a Curved Vane Spider
Diagonal Holder for an 8" Newtonian
For the past few years I have been experimenting with the concept of curved vane spiders as a means of eliminating diffraction spikes. Work in 2007 with the 10" Dall-Kirkhams showed that a design I had developed was not only mechanically rigid but yielded relatively little total obstructed area and, under high magnification, appeared to produce almost textbook Airy diffraction patters. The spider is fundamentally based upon the idea of the single 180 degree semi-circular vane but with the addition of a third curved vane pointing upward. This has the effect of solidifying the structure and allows for both thinner as well as less deep vanes. Less deep vanes have the advantage of not appearing thicker than they actually are due to slight mechanical misalignment or when viewing off-axis rays. A little manipulation and the whole can be turned into a nearly equilateral three vane spider that uses mounting holes spaced 120 degrees apart. The design is similar to some three vane curved types seen with the exception that the vanes are not rotationally symmetrical. Whether this is operationally significant is not as yet fully understood but the design I have developed is very easy to construct and appears to work excellently. It is possible that the lack of complete symmetry may in fact contribute to a further reduction in observable patterned diffraction effects.
Materials and Methods
The spider vanes are made of KS hobby brass that you can get in any hobby shop. For the 8" I used .032" thick by 1/2" wide 12" long brass strips. For 10" telescopes I have used .064" thick X 1/2" wide stock. For Newtonians with small secondaries, a 10" telescope might well work with .032" stock. It largely depends upon the telescope user and how careful he is with the telescope. But, needless to say, the thinner the stock the less diffraction that is produced. Remember, this spider still produces diffraction, it just smears it out evenly so it goes unnoticed. The hub is made out of a 3/8" X 16 pitch thread long brass connecting nut.
As can be seen above, the three vanes are formed by bending one strip to a radius of about 4 7/8" and having it extend from leg 2 to leg 3. I bent my first set by hand, slowing working it to fit a template cut out of cardboard, as shown below.
The second set I rolled at a machine shop on a tinsmith's hand cranked triple roller. This worked out nicer and it's quick (I had to make several sets) but a roller is not necessary, especially if you're just making one spider and you're careful. The vane ends are attached to the tube by making tabs of .064" thick stock and soldering those to the bent over ends of the .032" vane stock. The brass hub is soldered in the middle of the vane. A third vane, added for structural rigidity (seen below), is bent to the same radius and soldered to one of the sides of the six sided nut and continued upward to mounting tab 1.
Begin construction by getting a flat surface of some kind at least 12" square and marking a point in the center. I happen to have an 18" diameter aluminum plate but a thick piece of plywood would do as well. In some respects the plywood might be better due to the heat sinking tendencies of the aluminum during soldering operations. After the center point is marked a compass is used to draw a circle that is approximately 1/16" smaller than the ID of the tube. This smaller size will allow for shimming during centering operations later on. Note that the hub (3/8" hex connecting nut) is screwed to the plate so it will not move and be at exactly at right angles to the plane of the construction base plate. This will insure that the diagonal mounting shaft will be close to the optical axis of the telescope.
Next mark three spots on the circle exactly 120 degrees apart. This is where the mounting holes will be. The lower vane is bent to an approximate radius of 4 7/8" and set op as shown. It bears against the lower flat side of the hub where it will be soldered. Note that the end of the vane has been bent over so it will bear against the mounting tab and create sufficient area for a good solder joint. The tab was cut from the curved .064" strip seen above. This was a curved vane section left over from the 10" D-Ks. In practice, it does not have to be bent since it is such a short section of the circle.
The mounting tabs are made by first drilling a hole to receive a 10-32 bolt. When drilling small pieces of metal use a machinist's vice or hold it in a pair of vice grip pliers. Brass is very prone to sticking and grabbing and will tear out of your hands and maybe cut your fingers.
After drilling the tab is cut to the correct length, about 3/4".
The lower and upper vane and tabs are now soldered as shown above. The square piece of steel was used to hold the vanes in position. Small objects were placed against the tabs to hold them in position for soldering. I used a hard 50/50 solder and the new water soluble flux. I was brought up on the old rosin paste flux but found the new stuff to be even better. Just remember the clean the parts with a fine emery paper and smear on a coating of flux, put the parts together and use gentle heat - just sufficient to melt the solder. Avoid high heat, it over softens the brass.
