My (new) visual telescope -
I now have a nicely large telescope mounted in my dome - a 50-cm (20-inch) f/4·5 Newtonian on an alt-az mount. (It's not a Dobsonian - it has bearings and machined surfaces and real soon now a computer-controlled drive system. JD would not approve.)
The optics were made by George Livanos from a 30mm thick piece of plate glass. At f/4·5 it is faster than I like (I prefer f/5 or slower for visual use) but the price was right and I was willing to compromise. A 90mm minor axis secondary mirror is used. The primary is supported on a 27-point floatation cell designed with the aid of the PLOP software and made from aluminium triangles.
The OTA is made from 25mm aluminium RHS, 1.6mm thick. The top end and mirror cell are octagonal, welded from 3mm thick 25mm RHS. Everything is held together by 6mm stainless steel bolts. There is no allowance for quick disassembly as this telescope lives in my dome which is under dark skies. I have little need of a portable telescope.
The mounting is designed from the ground up to be computer controlled. It is not a Dobsonian. Sitting on top of my pier is a Y-shaped steel support for the ground board which is made from 20mm thick MDF, machined to 850mm diameter on a lathe. Onto this is mounted the centre pivot, which is a 150mm diameter bearing (from an aircraft landing wheel, I believe) which allows for a large hole in the centre through which cables and plugs may be passed without causing tangles whilst the mount rotates in azimuth. Onto this rests another 20mm thick MDF board turned to 900mm diameter which has a machined fibreglass surface which rolls on the ball bearings mounted on the ground board. This board is reinforced by several more pieces of 20mm MDF to ensure it doesn't bend when the telescope is mounted on top of it. It is intended to press a motor- driven roller against the outer surface of the lower board in order to drive the telescope in azimuth.
The altitude pivot is a large disc on one side, and a large bearing on the other. The large disc has a machined fibreglass surface and rests at one end on a ball bearing, and the other on a roller (which will be driven by a motor). These are held up by a steel RHS frame as can be seen on the above pictures.
Encoders have been fitted. The altitude encoder is quite standard, being simply attached to the shaft which holds the bearing onto one side of the telescope.
The azimuth encoder, however, is a very clever design by my friend Rob. As can be seen in the photograph, the roller is attached to a long tube which in turn is attached to a vertical plate. That plate is attached via a flexible coupling to the upper disc and is the heart of the design. To apply pressure to the roller against the lower disc, the vertical plate can pivot a little (although in practice it doesn't really move). A plate of flexible metal (phosphor-bronze) is sandwiched between aluminium plates (you can just see the coppery-coloured plate showing between the 2 aluminium plates). Only the flexible plate is attached to the vertical plate; the two other plates have mitred edges and don't restrict the vertical plate from pivoting slightly. A spring (just visible behind the encoder) applies pressure against the vertical plate, forcing the roller against the lower disc. If there are any centering errors (or bumps or hollows) on the lower disc's surface, the roller can pivot to compensate. (Without the spring, the roller would be just touching the lower disc.) The roller was machined to a smooth finish, but then bead-blasted to give it a better grip. The encoder is coupled to the shaft in this picture with a piece of plastic tubing - but this was replaced after the picture was taken by a proper coupling.
There is a 75mm f/4 Newtonian as a finder (~12×), mounted on the centre section.
My (new) imaging Telescope -
I have a new telescope dedicated to imaging which replaces my old 20-cm one (shown below). It is a 306mm (12-inch) f/5 Newtonian on a German equatorial mount. The mirror is on a second-hand Duran 50 blank (the tool for my f/6·5 visual telescope further below), and is (naturally) hand ground and figured. The focal length is 1515mm, which combined with the 17µm × 19·75µm pixels of the TC245 CCD chip in my Cookbook camera, gives an image scale of 2·3 × 2·7 arcseconds (a better match to the pixel size of my CCD than the 20-cm was), and a field of view of 14·5 × 10·8 arcminutes (you've got to pay for the resolution somehow!). The secondary is 66mm (2·6-inch) minor axis (removed from the old 20-cm, which now has an old 2·5-inch one). The tube is a proper Serrurier truss made from 25mm aluminium poles. There is a tube inside the trusses made of 0·8mm polycarbonate to keep the dust, dew and stray light out, which is lined with black flock paper to make it really dark inside.
