7" f/12 iSTar refractor 1: Toobs

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While working on the OTA of the 10" f/8 I came up with a surprising number of tube options. Some of these could easily be carried over to refractors:

I used a spiral wound ventilation duct on my 5" f/15. The appearance is very much a matter of taste but the tube was surprisingly stiff and fairly light. I thought it was all I could get at the time. I was wrong:

The alternative from the same ventilation materials source is the straight seamed, galvanized, steel duct. I managed to find one today lying outside a woodworking factory and the weight seemed not too bad at all for a 2 meter, 6'6" length of 160mm 6.3" diameter. The thickness is around 0.6-0.7mm millimeter. [0.025" = 1/42"]

Lindab - LRTR straight seamed ducting.

These straight seamed tubes come in a large range of sizes with matching joints just like the spiral duct variety. There should be a tube size to suit many refractors where aluminium tube is expensive or unavailable. It certainly beats cardboard or PVC drainage pipe for stiffness and probably weight too. PVC bends over time, particularly when it is warm and it holds a lot of heat.

The table alongside shows the LRTR ducting dimensions and weights in metric terms and is borrowed from the Lindab online catalogue. Note that this tubing is available in up to 3 meter lengths. That's 12' in old money. It usually has a small flare on each end for joining tubes in series with a simple clamp. The flare can be utilized as a physical retainer or sawn off to taste.

Provided the seam was placed above or below the normal orientation on an altazimuth mounting these ducts make an attractive telescope tube. Being galvanized means it wont rust easily but it will probably need a special primer coat before painting externally. Though a good example, without too much scuffing, looks well enough in the natural silvery-grey, metallic finish. I doubt that priming  matters much inside where a coat of matt black paint is protected from the weather and won't get physically knocked about.

The ends of the spiral tubes do need cutting carefully to achieve a perfectly square end devoid of jagged edges. I turned a plug from solid aluminium alloy for the tailpiece of my 5". The very thin steel does not have enough "meat" to take tapped threads for screws. The spiral ducts are usually fixed to their joints with self-boring, self-tapping screws.

I cycled 50 miles in vain yesterday while desperately trying to find a straight seamed, ventilation duct stockist in the city. Then discovered a local woodworking factory had some spare, but used, dust extractor pipes. So I am now the proud owner of two x 2m [6'6"] lengths of galvanized pipe at very reasonable cost. One tube of a nominal 160mm bore and the other of 200mm internal Ø. Weighing 13lbs-16lbs respectively.

They are certainly much prettier than the more common spiral wound variety. Both ends have a small, neat flare where a specially profiled clamp would be used to join tubes together, in series, to make any desired length. These flares provide extra stiffness at the open ends which is probably well worth having. The longitudinal seam is very nicely formed and really not too much of  an eyesore when you get used to the idea. The seam is low and narrow enough to be easily lost at the hinge or clamping point of many common, telescope tube rings.

The popular aluminium irrigation pipe, often used for refractor ATM in the USA, is probably heavier than this steel duct due its greater thickness. The nominal stiffness ratio is 3:1 in favour of steel over aluminium. These steel ducts are 0.6-0.7millimeters. [~1/40th"] So the steel ought to be about as stiff as the typical 1/8" [3mm] aluminium irrigation pipes. Local availability will dictate price comparisons. I paid pocket change for mine. The quality and finish of the welded aluminium, irrigation pipes available in the USA seems to be a matter of luck. Freight charges within the USA sometimes seem to exceed the cost of the tube and also makes them vulnerable to damage! 

I now have the option of making a 6" f/12 or a 7" f/12 refractor with plenty of room to spare for properly ventilated baffles. The 2m lengths of 12", 30cm tube felt just a little too weighty @ about 20lbs for my 10" f/8 mirror. Offering no real advantage over lighter alternatives. The image above shows the two tubes exactly as obtained. They should clean up nicely with very little effort.

Many refractors have main tubes of about the same diameter as the lens aperture. This makes for very thin light baffles just behind the objective. Which leaves no room for ventilation holes to avoid air flowing down inside the baffle's central apertures. Where the difference in refractive index with air temperature will directly affect the light path. The best arrangement is probably scallops cut out of the periphery of each of the baffles. Allowing cold air to flow down the inner tube wall unhindered and without affecting the optical homogeneity of the light path for which refractors are justly famed.

Interestingly, Dave Trott of folded refractor fame discovered that air currents were best excluded from his folded 6" design. He went to the trouble of sealing all the casing to stop air movement. I presume his focuser was given the same sealing treatment. I discovered the Vixen focusers are anything but airtight. They allow a constant rain of dust inside the OTAs when standing on their noses. 

The 8" tube looks rather "tubby" despite its quite considerable length. Only if it were stretched to an F/15, or even more, would it probably take on the classical refractor look. Though it should be remembered that the "classical" length of dewshield will add its own grandeur to the overall scale. A self-rolling, black, 'memory plastic' dewshield will avoid adding any extra weight at the "wrong" end of the OTA.

Many modern refractors are already far too "nose heavy" without adding a decent length of metal dewshield. Which usually means the tailpiece/focuser end of the tube far too long below the mounting saddle [or dovetail]. Which further exaggerates the problem of eyepiece ground clearance and forces even higher piers or tripods on longer focus instruments.

Despite their short focal lengths many of the "stumpy" 6" f/8 Chinese refractors were ridiculously unbalanced due to the heavy lens, cell and abbreviated dewshield. Making access to the eyepiece almost impossible when the OTA was pointing overhead on a typical Chinese tripod. Adding a ring counterweight, at the focuser end of one of these refractors, just made the OTA even heavier and increased the moment of the entire instrument. [Moment = mass x distance from the fulcrum or pivot.] Which makes equatorial mounting choice very expensive if the "wobbles" are to be completely avoided. The problem is usually one of very low frequency oscillation taking too long to damp down. The wobbles are usually excited during focusing. Ironically, it is the shorter but heavier APOs which most suffer from this because of their very short depth of focus.

