10" f/8 Curved vane spider: Failed.

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Here's a quick mock-up of my new idea for a single plane, tilting collimation, secondary holder. Thanks to Mr R.F.Royce for the tilting secondary inspiration. I didn't follow his idea of a titled mirror plate. Worrying about losing a secondary mirror on the way from the telescope storage would only put me off observing. So I'm playing safe for the moment and using a tubular mirror holder on a wooden plug, or base.

The folded U-shaped brass strip behind the oak plug is springy. A short length of garden hose is trapped between the heads of the fixing screws between the insides of the brass U-spring. This piece of hose is flexible enough to provide extra stiffness during the slight adjustment needed to align the secondary. It also kills vibration . I will try to find something more rubbery if needed. A couple of cupped rubber furniture feet might do. These could be captured by the heads of the fixing screws.

The cable clip is just something I found amongst my boxes of junk. I shall buy two new clips of roughly the same size for the finished item. This will increase the clamping force and avoid twisting on the curved spider.

The stiff, white, plastic, plumbing pipe (Pex) just happened to fit the cable clip perfectly. I stuffed a couple of offcuts of plastic Rawlplugs inside the white hose to centre the long bolt.  Tightening the wing nut expanded these plugs to form a nice tight, concentric fit on the bolt. The entire assembly can be rotated around the plastic hose in the cable clip. The Pex pipe's diameter has consequences for stability and the amount of friction it provides to avoid unwanted or accidental turning during transport.

A long wood screw, with countersunk socket head, tilts the secondary mirror cell against the springiness of the brass U-spring and hose off-cut. This screw is only temporary. On the finished item I shall probably use a long hex, socket-head screw and captive (T) nut.

The curved spider (bent stainless steel rule)  has only tiny fixing screws holding it to the cage pot at the moment. These will be doubled in size with Ny-loc nuts to ensure long term stability once clamped up tight. I deliberately used the smallest screws I had to avoid making large holes in the aluminium pot/cage which might be in the wrong position if I changed the design in a later iteration. Centring of the secondary can be adjusted by using blocks under the spider feet.

Note how the clip fixing screws have been arranged to fall within the optical shadow of the tubular secondary cell. Anything exposed within the light path will produce diffraction effects. Which must be avoided as much as possible for highest contrast on the moon and planets.

As can be seen in the tubular brass, secondary mirror holder. I have deliberately countersunk the screw holes in the oak former. As they are tightened down, the screws pull their heads into the hollows under the thin brass. The screw heads end up being flush or just below the surface of the brass for diffraction purposes. Any cosmetic "untidiness" will vanish after a couple of coats of flat black paint.

The inside of the aluminium secondary cage (or pot) will be lined with thin, black foam usually intended for collage. The spider and cell will be painted matt black. The secondary mirror support is already vibration free and will be better still with bigger fixing screws. Now I am closer to fitting the  mirrors to see how my creativity has worked out in practice.

Update: I found some 10mm cable clips and some 16mm in the shops but nothing in the middle. So I bought some 10mm clips and will try to find or make a smaller, central boss. I have stumps of brass rod in different sizes but no alloy. Though I may be able to find some alloy tube. I did, but it raised another problem. Or rather the reverse. It moved the secondary mirror off-centre because the cable clips were of much lower profile than the first one. Packing increased the height, returning the mirror support to the centre of the cage.

It would be relatively easy to make another curved spider vane. The low cost of stainless steel rules in discount DIY stores makes experimentation relatively inexpensive. The rules I bought are very free cutting. Allowing easy drilling and filing. The only possible disadvantage is the width of these rules. The slightest twist or sag in the spider means that the apparent thickness increases dramatically. Much is made of narrower vanes but these are also prone to flexure in use. I have some steel pallet strapping which would be ideal in tension used in a "normal" spider. It is hardened and not easy to drill but its very thinness means I would need more vanes for sufficient stiffness in use. Or would have to put them in tension.

A curved spider vane is self supporting. How well it is clamped and whether the bent 'feet' of the vane are properly supported seems to be vital to success. I have changed the tiny fixing screws for some 5mm stainless steel ones and this helped the curve maintain its rigidity. Sandwiching the vane feet with flat alloy plates helped the thinner vane but not enough. Secondary vibration lasted for several long seconds. Hence the change to the thicker steel rule. This is amazingly rigid but is thicker. How much more diffraction it causes I will only discover when I observe with the instrument.

I have removed the single central cable clip and fitted two more nearer the edges of the vane using much larger screws. Aligning the secondary mirror is a bit hit and miss. I keep peering through the blades of the primary mirror cooling fan and not liking what I see. Adjustment by rotation and tilting is proving coarse and unpredictable. I have made a couple of 1" square x 6mm Tufnol spacers to go under the vane feet. This stiffened things up rather nicely. The vane is now rock solid and needs very firm hand pressure to move at all.


I fitted a very long 1.25" focusing tube in an adaptor ring in the 2" focuser. This was to ensure my eye was on-axis with the secondary mirror as I tried to centre the reflection of the primary mirror cell below the focuser. Right now I think the weakness of the design is the folded brass U-spring. It flexes sideways so that it does not return to exactly the same place every time. This will not do! I need to seriously rethink this aspect of the design. I bought two different brass hinges to tilt the secondary but both were complete overkill. Perhaps I should return to a normal push-pull design?

Click on any image for an enlargement.

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