10" f/8 Planetary Newtonian: First light:

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(Images added with captions) 

I finally slid the mirror into its cell on the ends of two thin, wide battens rather like a fork lift truck. The battens were just thick enough to let the mirror slide straight into the lateral restraining clips. I could then tip up the cell and all was secure. Then I brought the pot gently back up to horizontal again to check whether the mirror wanted to take a nose dive. It seemed not.

With the mirror cover finished I felt safe enough to take the OTA outside. There are lots of overhanging trees so I didn't want to risk the mirror being "rained on" on the very first outing. I had already moved the MkIV on its temporary angle iron stand, using the sack truck, to a position where I could safely mount the OTA.

Running backwards and forwards from the focuser to the mirror to adjust the collimating screws on a 2 metre long mounted telescope is rather time consuming and frustrating. I quickly discovered that the secondary had to be moved forwards by over half an inch. I thought I had judged it well by sighting into the focuser but apparently not. Once that detail was fixed I was able to continue collimating the telescope well enough to feel able to pop in an eyepiece.

In this image the OTA has been wheeled effortlessly out to the mounting and lowered over the saddle clamps. Only the top saddle camp has been rotated, but not tightened. This secures the top of OTA against lifting upwards in the next set-up stage. The wheels automatically support the OTA at the correct angle,  parallel with the cradle.

A 35mm Meade 4000 Plossl for 57x seemed a good choice. This is my longest focus commercial eyepiece. As I pointed the OTA roughly at the trees just beyond our boundary I tried to focus. Imagine my shock when the leaves literally snapped into perfect focus! The image was strangely water white without any of the usual colour fringing and soft purple shadows I am used to on my refractors. The edges of the leaves were so sharp I felt could have cut myself. From 50 yards away I was able to examine the finest details on the leaves. Just as if I was holding them in my hand. Not bad for an overcast, cloudy evening and imperfect collimation.

Next I sought out something more distant to look at.  Not that this is easy from our hedge and tree ringed garden. A large copse of trees at a minimum of 460 yards (according to Google Earth) snapped into perfect focus. Focusing was strangely instant. I had imagined that a 2 meter long F:8 would be slow to come to focus. Surely I ought to be racking in and out to find the best focus. But it was anything but. It was either razor sharp or completely out of focus. Not what I'm used to at all.

Despite the rather poor light I was easily able to identify the difference between the leaves of the Ash and Oak trees completely without effort. The image was unbelievably sharp even at that distance and power. Very satisfying indeed! I had pegged a piece of black foam to the inside of the pot opposite the focuser. But had made no other efforts to improve contrast and kill glare. It was much too cloudy to expect any astronomical object to become visible later so I replaced the mirror cover and tidied everything away.

Here the OTA has been slid upwards to its balance position, the lower clamp rotated and both clamps firmly tightened. The wheels simply fall away as the OTA is lifted along the cradle.

I haven't weighed it but I find the OTA much heavier since I fitted the mirror. I popped my axle and sack truck wheels under the mirror cell and this made it effortless to cross the garden. I still haven't found any suitable alloy channel to retain the axle securely so steering was a bit haphazard. While it would be possible to use the new sack truck to move the OTA about the instrument is much longer than the truck handles. Making it rather awkward to handle. The OTA doesn't want to sit still either.


Update: A simple, short, channel section of light alloy, which matches the axle diameter, is pop-riveted to the undersides of the beams. This keeps everything under control and makes moving the OTA so easy you wouldn't believe it. Being a close fit in the channel the axle shows no sign of wandering from side to side and steers perfectly. It also deep enough to resist the axle lifting out when the OTA is pushed and pulled. The telescope will never travel far enough to put excessive loads on the rivets. If they work loose it will be easy to reinforce or repair the fixing.

I set axle height on the beams so the telescope stands upright on the beam plugs and cell with the wheels still in place. This makes it very easy to insert and remove the the axle into the channel with a push or pull of my foot simply by setting the telescope on its tail. No need for lifting nor fiddling in the dark.

A closer view of the Tufnol saddle clamps. It was heavily overcast and already raining again when I set up to take some pictures.

I nearly brought the wrong channel home because my recycled axle is smaller in diameter than all the latest sack trucks in the shops. Luckily I bought both options and the smaller channel fitted the axle perfectly.


First view of the Moon: A total disaster! I'd just finished dinner when I saw the moon, dragging its heels through the neighbour's trees, from an upstairs window. Like a fool I rushed to get the mounting out where I could best catch a glimpse over my own hedge. The MkIV promptly fell over as I struggled to get it aboard the sack truck. The only place I could put the MkIV pier was on a grass bump which made it wobble alarmingly! Then I had to move it back 6' to capture a clear view once I'd sighted along the OTA beam.

