a Curved Vane Spider
Diagonal Holder for an 8" Newtonian
The diagonal holders that I have seen generally are made much like Cassegrain secondary holders, there are three collimating screws much as with the primary mirror. This has never altogether made sense to me inasmuch as the Newtonian diagonal is fundamentally different by virtue of its 45 degree angular reflection. In point of fact, Newtonian diagonal holders only need two adjustment; rotational about the central axis and tip-tilt around the deflecting axis. This arrangement makes adjusting the diagonal easier in that one needs only to rotate the diagonal until the mirror is centered from side to side and then adjust the single tip-tilt screw until the mirror is centered longitudinally. With the threaded support shaft you can also rotate of diagonal over several times until the correct longitudinal position is achieved. Very simple. One should take advantage of the peculiarity of the Newtonian diagonal instead of ignoring it or fighting against it, and that advantage is that you can adjust in one direction and then in the opposite 90 degree direction without affecting the other adjustment you just made - not an insignificant matter. Also, getting one's hands and fingers and a screwdriver or Allen wrench into the restricted area of the tube (behind the spider!) and dealing with three screws is also an inconvenience. With the arrangement I have here one need only twist the diagonal shaft to the correct position and then turn a convenient Allen head bolt to finish the job, and you can do these things easily in the dark. I have been making and using these holders since 1991.
Above is a schematic drawing of the holder showing all its component parts.
Materials and Methods
The dimensions given and materials selected are for a holder for a 1.52" diagonal. The exact design shown here will work for diagonals up to about 2.14" minor axis. After that, you will need wider and heavier brass stock.
The picture above to the left shows a unpainted finished holder. The forward mounting plate and rear plate are made of 1" wide x 1/16" thick KS brass you can buy in a hobby shop. There is a 1" wide brass hinge at the bottom upon which the 45 degree diagonal mounting plate tips and is drawn together by the Allen head machine bolt at the top. The small 1" wide hinge was bought at a hardware store. Use acetone to remove lacquer from the hinge and get down to bare, clean brass. The 10-32 thread nut A is soldered to the mounting plate. The 10-32 nut B has its threads drilled out somewhat oversize to accept the Allen head machine bolt. This bolt draws the two plates together as needed to effect adjustment as required. Just below the Allen bolt head is a 3/8"-16 nut C. This nut threads into the 3/8 mounting shaft C. The only thing missing is a piece of buna rubber inserted between the adjustable plate rear plate just a little above the hinge. The rubber acts as a compression spring. The picture above right shows a finished and painted holder open to show the red rubber compression spring. A diagonal ready to be mounted sits above the holder. The holder is largely soldered together but to avoid difficulties and frustration with soldering multiple parts in a small area cyanoacrylate glue (super glue) is used. This glue is very useful if its adhesive properties are understood and one does not place excessive stress on the joint. This is the reason soldering used in certain high stress cases.
Construction begins with drilling out a brass 10-32 nut with a 15/64 or .234" drill. In order to give sufficient clearance for the 10-32 Allen bolt, which will work at different angles, the drill should be worked manually at the angles the bolt will encounter and thus elongate the hole slightly in a vertical direction. The picture above middle shows the back plate, drilled out nut, the 3/8"-16 nut that supports the 3/8 mounting shaft. Note that the 10-32 nut has been drilled oversize and is somewhat oblong vertically. In the case above, the assembly has been entirely soldered. It would be a bit less difficult to solder just the 3/8 nut and glue on the hinge. I have done this and it works well. The picture at the above right shows a plate and the threaded 10-32 nut soldered on a piece of hard oak wood. Water has just been squirted on the part to cool it. You should not use any more heat than is necessary, and it doesn't take much. All parts should be fluxed and assembled prior to soldering.
The combination mounting and forward face plate is made by cutting a groove with a band saw or hack saw or even filed with a three sided file. After making the groove the piece is folded over in a vise as shown. Using a 45 degree angle template carefully bend the piece until the angle between the pieces is a little less than 45 degrees. This will allow room for adjustment when collimating.
The picture above left shows the back plate assembly and the forward mounting plate assembly ready for final assembly. Above right is seen the forward amounting assembly with the support strut installed. This is held in place with cyanoacrylate glue.
Above left shows the parts assembled for gluing. The while material between the hinge plates is three pieces of index card soaked in wax and used to separate the hinge so as to keep them parallel while the glue sets. The wax keeps thing from sticking. Put a very little oil in the hinge to keep the glue out. Put the bolt in to insure that everything is properly lined up. Use very little glue. Basically, set everything up and let the glue seep into the space between the hinge and the forward plate. The glue is very thin and finds its way in immediately. Do not assume that cyanoacrylates set instantly. This only happens when it touches your damp skin. Otherwise it can take some time to get the stuff to hardened, perhaps 10 minutes or more.
Finishing and Painting
Painting is best done with Floquil engine black brushed on. Make certain that the holder is well cleaned with acetone to remove any lingering solder flux or oil.
Mounting the Diagonal
The diagonal is held on to the mounting plate with ordinary bathtub sealing RTV. Using RTV to mount optics is a bit tricky. In some cases it works well and is very convenient and in other cases it can be disastrous. Mostly, mounting large, thin mirrors has not worked well, though people still continue to do it. Large thin mirrors are extremely liable to flexure in any case and gluing them to a surface that has its own set thermal reactions is never going to be successful. I've even seen large mirrors mounted on wooden tip-tilt plates - amazing. In the present case the mirror is small, thick for its diameter and only a small area in the center of the mirror is used. You don't smear the whole surface of the mounting plate and the back of the diagonal.
The first thing to do is find the place where the secondary mirror is centered on the mounting plate. This is accomplished in several ways. One is to put a small piece of double sticky tape in the diagonal and place the diagonal on the mounting plate. (Place the mirror face down on Viva paper towel and the surface will not be harmed. Try not to slide the mirror along the paper towel.) Then you can hold up the holder and look down the mounting shaft toward the diagonal and see if everything is centered. The diagonal should appear round and perfectly centered around the shaft. If the mirror appears off to one side then the mirror can be removed and shifted over a bit. Another technique is to put spider hub nut in a vise and screw in the diagonal and rotate it about its axis, the diagonal should be centered and not move eccentrically. Make sure the diagonal is stuck down well to the holder. You don't want it to fall off. Once you have the diagonal in the correct position mark around it with a pencil as seen in the picture above left. You will now b able to put the diagonal back in its correct location. Cut two pieces of index card and fold over into two thickness as shown above. These are spacers to be used in gluing down the diagonal. You do not want to glue the diagonal directly to the holder without these. These paper spacers are .015" thick and will insure that the RTV remains .015" thick between the diagonal and the holder mounting plate. This accomplishes two things: it keeps the two surfaces from squeezing together completely, which can induce stresses; and it allows a razor blade, which is .010" thick, to be inserted between the surfaces and the diagonal easily removed if necessary. If you can't get a razor blade between the surfaces you can make a real mess and even possibly severely cut your fingers trying to get things apart.
With the card spacers placed as shown at the left, a small dab, perhaps 1/2" square, of RTV is smeared on the diagonal as well as the holder. You want the area between the card to be filled, no more. Of course a little will leak into the card but you can pull them out later with a pliers. Waxing the card will keep the RTV from sticking altogether. The holder is placed on the diagonal as shown and positioned along the pencil marks you previously made. The natural weight of the holder is enough to hold it until the RTV dries. After about three of four hours pull out the paper spacers and you're done.