become the basis for a universal mounting which could easily handle large and long refractors. I see my project as a generalized response to the almost complete absence of suitable and affordable equatorial mountings. This lack is blocking easy access to "classical" refractors. Or even long focus [so called- planetary] Newtonians. 

Without spending a great deal of money [many thousands of euros, pounds or dollars] there really wasn't much available beyond a Dobsonian altazimuth mounting for this category of instruments. This absence of suitable mountings is forcing very short focus telescopes onto manufacturers product lists. Which directly hampers their optical quality in an achromat and makes an APO the only [but very expensive] get-out clause. Larger APOs themselves are very heavy too. Which makes the choice of mountings even more stratospheric in cost for the complete instrument. This pushes them firmly into a niche product instead of enjoying all the benefits of [cheaper] mass production.

I have been pondering ways to properly stiffen the flange and stud assemblies. The entire length between the flanges really ought to be completely filled with an incompressible material. Leaving a central space for the shafts to turn freely of course. The large studs [threaded rods] would compress the packing material between the sturdy flanges. Producing a virtually solid housing at very low cost in materials with only very limited skill or tooling required. 

No heavy supporting plates are required as is vitally necessary with a successful pillar block mounting. The finished mounting will be no lightweight but should [ideally] be able to tolerate being left outside under a simple cover without rapidly losing its cosmetic appearance.

Despite the squareness of the flanges the housings need not be square externally. It would require a 200mm [8"] round pipe to enclose my 143mm square, bearing flanges over their corners. Though a smaller pipe could protect the packing material while leaving the flanges exposed. Perhaps not the most attractive option aesthetically and providing much less protection for the packaging and bearings. Though 8" disks behind each flange might look more the part this could require a lathe for a really neat job. Depending on the materials involved. I am trying [hard] to avoid the use of machine tools in my design despite owning a lathe and many other tools. 

My "prime directive" [if you like] is to build something from materials readily available anywhere in the civilized world. By this means the builder can bring their own skills, materials and inspiration to the table. Hopefully leading to a steady improvement over time with each new iteration. As has occurred repeatedly to the [originally] very crude Dobsonian "sidewalk" telescope. Scrounged or free materials being the only requisite at first but which eventually led to the exquisitely made, modern, lightweight, truss Dobsonian. Slavish copying rarely occurred with the Dobsonian because the basic design was so "right." The "hive mind" went to work in cooperation and competition to improve the Dobsonian from the basic cardboard tube and dumpster scrounged, rocker box DNA.

Those who demanded a higher standard and were willing to pay for it has resulted in a whole new industry of amateur telescope makers and commercial manufacturers. The same has never occurred with equatorial mountings. Superb mountings are available commercially but at a price well beyond many pockets. The best mountings are immensely strong, reasonably lightweight and have all the bells and whistles of Goto and accurate tracking to match the modern imager's demands Unfortunately these multiple demands have further escalated the prices of most equatorial mountings. 

The amateur cannot easily copy the opposed bearing, hollow shaft precision of an AP1200. Not without castings and a decent lathe and the finely honed skills to use it. So we must use mass and stiffness to our own advantage. The result will not be lightweight but it will stand up to the requirements of stability in a breeze. While simultaneously carrying an 8' long telescope with the really heavy lumps stuck on each end. The huge moment arm must be controlled and damped without introducing any friction. Otherwise the thing can't be driven or pointed with a high degree of accuracy. 

Reading back through the old ATM books, in 3 volumes, "lightweight" is hardly ever mentioned. Solid brass tubes for refractors are certainly mentioned as highly desirable. That was in an era when making the mounting was also the norm. Heavy steel telescope tubes, water pipe fittings, massive wooden beams and concrete castings were what was used back then. That was before light pollution forced many city based amateurs to carry their instrument off to darker sites. Thus began the demand for light weight in everything except up-market focusers. Where [very strangely] a five pound, oversized lump of CNC'd metal is considered perfectly acceptable and even greatly admired.

I quite like the idea of 200mm [8"] PVC pipe to offer long term protection for [say] solid birch plywood packing. Simple PVC caps could conceal the flanges and bearings leaving only the stainless steel shafts exposed. PVC pipe and fittings are readily available almost everywhere. 