I managed a slightly out of focus view of the moon with the 35mm. It would not quite focus inwards far enough to be perfectly crisp. So off indoors to fetch the 26mm. This focussed nicely but the OTA was too just heavy for the counterweights, so badly out of balance. Holding the OTA steady at about 80x while trying to resist its desire to slump was not conducive to a steady view. Then the OTA wanted to slide downwards on the cradle.

A temporary  paving slab lowers the centre of gravity and adds stability without massively increasing the overall weight. Ideally, it needs a bigger slab than the one shown here. Fortunately the base is a standard slab size. Two 15 lb counterweights now produce perfect balance. The MkIV movements have become silky smooth again. The OTA "trolley" ought to be parked out of the way in actual use.

I almost wish I hadn't tried tonight except for the lessons learned:

It badly needs feet on the pier to allow the sack truck plate to slide easily under the base. Or (better) make dedicated wheelbarrow handles with big wheels to move the mounting around more easily.


A screw adjustable "sliding" weight to balance the OTA itself both rapidly and securely with changing accessories at the focusser. Small changes in mass make a big difference in the OTA balance point due to the very long moment arm.

The OTA is actually pointing directly overhead here, though it doesn't look like it in the picture. The cell has four inches of ground clearance and an inch from the pier. I'm glad I built the slotted angle iron pier now to get a feel for the required dimensions without wasting materials.

Fix the location of the OTA's desired position on the MkIV's cradle by means of a pin or block to avoid it sliding downhill once the clamp plates are applied. Fine screw adjustment of the OTA's position on the cradle would be nice but probably unnecessary.

I need to shorten the OTA's optical length by at least 1/2" [12mm] to allow longer eyepieces to focus. The 35mm is probably as low a power as I'm ever likely to need.

Here I have added a pair of runners to lift the pier clear of the ground. Timber battens could be fixed underneath for a little more clearance and greater stability on uneven ground. The sack truck now slides easily under the pier without having to tilt the entire mounting on its pier. Access is only required at either side, west and east of the pier. Rather than north or south, where the axes would only get in the way. A temporary block of wood holds the mounting at a neutral angle for lifting and wheeling about. This also avoids cosmetic damage to the MkIV during transit. The polar elevation turn-buckle has been fitted and the mounting secured temporarily to the pier with large roofing washers. I still need to make a solid top plate to support the mounting base. The 40cm (16") concrete paving slab rests on the inner frame members, lowers the centre of gravity and provides stability.

I must add stronger springs to both mirror cells. Adjustment is not nearly positive enough.

Update: The Moon is much higher tonight but I haven't had time to work on the mounting or the telescope.

I have now bolted lengths of roofing batten to the slotted angle iron (Dexion) runners. This allows more clearance for the sack truck to slide underneath.  Care must be taken when moving the mounting on its pier to avoid the counterweights taking control. Even with the paving slab in place the weights try to tip the pier sideways. Allowing the weights to swing to their lowest position, beside one of the sack truck handles, helps maintain stability when in motion over rough ground. At least I won't break my neck on any pier legs! I was constantly falling over the legs and tall adjusters on the big refractor pier. I even thought about burying the legs flush with the lawn except for the complete lack of a view thanks to the trees and the house.

With the MkIV looking so scruffy and the slotted angle iron even worse it's all looking a bit amateurish and downright fuggly at the moment! Since nobody else will ever see it, even in the dark,  I promise not to tell anybody if you don't. ;-)

Click on any image for an enlargement.

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10" f/8 Mirror and cell

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I have applied some sticky felt pads to the cell backplate to support the primary mirror on a 100mm (4") circle. These felt pads sit on a waxy paper and are usually sold for protecting furniture against scratching by decorative objects, like vases, etc. The pads are firm, extremely sticky and easily stacked to the required height. Though I needed 3 layers the adhesive makes them completely stable. Being quite slippery the felt will allow the mirror to slide freely so that it does not distort on its supports. I shall add some pads to the 3 side supports, as well, to limit lateral movement without pinching. Hopefully eliminating the chance of the mirror tilting forwards without having to add diffraction-raising clips over the surface of the mirror.

This is one of the disadvantages of the new, smaller support circles suggested by "Plop". The mirror can easily tip on such a small support circle. Fortunately(?) I have a much larger circle of felt pads where the collimation (coach) screw heads sit. This circle was deliberately placed on a much larger radius to achieve slow motion collimation adjustments. The soft pads on the screw heads were just extra insurance against the mirror dropping while being installed or removed. Though they did raise the necessity for another layer of support pads further in.

The mirror support "gold standard" for many decades was a 0.7x Radius circle. Or 3 points 120 degrees apart on a circle 7" diameter under a 10" mirror. The new "computer calculated" standard is only 0.4R. Make of it what you will. The pads can always be removed and replaced further out if the they ever change their minds.

