10" F:8 Planetary Newtonian lower pier:

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I have constructed a simple slotted angle iron stand to check out the OTA's balance point, counterweight requirements and ground clearance.

The MkIV wants to tilt towards the north. So I have temporarily used a counterweight on the polar axis and cord to keep things steady.

The 20lbs of counterweights shown seem to balance out the OTA with a 10lb weight in place of the primary mirror. So near enough once a finder and eyepiece are in place.


The OTA insists on sliding downwards along the saddle. So I have added a cord to keep it in place while I play. I think I just need to file the channel (box) section to allow a slight clearance to allow tighter clamping.

At 40cm square, the base of the stand is not really large enough for serious stability. I also need to add a piece of 3/4" plywood on top to support the mounting base and allow the MkIV to be firmly bolted down. A simple way to add further stiffness to the truncated pyramid would be to line it with plywood bolted at intervals to the slotted angle iron. This would also kill self-resonance and vibration without adding a lot of weight.

I will probably add some wheels when I decide how best to go about increasing the base dimensions. I want the mounting and its stand to be as mobile as possible after struggling with my massive refractor pier for years on end. The trees in my garden force a tour down the drive to see much of the night sky. I have a pair of pneumatic wheels ready to make a trolley or wheelbarrow style mover. The refractor pier was so heavy it sank into the ground and was almost impossible to move.

My all-aluminium OTA is a rather strange looking beast but is very light and still seems easily rigid enough. I fitted a larger coach bolt to clamp the secondary cage more firmly to its support rails. It feels rock solid now. A large, 10mm wingnut applies the clamping pressure.

I have already cut down the height of the stand but the ground clearance is still too much even with the OTA vertical. This is the only position where it really matters.The bottom end of the OTA rises steeply away from the ground in all other pointing positions. A lower pier means greater stability and less ladder climbing so it is well worth pursuing.

I could probably lighten the counterweights by sinking the saddle between the two main spars. This would reduce the moment arm of the OTA relative to the polar axis. Clamping the OTA, while maintaining slide-ability for balance, would then need to be looked at afresh. The saddle casting would need to be cut at the ends where it is widest. Otherwise it is over an inch too wide unless I increase the separation of the aluminium spars. Clearance from the stand might then become a problem when the OTA is vertical east or west of the pier.

Balance can always be achieved by adding extra weights but I want to keep the OTA ultra-lightweight for maximum portability. The shorter the distance from the bottom of the OTA to its balance point the lower the pier needs to be.  Ultra-lightweight telescope designs have traditionally suffered from vibration and wind effects. I hope that my design avoids these problems in practice.

Notice how the massive, Fullerscopes MkIV mounting appears to have shrunk when supporting another long OTA.

Now with an even shorter stand the ground clearance is a much more sensible 4" (100mm).  The scrap piece of chipboard is only to support the mounting base temporarily to stop it wobbling. I can comfortably reach the eyepiece with the OTA vertical while I am standing on a standard 11"/ 28cm high, plastic beer crate. At all other elevations and orientations it becomes much easier to reach the eyepiece. Quite surprisingly so. The expectation of always needing a stepladder to observe has safely diminished. 

An update: Only to say that I have been suffering from very sore joints lately. Which are made much worse by heavy lifting. I repeatedly lifted the entire MkIV, including the 20lbs of counterweights and the telescope. I paid the price for my foolishness in pain and hospital appointments to assess the cause. After years of struggling with a solid wheeled sack truck I have now bought a new one to aid future movement of the mounting. I shall return to construction when I find renewed inspiration. 

2nd update: I filed notches for the cell fixing screws in the mirror backplate. The backplate is now free to move backwards and forwards freely over the screws. I added split washers to ensure the screws would not work loose. 

I also removed the split spring washers from beneath the wing nuts on the cell adjustment/collimation screws. The springs provide enough tension to stop the wing nuts from rotating of their own accord. 

I added a very large washer to the upper cell clamping screw and this solved the problem of the secondary cell sliding slowly downwards. I may use a Tufnol clamping plate instead of the present, relatively thin plywood. The plywood tends to take a set when under continuous pressure. Which leaves it bowed and unable to clamp the upper cell firmly against the alloy angles. 

Third update: I tried making a folded U-spring from hammer hardened brass to allow a single plane secondary adjustment. Earlier fiddling with a brass door hinge were inconclusive. I was on the point of turning a new central boss from Tufnol to hold the spider and secondary mount together when the lathe drive belt broke! So, after stripping the headstock and lay-shaft to fit a new V-belt I gave up for the day. The linked V-belt I have been using is undersized. Causing noise without a solid drive. 

I shall have to search out a source of new belts in 13mm 'A' size. The prices for Nu T-link belts in Denmark are absolutely ridiculous! Over £115 + 25% VAT per meter! That must be over £143 per metre! $256US! I hope I can find a plain V-belt which will keep me going. Though this requires dismantling the headstock, back gear and lay-shaft each time the belt wears out or breaks. Better that, than be ripped off! Just found eBay UK has T-link-type V-belts for 1/8th the Danish price! The US price is much less again but not worth the trouble with postage charges, customs, plus 25% VAT on top of everything. 

Click on any image for an enlargement.

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10" F:8 Planetary Newtonian: Further progress.

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After a rather long hiatus, with little real work being done on the OTA, the sight of the moon from my bedroom window inspired me to restart work. The sky seems to have been cloudy for months. Some of the new images below have been posed for the camera while I continue work on the OTA. I have removed the various fixing bolts to work on the individual components. The desirability of an ultra-lightweight OTA has been carefully maintained. 

  

The OTA is now taking more permanent shape. A fan cooling port has been cut in the base of the mirror cell 'pot.' A standard 80mm 12V computer fan will sit inside between the cell and the mirror backplate. To give an idea of scale the main spars are 2m or 6'6" long.