Round 8" PVC pipe would certainly match the sheer scale of the mounting and refractor. In fact it would perfectly match the diameter of the steel main tube. Above all, the finished project should look sleek and "professional" without requiring hundred of hours of "titivating." Hopefully the finished result would achieve a neat, weatherproof finish of which one can be reasonably proud without needing a professional spray job. Just because something is affordable and requires low skill levels does not mean it should be tatty or look like a pile of junk. Or worse, like a modern art sculpture thrown together by a welding 'amateur' reinventing the wheel. 

Square PVC pipe is much harder to find despite its apparent desirability to appear in keeping with modern trends in high-end equatorial mountings. Non-availability makes it a non-starter. Though roof flashing aluminum could be bent around a square plywood form if that is the desired format of the builder.

The would-be builder should note the wide availability of dirt cheap, high quality flange bearings in many different bore sizes.[Metric and Imperial] Thick wall pipe could be adapted to the axis shafts if that is what is available to you. Though thin wall tube might be too risky. Better to fill thin but smart, stainless steel tube with cheap "iron" water pipe to increase the stiffness if in any doubt at all. 

Finding suitable shafts might be the best option before choosing your flange bearings. I was incredibly lucky and found a local engineering firm willing to sell me stainless steel shaft at cost. Nor is stainless steel absolutely necessary except for its superb longevity under the usual damp conditions where amateur astronomy is concerned. I fought with rusty steel shafts for years before finding stainless steel shafting in my own neighborhood. It is readily available on eBay too. Albeit at a very high asking price plus freight.

Rusty, ordinary steel shafts not only stick fast in your precious bearings but will make counterweights and locking bushes very difficult indeed to remove or adjust.  Gather your courage and ask your local engineering companies and metal stockists to see is they will help with stainless steel shafting [or thick pipe] for your telescope mounting project. You have nothing to lose by turning up at their door and asking politely.  Check your local small ads websites for oddments of SS shaft or even the bearings. Though these tend to be far more expensive than buying new! Businesses do go bust and could provide your shaft materials if you ask at just the right time. 

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2" shaft mounting Pt4 : The vital PA-Declination socket.

I was unable to buy QD self-tightening bushes from the bearing stockists. So will have to look elsewhere for those. Ever open to a challenge, the absence of ready-made bushes might just lead to a better way to join the two axes. I'm trying to imagine a way to bring all four studs into play to spread the loads more evenly. While simultaneously enjoying a high degree of easy disassembly without flexure. Some sort of solid block encompassing the entire declination assembly would be best. A 6" cube, with four, through holes for the studs and one for the declination axis, should work. The block will be clamped by nuts on the existing studs which could leave the skeleton design open to view. Whether it actually wants to be open is quite another matter.

Once an ideal position has been found for the declination axis junction the individual rods could be covered in metal tube or pipe.

Am I throwing out the potential for stressed skin reinforcement of the axes assemblies by not covering them? I really don't think so. This would require the assemblies are actually able to bend under the likely loads applied by the telescope. Which are mostly static and minuscule compared with the capacity of such massive shafts, studs and bearings.

The weakest point is always going to be the "socket" where the polar axis fits the declination axis. Each set of four studs, bearing flanges and the shafts are arguably stiff enough as a unit provided the two axes can be joined in a way which denies any local flexure. Meanwhile, the 2" declination shaft blocks the PA shaft from passing right through the Declination assembly. So I need to use every millimeter of possible socket depth to good effect without direct contact occurring between the two shafts.

Where to find a [minimum] solid 6" metal cube? This is going deep into Unobtainium territory! It would also be far too large to swing eccentrically in my lathe for boring the very deep through holes square to the cube. Any collection of parts, to build the desired cube, would need to ensure the socket itself was made of hard and inflexible metal. This area must be a really solid part of the entire built-up unit. It must also be slotted to allow clamping around the PA shaft to permit easy, later disassembly and reassembly.

Birch plywood could easily be laminated into a suitable cube but [I think] it falls down on the ideal stiffness around the PA socket. Drilling the individual laminations for the rods and declination shaft would be easy enough using the pillar drill and a simple jig for accuracy. Solid, laminated plywood is easily stiff enough for the cube when further loaded by compression from clamping nuts [or load bearing plates] on the declination studs. The block would also further stiffen the entire declination assembly against bending or the threaded rods rotating around the shaft.