Here the cell backplate has been removed to confirm the mirror fits perfectly between the felt tabs on the three lateral restraint brackets. The pictures I took with the mirror in place were fuzzy. Probably the camera focusing on what was in its reflection. It is raining today so I couldn't go outside where there was more light for a greater depth of field.

The doubled up collimation springs and the wing nuts are also shown. The springs fit between the back plate and the (pot) cell, resisting the downward pressure of the mirror. Adjustment of the wing nuts aligns the primary mirror accurately with the secondary mirror in the telescope tube.(i.e. Collimates the Newtonian optics)

One day, if I'm in the mood, I might sink the lateral, mirror support angle brackets below the mirror cell backplate. This would lower the mirror a little if it was ever necessary for improving the balance of the OTA. Which I seriously doubt will ever be necessary.

Now I just have to work out how to get the mirror safely into its cell! There isn't remotely enough room for my fingers between the 10" (254mm)  mirror and the very deep, 27cm (10.6") inner pot diameter! The best method might be to lay the pot on its side and slide the mirror in on a couple of thin battens. If I get it right the mirror will ride in as if on a fork truck. Once the mirror is safely between the edge restraints I can bring the pot upright. All without having touched the mirror surface. The cell pot is held to the main beams by two screws with split spring, locking washers under the nuts for security. The notches for the screw heads are seen on the left of the backplate in this picture.

I have obtained some M4mm threaded rod and some 40mm expanded polystyrene sheet. The first is for the collimation tilting screw on the secondary mirror cell. The foam is for a plug which will be fitted to the original lid of the mirror cell pot. This will seal and protect the mirror from falling objects, dust and damp when not in use.

I shall have to do some homework on the latest thinking on first surface mirror protection. Some telescope designs had a soft pad resting directly on the mirror. I think the 18" Fullerscopes at Charterhouse had a direct contact pad.  Usually blotting paper and chamois were involved to remove any moisture. Some people sealed the top and sometimes the bottom  of the closed telescope tube. It is decades since I last had a reflector so ideas must have changed. As have protective coatings, I would imagine. My lid and almost airtight plug might seal in the dew from the last observing session.

Should I cover the plug with absorbent material? Taking the protective lid indoors to get warm while I am observing should help to remove dampness when it is replaced after observing. I could cover the foam plug with real chamois to increase absorption. Can dry chamois absorb moisture from the air? I imagine so. The fan opening in the rear of the cell will allow some air movement to remove remaining moisture. Do I need a cloth dust filter on the fan? Am I being paranoid? Probably. I knew it was a mistake to turn back to the dark side of Newtonians again. Where will it all end? I could even start making mirrors again! Eek!

Well it seams little has changed since I used a plastic, snap-on lid, food tub. I built it right into the 9-point mirror cell to house my F3.8, 8.75" primary mirror back in the 70s. I was quite impressed with my idea at the time. It fitted the mirror perfectly and protected it from almost anything likely to happen to it in the truss skeleton tube. Then there was my use of expanding rubber Rawlbolts to hold the aluminium tubing to the supporting rings Perhaps I should have patented my ideas and become hugely rich? I could have become a household name and moved with the stars! ;-))

Cutting the polystyrene sheet for the lid plug went well. I used a home made beam compass to mark the circle and then cut round with a fretsaw. Nipped outside to blow the dust away, et voilà, a perfectly fitting circle. Now I just need to buy some genuine chamois. I wonder if there's any point in padding it with cotton wool for greater absorbency? Probably not unless I make it a mirror contact pad. Now I have to think about a suitable glue. I have some water based contact adhesive...

Or should I use a clamping system so the chamois can be removed and rinsed if it gets dirty? It might get accidentally dropped or placed face down on a dirty surface, or even the ground. Large washers would stop screws from pulling through. Countersink the screws and washers to avoid contact with the chamois if they should ever rust over time. Clamping is best I think. Trim the excess chamois and trap it between polystyrene and the aluminium lid. The chamois will increase the friction of the plug in the pot. Reducing the risk of it falling out while the telescope is being moved about. I may need to reduce the polystyrene plug slightly to get a nice fit in the pot.

Self tapping screws would be neater on top of the lid but wouldn't hold in the polystyrene. I could use a circle of thin plywood between the lid and the polystyrene. The chamois would still be trapped as before but between the plywood and the lid. The screws would bite in the plywood. The polystyrene would be glued to the plywood.

I know I'm rambling here but thinking it through like this saves making mistakes with time lost and materials wasted. Think twice- cut once.  Until I finish the cover I'm not keen to put the primary mirror in its cell. It just feels too vulnerable with the lightweight lid only located by a loose, pressed taper. You know what they say: Location-location-location.