To avoid having the tabs fall apart while hitting the parts with the solder, you might pre-solder the parts as shown above (known as tinning) and then assemble and heat. The solder will melt on both parts and they will flow together without having to touch them. I pre-solder every surface at every joint and then just place them together and heat. Soldering small parts can be a little tricky and mastering the technique takes a little practice. It's mostly about the judicious use of heat, not too much - just enough, and making certain that the surfaces to be soldered are clean and properly fluxed. Get a torch with a small flame. The ones that Ace Hardware sells with the right angle head is good.
Mounting and Centering
After everything is cleaned off with acetone (you have to get the flux residue off) and before painting, the spider is tested for a good fit. It should be a little bit smaller than the tube, say, 1/16" between each tab and the wall of the tube. If the spider is properly made it will be very close to center when screwed into place, but you should allow some adjustment room for final centering; and remember, you have to put in the flocked paper yet. Start off by drilling the holes. The first fit of the spider can be made with 6 penny nails stuck in from the outside as seen above. This keeps the spider from falling out while you trial insert the bolts and nuts and shimming washers. Do not force the spider into position, neither squeezing it in nor screwing down nuts and pulling the spider out of shape. It should fit easily. Use washers to take up space between the tabs and the walls of the tube and use washers outside to create additional bearing area against the cardboard tube. By the way, make the holes a little oblong longitudinally so the spider can later be adjusted to point exactly at the center of the mirror - the washer will cover the extra hole space. The holes can be drilled accurately to a plane at right angles with the length of the tube by wrapping a piece of heavy paper or aluminum roof flashing around the tube until it lines up with itself and then marking the position off with a pen as seen above.
Before drilling the mounting holes you should study the geometry of the optical system very carefully and know exactly where the holes need to be. Thoughtful time here can pay dividends in a good looking telescope. Carefully calculate the distance between the center of the diagonal (where the light reflects outward to the eyepiece) and the center of the spider mounting holes. (Picture above right, distance HD.) Then you must calculate the length of the cone of light cut off by the diagonal. With the Moonlight standard Newtonian focuser I generally allow about three inches from the outer surface of the 10" tube to the focal plane. This results in a total of 8" from the center of the tube to the focal plane; thus, 8" will be cut off from the cone of light produced by the mirror. Subtracting this amount from the total focal length the distance from the face of the mirror to the diagonal at the central reflection point will be 40" for an f/6, 56" for an f/8 or 32" for an f/5.
The picture above shows a little device made of a piece of index card cut about 6" long and two inches wide. It is then folded over to make it 1" wide and one end is cut off as shown. A pencil mark should be made a point one half of the inside diameter of the tube as measured from the pointed end. The pointed end makes it possible to more accurately place the card at the side of the tube as shown below. Place the pointed end of the card at the right side wall and note where the pencil mark falls on the center of the hub. You can make two or three marks near each other and note which one falls nearest the center. Place it 180 degrees opposite and see where it falls. This will tell you how to move the spider to get it exactly horizontally centered. Repeat the process for vertical centering. Check the horizontal again and so forth until everything is equal all around. The next thing is to adjust the individual vanes longitudinally until the hub hole points directly at the center of the mirror. Do not tighten any of the mounting bolts fully until this is done. At this point the holder may be inserted and the diagonal adjusted and the telescope collimated. Of course, the spider will have to be removed for painting and the installation and adjustment process repeated but at least you'll know that everything will fit and work properly. Make a note of what washers were used where for shimming so you can re-assemble easily. But I'd still re-check centration after re-assembly.
Painting and Finishing
The spider was painted using a flat black spray suitable for metal. I have also used brush on Floquil engine black from the hobby shop. Make a holder from a dowel or pencil or old brush (as seen above). Wrap tape around the end until it's a press fit in the hub hole. Then you can hold it in a vise or on a bench with a weight on the stick until dry.
Finished spider installed.
Detail view showing adjustment knob and central obstruction stop. I have always used a central stop disk. Whether they are significant or not I'm not certain but I feel better thinking that they produce a clean obstruction. This stop was made from a piece of 1/8" thick Plexiglas cut out with a drill mounted hole saw and the edge turned smooth with a file and then burnished smooth with a piece of wood and finally spray painted flat black. I have also used 1/16" sheet brass.