I keep coming back to the PA socket problem. There is no point [at all] in building a massive mounting like this if the socket flexes even slightly under local loading.

Perhaps a much large diameter, thick wall, metal tube could be fitted into the plywood block? A solid brass socket of greater outside dimensions would work. It would spread the loads into the plywood cube thanks to the increased surface area. Pressure per unit area could drop compared to a plain 2" hole bored straight into the plywood. The plywood cube could be turned on the lathe face plate to ensure squareness and a really accurate fit of the brass PA socket. The brass socket ought to be split on one side to allow a clamping effect via a bolt. The brass would not corrode onto the stainless steel shaft as it might well over time with a steel, stainless steel or an aluminium socket bush.

Extending the plywood block downwards towards the top PA bearing would allow a greater depth of socket. The measurement from the edge of the bearing flange to the shaft is only 45mm or  1.75". So the plywood cube wants to be bigger overall to extended the socket's depth. The block must be made at least made larger than the bearing flanges alone. This will help to move the compression loads from the thinner edges of the bearing flanges away from the more vulnerable and compressible edges of the plywood block. 

Area of the socket wall in the plywood assuming 2" axle depth. = 2" x Pi x 2 = 6.3 x 2 for the bare shaft = ~13"^2.

The maximum possible bush diameter is only about 90mm to clear the studs. 90:50 = A ratio of 1.8:1.

Not a dramatic increase but probably worthwhile and providing a far more robust socket for very little extra effort. Let us assume a socket 3" deep: 6.3 x 3" = ~19"^2.  90mm = 3.5" x Pi x 3 = 33"^2.

The studs must obviously not protrude into the volume required for the bush itself. [See images for actual dimensions.] If the socket was turned from a larger diameter stump of brass then a flange could be provided in situ to help spread the end loading into the plywood block. This would avoid the bush ever breaking through to rub on the declination shaft over time. If the original bush material were of suitable diameter then a much larger flange could be incorporated into the design.

I do have some 7" aluminium alloy bar which could be used for this very purpose. Though I'd prefer not to be buried under a huge pile of swarf by turning a "top hat" shape out of it. A 7" disk could be set on edge and bored to match the 2" PA shaft on a radius. This needs considerable further thought.  Perhaps two plywood blocks could sandwich a thick disk of aluminium set on edge to provide the hard socket material? Perhaps a simple brass socket bush is best with only a modestly wide flange. Again that removes the option to use a flange [plate] as a bearing surface to reduce flexure at the PA-Dec junction. Perhaps with PTFE/Teflon as a low friction interface material. Too many options for a hasty decision.  

I have been back to buy another length of polar axis shaft but chose 60cm, 24" long this time instead of the previous 40m/16". The new image shows 15" between flanges with plenty of overhanging shaft both ends. It occurred to me that I needed more space for the thickness of the large wormwheel flange as well as greater spacing between the bearings.

There is also the problem of the RA wheel's large diameter causing collisions with the telescope. Placing the wormwheel at the bottom end of the PA shaft makes good sense but it will be more vulnerable to attack when the telescope is moved about the sky.  Placing the wheel between the PA and the declination junction looks neater and provides more clearance for the telescope tube. I doubt that torsion effects in a short, 2" shaft really matter. So that does not enter the wormwheel position choice. I would like to place the worms on the side of the bearings so slow motion extension shafts can reach the eyepiece. The worm housing will add extra width which must not contact the telescope itself.

Both axes assembled for a quick photo between thundery showers. Shaft lengths are 24 and 32" with the bearing assemblies so far weighing 37 and 45lbs respectively. The finished distance between the bearing flanges will depend how much shaft overhang is required by the wormwheels and PA-Dec and Dec-saddle sockets.

Making the socket bush much larger in diameter might weaken the plywood block by reducing the containing wall thickness locally. The existing studs could be used to compress a slit in the block to coincide with a slit in the brass bush. This would avoid having to add a separate compression cross bolt and kills two birds with one stone. The orientation of the plywood laminations needs some careful thought. A vertical, multi-layer sandwich makes most sense to allow each lamination to be separately drilled for the studs and declination axle. No other orientation allows easy drilling over such a depth. The downside of this is the high visibility of the laminations. Though a thin, aluminium covering could be used if the laminations are not "pretty" enough.