Update: The only real chamois I could find cost £18(equiv) in a builders merchants. I'll keep looking!

Chamois for £10 equiv big enough for the job has been well rinsed and hung in the shade after washing in pure soap flakes.

Plugging on: T-nuts and screws to hold the polystyrene to the lid. Over long screws to pull the nuts into the foam. The extra freedom will allow me to tuck the chamois under the lid. Then I shall swap to shorter screws to clamp the chamois in place between lid and foam. Well, that's the theory. If the clamping pressure is not enough I'll find some thin  ply for the T-nuts to bite into.

The cover seems to work well. Snug, not too much friction but stays firmly in place. I used a large spatula to push the chamois into the gap between the lid and the polystyrene. Then re-tightened the screws. Provided I don't snatch the lid off, perhaps creating some suction the lid comes off easily by hand. I have made no special effort to reduce folds and creases. I'll do it properly when I feel the need.

Click on any image for an enlargement.

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10" f/8 Beast Transport?

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More thinking aloud:

Since I can no longer lift heavy weights I have been considering how the OTA and mounting can be moved about to observe. The trees in and around the garden make it impossible to enjoy the whole sky for observation. Having the house on the southern boundary only adds to the obscuration.

Normally one needs a bit of altitude to get the best seeing high overhead. Though it is still very useful to be able to try observing an object lower down in the sky. Our present situation does not remotely offer that freedom.

I looked at the OTA and decided that, despite its relatively light weight, it would still be large, awkward and weighty for me to carry very far. I thought about adding wheels but did not want to go cutting holes for axles. Nor screwing plates to the beams to hold axles.  Their weight would contribute directly to the OTA's own weight and its balance point. Not to mention the cosmetic absurdity of having large wheels permanently attached to the bottom of the OTA! Nor did I want to be grovelling on the ground to remove wheels in the dark to begin observing.

Then I noticed there is a reasonable gap between the lower mirror cell (pot) and the beams. All I need is a piece of aluminium channel between the pot and the main beams. I can place the channel over a simple bar axle fitted with pneumatic tyres. I have just the thing already. Which I use routinely to move long planks. The axle and wheels fell off an inexpensive sack truck which rusted away quite rapidly out of doors.

The OTA can then be wheeled to wherever the mounting is placed at the time. The top secondary cage is so light that it would not be difficult to move the OTA as far as desired. There would be no need to have any sort of fixing for the axle. It just needs to rest in the downward pointing channel below the mirror cell. The pneumatic tyres would absorb the worst of any bumps as I trundle about the garden.

The very low pier offers enough stability to allow a sack truck  to easily move the heavy MkIV mounting around. There is mostly lawn or gravel so there are no serious hurdles. It would be best if there were short, solid legs on the pier. To allow the blade of the sack truck to slide fully under the pier.

Lifting the heavy mounting from vertical, back onto the sack truck, can then be completely avoided by temporarily lashing the mounting to the sack truck. A simple loop of rope, or a strap, fixed around a high point on the mounting and the truck's reinforcing bars would serve.

It would also be useful to have a strap to hold the counterweight firmly to the pier for transport. The mounting on its pier would then lift as one without anything swinging about. Nor the mounting needing to be bodily lifted and then leaned over against the sack truck handles before I can finally pull back on the handles. The tall truck handles would provide all the necessary leverage to easily get the mounting off the ground and comfortably balanced.  

Anyone with sack truck experience will know how difficult it is to get a heavy object off the floor when it has to be lifted bodily onto the truck's floor plate. The secret is to get the bottom plate (or bars) well under the object. The closer the object can be brought to the upright bars of the sack truck the easier it is to lift.

The danger is when the object refuses to lift and flops forwards, away from the sack truck. A simple securing strap will ensure this doesn't happen. The great length of the handles, compared to the much shorter lever of the floor plate, can then be brought into play. Human strength can then overcome very heavy weights.

Once in balance the pneumatic tyres will roll easily with any reasonable load. Balancing the weight is the trick for easy movement. If the handles are lowered too much it throws the weight directly onto the sack truck handles. Too upright, and the load wants to pull on the handles and tip itself off the front.

I have moved some remarkably heavy objects with sack trucks over the years. The most fun was probably moving my heavy, all steel, lathe cabinet to my last workshop over crazy paving, slate paths with several steps. Working alone teaches you all sorts of tricks and skills to overcome such hurdles. The simple lever, rollers and wheels allow even a puny human to achieve truly remarkable feats.

One just has to match the lever to the loads. Like dangling, high in the air, from the end of a 20' long, steel pipe. Just to rotate a rock the size of a Mini. That was while I was making a car parking space on a steep slope beside the house. It can be done but the lever itself is often too heavy to lift easily. :-)  

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Click on any image for an enlargement.

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