A 6-inch f/5 Telescope.


The 6-inch RFT (61Kb)

This telescope was made almost entirely from spare bits and pieces - its total cost was about A$30. This is mostly because I have a large and well-filled junk box - most people couldn't build it this cheaply. I tried to make it as simple as possible and yet be innovative in its design where I thought I could improve on "standard" parts.


Optics

Nothing fancy here - a standard 6-inch f/5 pyrex paraboloid mirror. I made it on the back of a tool of a friend's mirror. The tool was a piece of plywood covered in ceramic tiles and ground using standard techniques. It was polished on polishing pads (thanks Bratislav!) and figured on a pitch lap on the same piece of plywood (once I'd removed the tiles). Figuring took approximately 5 minutes (in two sessions) and the mirror must be the poorest I've ever made - barely 1/4 wave, but good enough for the low powers that this telescope employs. Grinding took several days while polishing another 2. I had to rush because I could get it aluminised (free!) if it was ready by a particular time - which was less than a week after I decided to make it.

The f/5 mirror yields better images than did an earlier 6-inch f/4 I had made simply because f/5 is much kinder on eyepieces than f/4. (The figure on the f/4 mirror was actually much better than the f/5 - it is the focal ratio that is the issue here.) The slight extra tube length is not a problem and I would definitely recommend never making any visual telescope faster than f/5.

The secondary mirror was from a long-forgotten project that I found in my junk-box. It has a small chip on one edge, but it doesn't matter for this telescope as it is well out of the on-axis field. It is a 38mm minor-axis mirror - larger than is necessary - but as I had it on hand, I used it. A 34mm one is the ideal mirror for this instrument.

The tube

The tube is a home-made fibreglass and resin job built for the already mentioned f/4 mirror made long ago (~1973). That mirror was sold although I still had the tube. The original tube was made by my elder brother and I (along with tubes for 2 other of our telescopes - an 8-inch f/7 and 4¼-inch f/4). Fibreglass tubes are recommended by Howard in his book "Standard Handbook for Telescope Making" - our bible during construction of our telescopes (and still an excellent book but unfortunately out of print now). The tube had to be extended for this longer focal length mirror, a technique which came back easily to hand despite the intervening years. However, I couldn't match the colouring that had been used and so I have simply painted the outside of the tube a dark blue.

Internally, the tube is lined with black flock paper (from Edmund Scientific) to make it really non-reflective. Black velvet is probably better, but would cost more than I paid for the whole telescope! Besides, I had some left over from other projects and this project was intended to use up left-over bits.

The focuser & secondary holder

Getting the focuser right is very important in small telescopes - not only must it satisfy all the usual requirements for a focuser, but it needs to have a very low profile in order to minimise the size of the diagonal. I have always believed that a lateral-sliding focuser is the best way to achieve this but I'd never made one, assuming that they required exacting machining in order to work properly. While mine isn't perfect, it works very well and required no machining. It is made almost entirely from scrap parts.

top view of telescope showing focuser (55Kb) side view of focuser (31Kb) internal view of focuser (37Kb)

I can adjust the position of the focal surface relative to the tube over a 40mm range, from being level to the tube surface to 40mm above. This is perfectly adequate for visual use and all my eyepieces come into focus somewhere within these extremes. Because the eyepiece I intend to use most on this telescope - an old Unitron 20mm Erfle - comes to focus with the focal surface close to the tube, it needs only a 34mm minor-axis mirror (22% obstruction) to yield a 12mm fully-illuminated field (almost one degree). This is excellent performance and very difficult to achieve with normal up-down focusers.

The heart of a slide focuser is the slide. Instead of precision rails and ball bearings, mine is made from the discarded rails of a computer print-out binder. In the old days of computers, you used to file print-outs of programs (on 15-inch wide paper) into special binders. They had either cardboard or plastic covers (later Kydex covers - good for top ends) and thin plastic spikes which went through the end sprocket holes of the paper to restrain them. The spikes were tucked under little bits of metal which slid in/out on metal rails. (If you don't know what I'm describing, you'll just have to take my word that these things were extremely common around computers 10 years or more ago.) Anyway, I had already cannibalised the covers of these binders for the top end of my ball-scope and I was looking at the rails wondering if they should be thrown out or put in my ever-growing junk box when I thought "rails...slide-focuser...hmmmm" And here it is.

(Refer to the above 3 pictures to help with the following description.) The rails are anodised steel, as are the sliders, and cut down to about 17cm in length. They are kept at the right separation by two cross members of 10mm wide, 2mm aluminium. Holes through these two pieces are used to bolt it to the tube. Longitudinal pieces of 1mm aluminium bent into a right angle add extra stability and allow a mounting point for the driving mechanism. The moving part is a plate of 2mm thick aluminium approximately 70mm wide and 100mm long. A 1¼-inch hole is at one end of the plate, with an aluminium tube over it to hold the eyepieces. Normally, such a tube would need to be machined but I had one in my junk-box from a previous project. Motion is provided by a rack-and-pinion drive from (you guessed it) my junk-box. My father made this for me for my very first telescope, a 4¼-inch f/12 Newtonian, which was de-commissioned many years ago. The pinion gear rides on a chrome-plated shaft removed from a floppy disc drive, which in turn rides in brass blocks which simply have suitable holes drilled in them. The hand knob is from a JMI NGF-2 focuser; the knob is removed when fitting a motor drive and placed in a junk box to await re-use. (The shaft end of the knob was used as a coupling between the focuser and encoder on my 20cm f/4·5 Newtonian imaging telescope - nothing wasted here!) There is one other important feature in this mechanism which makes it a delight to use - a 6:1 reducer. This is a commercial part, sold to amateur radio builders as a reducer for the tuning knob on their radio - current price is of the order A$20 (but I had one in my junk box from a previous project). It is gear-less, the reduction being done by friction-coupled balls turning between the input and output shafts. The addition of this mechanism turns the focuser from an ordinary one into an exceptionally nice one.