Heavy, load-spreading roofing washers [or thick aluminium plates] are assumed to butt against the plywood block while the nuts on the studs are used for block compression. Otherwise the plywood block will probably compress over time under the considerable, local pressure applied by the nuts. Which can easily amount to several tons per square inch. A slit in the block should also compress the plywood tightly around the metal bush to achieve greatest overall stiffness. The brass socket would then become a solid part of the block and thence the PA shaft. While the socket flange would protect the plywood when placing the heavy declination axis assembly over the exposed polar axis during assembly. Though even this could be avoided by assembling the various declination parts together once the socket and block are safely in place on the polar shaft. The declination assembly must not be allowed to rotate on the metal socket or there will be no RA drive!

Having discussed the socket to death: The problem with a turned socket is that it is not a tool-free option for most telescope mounting builders. I am going to have to find a serious alternative which is readily available in different sizes from the everyday engineering world. The problem is the huge range of prices for clamping bushes. Something as simple as an exhaust pipe clamp won't provide a guaranteed square [perpendicular] mating surface. I think that is going to have to come from a flanged bush.

This is a Tollock locking bush from RS Components. I searched their UK website to find these. Then used the Tollock name and serial number to search the Danish website. £20 [equivalent] +VAT + P&P isn't the end of the world. Downside is the need for screws in the face which ideally which wants to attach firmly to the Declination housing. Presumably the screws  are not captive so could be replaced with longer ones to capture a larger adapter plate of some kind. The plate is then fixed to the Dec housing or to a very large dovetail strip. In the 50mm bore size the outside diameter is only 90mm.  I see that as a potential Achilles heel and would prefer a larger "flange." I'll keep looking for a larger version if one is available.

This is crackers! After searching for other bushes I returned to the original Tollock page on the RS website and the price has just doubled! The same has happened to the RS UK price when I go back to their English website. What is going on?

I thought of standard pipe flanges but that would require welding or thread turning and then facing on a lathe for accuracy. Shame that QD bushes aren't readily available as they are elsewhere. [US & Aus.] Does the American market have a monopoly of wholesalers who won't sell to the general public and hide their prices from non-registered customers? I can understand their reluctance to waste time on individual sales in small numbers requiring expert support. I just wish I knew which of the multiple names for clamping bushes [in Danish] would find what I am looking for. There seems to be no standardization between suppliers!

Click on any image for an enlargement.

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50mm shaft, flange bearing mounting. Pt 3. Rapid progress.

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After scouring the small ads and ignoring sales of 6 meter lengths of stainless 50mm shaft a local engineering company kindly sold me enough [82cm, 32"]  for my intended declination shaft. I'm hoping my 40cm, 16" stump of 50mm shaft will be long enough for the polar axis. These shafts are heavy! It took two of us to lift a ~3m length of bar comfortably onto the bandsaw rollers.

Now comes the hard part. I have to decide if pillar blocks or pillow blocks are the way ahead. Pillar blocks will require a stiff and flat support plate to carry the loads directly down into the pier. Nope.

The images show my final choice. Self-aligning and rubber sealed  flange bearings secured by heavy studs to the ends of a [virtual] box. The inner nuts are unnecessary but avoid the components sliding around. The shaft turns with silky smoothness and almost no friction at all. Worries about a tight fit were groundless. Provided the shaft was square to the bearing it slid through without shake.

Flange bearings [pillow blocks] can be clamped to the ends of an almost solid [plywood] box via the threaded studs. The box avoids flexure modes in a potentially undersized flat plate. The PA box can be pivoted to allow adjustment for polar altitude. A turnbuckle or screw jack can make fine adjustments easy. One slight worry is indentation of the plywood at the polar axis junction with the declination box. Adding an alloy sheet in the 'sandwich' won't help unless that sheet is considerably larger than the flange to spread the loads.

I am quite tempted to see if another local engineering company has any square steel tube large enough to fit over the studs but resist the pressure from the flanges. The ends of the tube would have to be carefully squared to avoid twisting the bearings via their flanges. Though the bearings are self-aligning it still pays to have them square to each other..