The focuser is a JMI NGF-2 (2½-inch long focusing tube, flat base) with motor drive, and my own encoder fitted to the other end of the shaft (again removed from the 20-cm). The encoder is the key to efficient focusing with a CCD. I made the encoder from a precision 10-turn 5K potentiometer. This gives a bit over 1000 ohms over the 35mm travel; which equates to roughly 1 ohm = 0·003 mm, quite acceptable for my pixel scale. With a 3½ digit multi-meter one gets 0·1 ohm resolution and so provides excellent resolution. It allows one to accurately focus by taking an image either side of focus at known encoder values then predicting the encoder value of exact focus. (This type of encoder will only work where there is absolutely NO backlash between the shaft and tube. The standard SCT focusing mechanism and regular rack & pinion focusers won't work.)
Here and Here are pictures of the telescope without the baffle tube in place so that it's structure can better be seen. The top and bottom ends are octagonal, welded from sections of 25mm RHS aluminium. The central section is made from 2 widths of the same 25mm square RHS, made wide to fit better onto the mounting plate of the mounting. The poles have threaded inserts at each end and attach to the centre and end sections via 3mm right-angle aluminium plates. Here is a closer view of the mirror cell which also shows how the truss poles attach. The mirror cell is a standard 9-point design, but attaches to the tube at 4 points rather than the traditional 3. This allows a right-angle approach to collimation.
The spider uses an asymmetric design which gives much greater strength than the standard design. Here is a somewhat over-exposed view of it (as the spider hadn't been painted in this picture). It consist of a 50mm length of the 25mm RHS used in the tube ends and 2 pieces of thin metal strip, bent at 45° each end, threaded through the RHS and attach to the tube. This crude diagram should explain what I mean... This spider is by far the strongest of any that I've ever seen. By making it this way, the spider has much greater resistance to twisting or vibrating than the traditional axially-symmetric loaded design. It is a little more difficult to build and mount than the standard design, but it's well worth it. Another advantage of this is that the spider is centred within the tube, but the secondary mirror is given the correct offset. If you look carefully at some of the images taken with my 20-cm, you can see the result of offsetting the diagonal by moving the spider which was necessary with the commercial system I used on that telescope - doubled diffraction spikes.
I have used the 50-mm finder from the 20-cm as well. The 10-cm SCT guidescope mounts from the centre section on the other side from the finder. Its blue dew cap can just bee seen peeking out from behind the telescope in the above picture.
The mounting is a very old home-made German equatorial, but the original HA drive has been replaced by an Anssen Technologies (an Australian Company) 30cm worm and wheel main drive and a 15cm w&w drive for declination. Both use fully controlled stepper motors capable of tracking, guiding and slewing. Acquisition with the CCD is now considerably quicker.
The telescope has spent a lot of time on John Gleason's Paramount ME, as shown above right. Many mounts these days make exagerated claims of their load carrying capacity on the assumption that you are mounting a short SCT-type system. A Newtonian weighs much the same, but is longer and hence the loads on the mount are far higher - which usually means that the mount can't cope. The Paramount can support the load and length of this telescope and is a joy to use. Coupled with an SBIG ST-8e this telescope and mount have produced some nice images (at least I think so!).
My (old) imaging Telescope -
This telescope has been replaced by the above 30-cm one, but I've left this description here as it was used to evolve some of the ideas now used on its replacement. It does still get used as it has a wider field and is slightly faster. Coupled with a coma corrector it works nicely with the ST-8e CCD.