The secondary mirror must be attached to the moving plate and positioned so that its centre (optical, rather than geometrical) is directly under the eyepiece holder. I use a 'U'-shaped piece of 1mm thick aluminium about 20mm wide to link the plate to the secondary holder. A suitable width block of aluminium on the plate keeps it at the right spacing, while the bottom of the U wraps around a 20mm diameter aluminium tube (a cut-off portion of one of the truss tubes on the ball 'scope). A single screw and nut holds the two together and allows for rotation of the secondary should it be necessary. Slots in the top of the U allow for positioning the secondary mirror accurately under the eyepiece. Mounted on the same block on the plate is a piece of plastic used to shield the secondary from light getting through the slot in the tube in which the focuser slides (well shown in one of the above photographs). Getting back to the secondary holder, the end of the tube has a small flat 2mm aluminium plate glued to it through which 3 collimation screws are located. The secondary mirror is attached with silicon sealant to another 20mm tube cut at 45°. The other end of this tube also has an aluminium plate glued to it through which the other end of the collimation bolts go. The spring-tensioned collimation bolts are arranged not at the "standard" 120° spacing, but rather so that adjustments occur at right angles (up-down and left-right as seen through the focuser). One screw acts as a pivot and is only touched if the whole assembly needs to be moved towards or away from the primary mirror, only the other two are used when collimating.

Primary Mirror Cell

The primary mirror cell is one 4mm thick aluminium plate, the mirror resting on a 3-point support and held laterally by three posts attached to the plate. Through each post is a bolt (with lock-nut) to securely position the mirror. The collimation bolts are on the outside of the tube and move the plate relative to the tube. The photograph below shows two of the bolts and the attachment points on the outside of the tube. They are easily accessible while looking through the eyepiece, a boon for collimating.

mirror cell and mounting (31Kb) Mirror in cell (24Kb) Attatchment point of mirror cell to tube

The Mounting

A simple alt-azimuth mounting is ideal for this type of telescope. This was one mistake I made in my earlier version which had an un-driven equatorial mount. I quickly discovered that an equatorial without a motor is a pain - it still has to be pushed about but the eyepiece always seems to be in the wrong position for comfortable viewing. I had always intended that the telescope would be used with the observer seated and the mount was built with that goal in mind.

The mounting represents most of the cost of this instrument as I had to buy most of the parts for it - not enough large pieces in the junk box this time. As can be seen from the above pictures, the tube is held in a cradle that can be quickly opened to allow the tube to be rotated to bring the eyepiece to the most comfortable position for observing. It also allows the tube to be re-positioned to accommodate different weight eyepieces. This was the hardest part to make, taking me almost a whole day to cut out and assemble. Not that it was difficult, just that it was fiddly. It is made from ½-inch plywood cut out with a jig-saw. The outside is octagonal, the two rings being separated by 15cm and held by 4 plywood plates. It was assembled with the top, bottom and side pieces whole before cutting the top and bottom pieces for the hinges and latch. Because plywood can't be end-screwed, all the pieces are held together by small aluminium angle and 3/16" screws and nuts. This is where all the time went - there are 50 screws holding everything together which meant that almost 100 separate holes had to be drilled. But it was all worth it in the end as it is a delight to use because it is so easy to adjust.

The rest of the mounting is pretty standard Dobsonian technology. The altitude bearings are 12cm in diameter and are end caps for sewer pipe and cost A$4 each. They ride on teflon blocks attached to the mounting. Because the tube assembly is so light, the teflon blocks had to be moved far apart to increase friction or else the tube moved far too easily. The height of the mounting was set by the height of the eyepiece when at the zenith and the observer seated in a chair. This means that it is higher than one might normally make it (although if one did make it as low as could be done then it would be very uncomfortable to use because the eyepiece would be so close to the ground). The height to the centre of the altitude bearings is a bit under 70cm. The side-boards are left-over pieces of craftwood (a dense, re-constituted timber product) screwed to a piece of chipboard which was covered in formica (left over when our kitchen was built). The front board is another piece of ½-inch plywood. Note the handle cut into its face to facilitate easy carrying. The usual 3 pieces of teflon attached to a plywood base finish it off. All the wood was stained before 3 coats of varnish (had to buy a new pot of varnish - more expense!) was applied.

In use

The telescope is a joy to use. From the chair, one tends to stay in one field for a while before moving the chair and telescope around to another region. Depending on your observing style, you can either call this a problem or perfect observing. You'll notice that there isn't a finder on the telescope. I don't think that it needs one because I just sight up the side of the tube and then look through the eyepiece. If the object in question isn't in the field, then it will be close enough to find without a long search. (Remember, you don't go searching for 18th magnitude galaxies with this telescope. It is for low power views of bright objects.) Some people might think that a Telrad is called for with this telescope, and I may make one for it one day.

With the usual 20mm Erfle it yields 38× and a field of about 1·7° - perfect for comets, eclipses and just wandering through the milky way (the Eta Carinae region is superb with nebulosity filling the whole field). However, a 16mm Nagler yields 47× and a field of 1·7° too, with better images at the edge of the field and so has become my eyepiece of choice with this instrument. If I had made the focuser 2-inch instead of just 1¼-inch, then I could have used a 20mm Nagler for a 2° field, or my 32mm Widefield for 24× and 2¾°! It is tempting to re-build the focuser for this last combination because such wide field views are truly stunning and is what RFT viewing is all about.


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Page last updated 1997/05/24
Steven Lee