Updating a telescope

by Nils Olof Carlin (member of SAAF). Click for larger images! The one above shows solar viewing at the Mariestad star party, sept 2nd, 2000)

The telescope in question is one that I built for my brother 5 years ago - in one weekend! The optics, a 10.1" Coulter mirror with secondary, were bought second-hand. The tube is made of 1/8" plywood, of the cheap kind used to cover walls, with a printed paper finish. This makes for a light tube - the total weight, including finder but not rockerbox, is 23 lbs (10.5 kg). There is an internal "diagonal cage", that holds the focuser, spider, finder etc. The spider is my "hacksaw" design described elsewhere, with vanes of 2 mm aluminium (perhaps overkill, but spikes are no big problem).

After these years (I have spent a couple of them with the ATM list), it was obvious to us that a few things could be improved:

• The focuser: The original one was made from a 55mm camera objective. It was a bit too narrow to admit a Barlow, and the focusing range was a bit on the short side. And worse: it was mounted on "top" of the tube, and you would have to lean over the tube to reach it - and I prefer to sit comfortably while observing. Besides, there was no good place left for the finder.
• The finder: This is one half of a cheap 10x50 binoc with crosshairs (from thin copper wire, threaded through holes made with a heated sewing needle in the plastic field stop). The original spring-loaded mount was just too flimsy and went out of alignment too easily.
• The mirror cell: the original was too closed to cool well. Also, I could not reach the collimating screws while looking through the Cheshire - I guess I am spoilt, but I feel this is just too awkward.

I wouldn't cram this update into one weekend, but on the other hand I like to keep things simple. If there is an inexpensive way that works OK, I prefer it. So here are some of the old and new features - simple and inexpensive to make, and useful, I hope.

• The focuser: We wanted easy access to the eyepiece, at any altitude, sitting comfortably. To me, this means a focuser mounted at 45 deg to the altitude axis, at the left side as I like to have the telescope to the right of me. I had to cut off a corner of the internal diagonal tube to make room for its base. I have tried before -with no success then- to make a Crayford-like focuser with Teflon bearings, but having seen one made by the Uppsala people, I tried again. The focuser board is detachable, and held by 3 screws. Standing on it is a 50 mm (2") high "V" of 12 mm (1/2") plywood, with inner surfaces of laminate (the same as used on the trunnions)for the drawtube to slide against. The focuser drawtube is a piece of 40 mm PVC drain tube, about 70 mm long, with a 1.25" i.d. insert for the eyepiece. A 4mm screw in a threaded hole holds the eyepiece. Below the insert, the inside of the tube is covered with black self-adhesive flock plastic. I have taped a piece of Teflon sheet (the kind that has recently been popular for altitude bearings) around the tube, to make it slide smoothly. The focuser axis in this image is a cheap screwdriver with about 3 mm dia. shaft, cut to size.
  
  Since then, I replaced it with another one, with metal shaft and 2.4 mm blade, see picture. It is mounted in two bearings made of Teflon - I had some small scrap pieces handy, but just about any plastic would do, I believe. The bearings are fastened to an aluminium angle screwed to the focuser base, tight enough to give enough pressure. On the shaft between the bearings is a piece of rubber tubing, originally for bicycle valves. This is the key to success - it gives enough driving friction, and needs no "flat" face on the tube. It also holds the shaft in place (as long as you don't pull hard). I have made another much like this (with ball bearings), and I have found the rubber lasts surprisingly long (but not forever), if the shaft is not too large and the pressure is not extreme.

• The finder: It is made from 12 mm plywood and wood, joined together by 8 mm bolts and wing nuts to each other and to a plywood base, part of the internal diagonal cage. Spring washers (not shown) makes it easy to adjust, and the wing nuts are tightened to lock. This image shows the first version as it is a bit simpler, but if you look at the tube extension image, you see a version where the finder is rotated 90 deg to make the eyepiece better accessible while seated for observing. That way, I can go from the eyepiece to the finder by leaning a little back, without having to leave my seat. The holes from the hinge are useful for coarse aiming, but later, a red-point 1x finder will be added. I have fastened the flat side of the prism housing using two self-tapping screws - I made sure they did not hit the prisms! The dewcap is a plastic medicine bottle, with the protective lid on during solar observing.

• The mirror cell: I would not consider a mirror cell that I can't collimate while looking into the Cheshire or eyepiece. The original one had a 9 point cell, with a closed back that took a bit too long to cool. Thanks to PLOP, the program by David Lewis, we know that that 6 points is even better than the traditional 9. It is certainly much simpler mechanically, the way I did it now. The optimum radius for a 6 point cell is about 0.6 of the mirror radius, but I used 0.7 for a mechanically simpler design, the performance is still more than adequate according to PLOP. The square wooden frame is screwed to the tube sides. The 3 "see-saws", carrying two cork pads each, are made from 20x20 mm aluminium "T" profile, and fastened with wood screws acting as hinges (only one is visible here). I haven't (out of laziness, perhaps) jumped on the RTV bandwaggon, so I use a sling arrangement - not the usual one, but instead two slings between opposite corners, each making a 90 degree turn:
  
  The difference here is that the mirror position is strictly defined, it cannot move sideways as in the usual sling (this would even work for an eq mount at >45 deg latitude!). The slings are made from old plastic packaging straps, about 10 mm wide, and do not stretch -one is in front of the other, they cross at the lowest point. They are fastened near the corners, at the fixed end with a screw and a washer and with a clamp that allows centering the mirror before tightening. As they are black, they do not show well on photo. The mirror is kept from falling out of the cell by 2 safety clamps, and kept from rattling by a small piece of foam plastic at the top. There is a baffle screwed to the inside of the frame that restricts light leakage past the mirror. One of the "see-saws" is on a crossbeam fixed across the frame. The other two are on adjustable beams, forming a triangle with the fixed one. They are held by screws (acting as hinges) to the fixed crossbeam at one end. At the other, they are held to an aluminium L profile by knurled knob screws - easily within reach from the eyepiece (with longer tubes, I use hex nuts that I adjust with a wrench head on a piece of plastic tube). Here, they are spring loaded. You can see 3 plastic feet for the tube to stand on (and a small lead counterweight).

• The tube extension: This feature is left from the original model. 4 flaps cover the tube opening in two complete layers. Each flap is divided into 3 parts joined by fabric "hinges" (thick fabric and strong glue), and the middle part is hinged to the tube (or rather to the diagonal cage). I have used fabric here, too, but metal hinges will probably be more durable (they could be epoxied to the thin flaps). In use, the side flaps are held together by Velcro pieces, forming an octagonal tube whose length is half the tube width, in this case 6", 152 mm, making the tube 6" shorter for transporting (it is 41", 1030 mm now). This image shows it closed, you can see it open on other images.

• The altitude trunnions: These are the original ones, made from an aluminium cooking pan. The sliding surfaces are shown with brown tape, but I have since put on self-adhesive laminate strips instead (the kind to iron on the edges of fiberboard shelves). The surface is stippled, and perhaps somewhat similar in texture to Ebony Star.

• The solar filter: It is made from Baader AstroSolar film, fastened to a plywood frame with a thin string of glue (contact cement). It is secured in place with 2 screws (it must be removed before the tube extension flaps can be closed). As you see, the filter is not quite wide enough to cover the full aperture, but this is no problem.