125mm, 5" f/15 beside the 8" 20cm x 2m tube for scale.

N.B: The 5" has 12" of dewshield extension in front of the lens.

The poor, old, classical refractor has even more problems due to its much greater length and inevitably higher moment. Often the "leverage" of the OTA will completely overwhelm the most popular and affordable mountings of today. They are simply not suitable for a number of reasons. The scale is usually completely wrong for a long OTA with the really heavy "bits" at the extremes of the very long tube.

Most refractors, in the past, were housed in domes for the protection of the inevitably massive, cast iron and brass mountings. The main tube was often made of riveted steel since weight hardly mattered. They were never designed to be portable. In fact a ponderous movement was much admired as a sign of quality. Traditional domes do have seeing problems as warm air flows out of the open slit. Particularly when the dome is crowded with people on an organized visit. Roll-off roof observatories reduce such problems at the expense of a serious lack of shelter for the observer and dewing of the instrument.

Fortunately the "Berry style" offset and counterbalanced fork provided an easy and inexpensive alternative to the historically massive, equatorial mountings. The Berry mounting can be cheaply made out of timber and plywood but at the expense of it being "only" an altazimuth. Fortunately, motor-driven platforms can now provide "equatorial drives" if required. These were mainly developed for large Dobsonian Newtonians. Though these clever, motor driven platforms have equal application to larger refractors if suitably adapted with stability in mind. Possibly best applied just below the offset fork bearing plate, rather than carrying the entire tripod or pier. Which is likely to cover a considerable area of ground to stabilize larger refractors.

I made a Dobsonian inspired equatorial from 3/4" plywood plates, several decades ago but it suffered from high friction. The tube and declination axis were too overhung for comfort and required a high torque on the central polar axis pivot bolt. A Dobsonian reflector relies on gravity and the entire mass is over the ground board so only a central pivot pin is required.

The tube of my 5" was made from two, glued layers of 1.5mm rolled marine plywood on baffle formers. A PVC push-pull focuser sliding in a 2" black PVC extension was used. Simply because the thin plywood only came in 1.5m x 1.5m sizes and the focus was 75" or 1.875m. 

Since that mounting failure, I have often considered a "German" equatorial using PTFE/Teflon bearings running directly on massively over-sized pipes for axes. Conventional plumber blocks and solid shafts add enormously to the weight of a mounting without necessarily adding much stiffness unless the shafts are made seriously large in diameter.  Which only escalates the weight problem further.

Thin-wall, larger diameter pipes riding against Teflon pads in plywood housings could just as easily replace the heavy, cast, bearing housings and solid shafts. Easily achieved for a polar axis but controlling end float requires more careful thought in a declination axis. An offset fork with captive Dobsonian bearings could easily take care of that problem but would require an offset counterweight. Spring loading the bearing caps would avoid tightness through imperfect roundness of the large diameter PVC declination axis surfaces.

A suitably large spring to go over the declination pipe, to resist end loading, would be hard to find and even harder to make. Smaller springs working against Teflon pads on a part of the declination axle/structure should work. Equal friction/resistance for all pointing angles is the desired goal. Friction will normally change as the OTA rests all its weight above the polar axis 'T' structure compared with a horizontal declination axis. The traditional English Yoke mounting would suit Teflon against plastic pipe bearings to perfection. The north bearing could even become a thick-wall plastic pipe to allow access to the North Pole. Much easier than adding a "horseshoe" with its attendant concentricity requirements. The Yoke spaces the north and south bearing far apart to reduce flexure. A mounting only needs ball or roller bearings when it is made compact for portability.

The Berry offset fork mounting for refractors provides superb balance between friction and freedom of movement. Only around the zenith pointing angles does resistance to OTA movements rise dramatically. This is only due to the loss of leverage when the OTA becomes parallel to the vertical axis. At all other pointing angles my 5" f/15 moved with an equal and delightfully light force in any direction. From distant memory I think the force required was only was only about 1lb. I used a small spring balance to confirm this at the time. In a fit of desperation I raised the fork tines to compensate for the lack of tripod leg length. Beggars can't be choosers when the scrap heap provides such rare treasures. Most builders seem to keep their forks rather shallow but extend them forwards to take the counterweight without the risk of flexure in the fork structure. 

From memory I used a 6" PVC drainage pipe for the altitude bearing surfaces. The three PTFE azimuth pads were probably pinned on a 6" circle supporting a disk of Formica. A lead counterweight, on a 2" alloy extension pipe, was fitted first and removed last prior to every use. This was to avoid the fork tipping under the weight of the 6" PVC main tube. I should have used a captive bolt and wing nut since I usually removed the fork to bring it indoors to protect it from the weather. The metal tripod stayed out of doors.

A 2" prism was permanently fitted inside the tube on a 3 point adjustable cell with a simple brass push-pull focuser with RAS thread. I only had two simple lenses for eyepieces back then. Though dismantling my utterly useless Dixon's Prinz binoculars provided a better eye piece @90x including a spare for the PVC, 10x50 finder using the Prinz objective. 2" plumbing pipe fitted the tapered objective housing perfectly. The scaffolding poles lacked torsional stiffness where they joined the azimuth baseboard despite the heavy alloy angles which I had used to hold it altogether. My suburban garden was heavily lit by a multitude of street lights! I made the achromat myself from BK7 & F2. It all seems a very long time ago now!

Click on any image for an enlargement.

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