I have ordered the wormwheels from Beacon Hill and should have them within a month. I decided to go with an 11" for the RA and 8" for the declination. Though Barrie tells me these are nominal sizes and it sound as if they will end up 3/4" larger in both cases. Which is all the better provided bits of the telescope and mounting don't strike the RA wheel. This may need a slightly larger overhang beyond the bearing nearest the saddle to allow for when the telescope is vertical on either side of the pier. This is where closest approach to the PA wheel is most likely.

Beacon Hill have settled on 287 teeth for their wormwheels for a 1/5th RPM motor. These will need new motors so I can't just swap the old ones over from the MkIV which uses 359T on both their 7" wormwheels. The slightly longer, thread pitch of the BH worms will make the aluminium teeth on the wheels thicker and more robust. I'll be glad to get away from the constantly rusting MkIV worms! No oil or grease has ever proven to stop the rust with the mounting living outdoors but protected against the weather. The worm housings and their adjustment have been a regular problem.

Now I need a price for a large diameter, taper locking bush to give me a really solid and accurate connection between the RA and Declination shafts.

The bearings were incredibly cheap at under £10 equivalent. The galvanized threaded rods and nuts were more expensive! I was able to obtain the 50mm stainless steel shafts for half the price of the studs. Online sales prices for 50mm SS were 7 times higher per meter!   

My idea for alloy channel stiffening didn't work out. The channel was too narrow to go over the studs and too wide to go between them.

The scale and sheer weight of the present assembly is quite unbelievable! 16kg as shown.  The long studs will be halved to build the declination assembly. The present overall length to the fronts of the flanges is about 11" for the Polar Axis.  Though I may go back to my source for a longer length of shaft now I know it is affordable. The flanges are 143mm square or about 5 3/4". The shafts are 50mm in diameter or about 2". The threaded rods [studs] are M16 or about 5/8" in diameter. The massive shaft itself resists flexure while the studs add their own stiffness at a distance which rises as the square of the radius from the center of the shaft. I shall really have to find a suitably stiff pier to maximize the upgrade to 2" [50mm shafts].

Click on any image for an enlargement.

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11th JUne 2016 More imaging.

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It was a warm Saturday and late afternoon when I decided to try my luck at imaging again. I set up the 7" and 90mm refractors on the MkIV and MkIII mountings respectively. The Moon was at about 40 degrees altitude to the south but almost lost in the bright blue sky with high cloud. The Sun was sinking, boiling furiously  and blindingly bright but displayed several spots or features.  A large, dark spot near the limb looked worth trying to capture with the Neximage5 camera. 

Both mountings are being difficult with serious problems with both motor drives. I had to keep adjusting the slow motions on the MkIII to bring the spot central in the camera's narrow field of view. Despite the wild thermal gyrations the large spot looked pleasingly sharp on my computer monitor. The Solar image shows a hurried tour through Registax and PhotoFiltre with slight cropping. This was recorded with the 90mm f/11Vixen M90. I would have been delighted to have captured such a view with my usual afocal snaps but am left slightly disappointed with my results via stacking. The software needs considerably more practice to optimize the results. Unfortunately and I am really not getting remotely enough practice to become familiar with it.

The very light nights at this time of year at 55N make nighttime astronomy almost completely pointless. Only at 2am is the sky quite dark.  Even at 11pm, the sky is still quite blue as Mars and Jupiter struggle for attention.

This Moon image is quite unbelievable considering the very low contrast against the sky and thermal agitation. The Registax Wavelet process dredged this image out of the ridiculously soft mush right at the bottom of the barrel.

I have not tried using PIPP but did apply PhotoFiltre to gently increase the contrast and crop the edges slightly after Registax. I was careful not to overdo it to avoid that "over-sharpened" look. I keep wondering how good the images would be with dark skies, some really good seeing and a rock steady mounting with a steady drive. Presumably the scale could be considerably increased, centered on a single crater, with much smaller lunar detail easily visible and everything nicely sharp. I am certainly not kidding myself that I have dented the potential of the Neximage5 camera even with my present optics and mounting problems. Nor have I managed to rid myself of the "TV lines" superimposed on the images.

Click on any image for an enlargement. 

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