The mirror cell backplate has been made from scrap alloy. Actually the base of the secondary cage 'pot.'  Not having an 80mm, metal cutting, hole saw, I had to chain drill the large ventilation holes. Then file to a scribed line to smooth things out. This is a very noisy and time consuming business depending on pride in workmanship!

I weighed the alloy disk and it exactly matched a similar circle of 3/4"/18mm multiply, Birch plywood.

The mirror back plate was made a deliberately close, but easy sliding fit in the cell pot. The idea is to make the fan push the air around the mirror blank itself. Not out and around a leaky back plate. Which would rather defeat the exercise of stripping away the thermal boundary layer clinging to the mirror. Any leakage around the mirror back plate would just take air away from the main flow. Possibly leading to a reduced draught and stagnant air hotspots. The tightly fitting backplate will also ensure that the mirror does not shift bodily off-axis with changing telescope orientations.

I cut the aluminium circle roughly to shape with an electric jigsaw. Then allowed the disk to spin it on a central nail in a wooden board, while an angle grinder, with a coarse sanding disk, smoothed the edge to size. Industrial leather gloves and ear defenders were essential. It made quite a pleasant change from listening to the neighbour's endless chainsawing. ;-)

Three galvanised, angle brackets have been fixed to the backplate to retain the mirror laterally. Short lengths of bicycle inner tube have been stretched over the raised legs of these brackets to gently restrain the mirror from flopping forwards at very low altitudes. It is now considered very undesirable to have clips hanging over the front aluminised mirror surface. Causing a massive contribution to overall diffraction. However, the mirror aluminising shows three tab shadows so I am already doomed to have diffraction!

Having the side supporting "rubber bands" near the front surface of the mirror blank will avoid any tendency to become "top heavy." The slight flexibility of the side supports will avoid mirror damage as the OTA is carried in and out of storage in the dark. Though a well shielded outside light may be in my stars.

The original pot lid will be padded with a disk of polystyrene and placed over the mirror cell for safe movement and storage. This will avoid having to reload the main mirror each time I want to observe. Hopefully avoiding the attendant risks to the mirror in carrying it out to the telescope in the dark, over various unseen hurdles, closing doors with elbows, etc.. The mirror will also enjoy an unheated, secure environment prior to every use. While most telescope mirrors must be cooled down from an indoor, centrally heated situation, mine will always be ready to use.

The secondary cage 'pot' now rests on-axis on top of two pieces of aluminium angle. These rails are pop-riveted to the tops of the main spars and have the same semi-matt, silver finish as the main spars.

The previously rather flexible, curved spider vane has been upgraded to a thicker stainless steel rule. The secondary mirror support is now perfectly solid. There proved to be no need to add the intended second curved vane for stiffness.


Obtaining cage rotation is as simple as elongating the single fixing hole into a long slot around the circumference of the pot. However, this assumes the secondary mirror is aligned truly on axis so that it rotates around the optical axis when the whole secondary cage is rotated. Otherwise the collimation will go awry with any secondary cage rotation. I will leave the cage fixed for the moment to judge the desirability of rotation. A simple 150mm x 40mm x 3mm strip of  aluminium clamps the secondary cage to the rails without localised pressure or cage distortion.

A view of the mirror cell innards. Three restraining brackets locate the mirror laterally. Coach bolts ensure the collimating screws do not rotate. The 12V computer cooling fan will blow air onto the back of the mirror. The 3-point mirror supports have yet to be fitted.

The art of designing 3-point, mirror support geometry has changed dramatically since I last built a mirror cell! 'Plop' and numerous websites, now specify 0.4 x the mirror Radius. How we ATMs survived, with the gold standard of 0.7R, I have absolutely no idea. 0.4 x 10/2 makes for a tiny 4"/100mm circle. This is not much larger than the 80mm ventilation holes in the backplate and cell.

The mirror cell from the rear/ outside. The three wing nuts are for adjusting primary mirror collimation. They have been deliberately placed on a large circle to ensure fine adjustment.  I have used split washers to maintain settings. Though the springs probably provide adequate locking on their own. The coach bolts have doubled springs trapped between the cell and backplate to ensure positive movement for even the smallest adjustments. Any slop in the mirror backplate adjustment can make collimation difficult. e cell and backplate! I just hope they are right!!

The almost completed OTA now weighs 7.3kg = 16lbs without the optics. These will add about an extra 9lbs. Making the final OTA somewhere around 25lbs in total. Which should be manageable.

I plan to split the secondary mirror supporting block to turn it into a clamped see-saw. This will allow two, through, clamping screws to obtain secondary alignment. There being no need to adjust the secondary in all planes. Which merely confuses the issue and makes collimation more difficult. Royce suggested this clever idea on his website.

I still need to address the construction of a shorter and more portable pier for the MkIV Fullerscopes mounting. The original is over 6' high for use with my refractors and is of truly massive weight and  proportions. Since the MkIV mounting rests on a heavy steel flange, which itself is welded to the pier pipe, I see no point in cutting the 7" tube any shorter.

So it looks like I'll have to make a temporary pier of plywood and heavy timber once I have the final balance point of the OTA. In theory I could now put the optics in for a trial but will wait until I have a decent support for the mounting. Such a long focus OTA does not really/probably lend itself to a Dobsonian mounting.

My wife has been having gentle fun at my expense with plays on words involving my main telescope building materials:

Pottering about, Panorama, Pan-European telescope, built from Pandora's pantastic box of parts, ATM Pandemonium, Panning the night skies, Panic observations, maximum planetary potential.

I think I shall call my new telescope "Panerphernalia."  :-)  

Click on any image for an enlargement.  

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