I have a dedicated telescope for imaging - either photographic or CCD. It is a 20cm (8-inch) f/4·5 Newtonian on a German equatorial mount. The mirror is on a second-hand Pyrex blank, and is hand ground and figured. The focal length is 905mm, which combined with the 17µm × 19·75µm pixels of the TC245 CCD chip in my Cookbook camera, gives an image scale of 3·89 × 4·49 arcseconds, and a field of view of 24·3 × 18·1 arcminutes. The secondary is 66mm (2·6-inch) minor axis, designed for a wide field of view for photography, but since I got the CCD it hardly ever sees my ancient Pentax MX 35mm camera. The tube is rolled steel, and I use a 10 × 40 finder. The inside of the tube is lined with black flock paper to cut down on internal reflections.
The focuser is now a JMI NGF-2 (2½-inch long focusing tube) with motor drive, and my own encoder fitted to the other end of the shaft. The encoder is the key to efficient focusing with a CCD. I made the encoder from a precision 10-turn 5K potentiometer. This gives a bit over 1000 ohms over the 35mm travel; which equates to roughly 1 ohm = 0·003 mm, quite acceptable for my pixel scale. With a 3½ digit multi-meter one gets 0·1 ohm resolution and so provides excellent resolution. It allows one to accurately focus by taking an image either side of focus at known encoder values then predicting the encoder value of exact focus. (This type of encoder will only work where there is absolutely NO backlash between the shaft and tube. The standard SCT focusing mechanism and regular rack & pinion focusers won't work.)
The focus encoder
I have strengthened the tube around the focuser with a band of 2mm steel inside the tube to minimise flexure during long exposures. I have confirmed this by moving the telescope to several parts of the sky and measuring the focus (the encoder helps considerably for this exercise). There are still temperature effects to worry about, but they are smaller than flexure with a heavy camera. Having essentially an absolute encoder means that I can position the CCD to just about the correct focus at the start of a night for doing twilight flat fields. Once focus has been achieved, focus offsets for different thickness filters just mean moving the focus the required amount instead of a new focus run. I couldn't live without a focus encoder.
The mounting is a very old home-made German equatorial, but the original HA drive has been replaced by a 10-inch Mathis worm & wheel set driven by a 1 rpm mains-synchronous motor. Declination slow motion is a 26-inch radius tangent arm with a 40 TPI screw and 1 rpm DC motor. This arrangement is fine for guiding, but I want to replace both drives so that I can move the telescope in a faster motion for positioning object on the CCD. I guess stepper motors are the way to go here.
I've made a few changes since the top photograph was taken. I've swapped to a Celestron 8×50 finder and I've changed the drive system. Here is how it looks now.
The finder comes close to my ideal - it has adequate light grasp, good images and is light and well made. It has a proper etched glass reticule allowing for decent illumination (the extra lines are a bit wasted in the southern hemisphere) and a separately focusable eyepiece. I've added a dew shield to help stop it from fogging up so quickly.
The drive system is an Anssen Technologies (an Australian Company) 30cm worm and wheel main drive and a 15cm w&w drive for declination. Both use fully controlled stepper motors capable of tracking, guiding and slewing. Acquisition with the CCD is now considerably quicker.
My portable telescope -
I've sold the above telescope - after almost 20 years of service. You can read more about it here.
My RFT -
This is a fantastic little number, especially for bright comets and eclipses. I bought the optical tube assembly but have subsequently modified it.
Originally it had a straight-through eyepiece position and was mounted on a strange sort of alt-az mounting (rather like the sort you find in public places for scanning the horizon), but this is very awkward if you want to observe high in the sky. My first modification was to make a simple diagonal by adding an old Newtonian flat in a tube. I put the thing on a simple Dobsonian-style alt-azimuth mounting (it looked like this). The large, focusing eyepiece seemed to be an old WWII job (possibly Kellner design). It gave about 13× and a field of view of 3°. The exit pupil was overly large at 9mm, so one didn't use all of the aperture; but the images were fine and I was happy with it for a long time.
I decided to upgrade it when I discovered that I had a large, old rack-and-pinion focuser that fitted it perfectly. I made a 2-inch star diagonal using a mirror from an old photo-copier and some scraps of aluminium. I had to modify the tube to accomodate the new diagonal, and while doing so I inserted some black baffles inside the tube. I've also added a long dew- shield, held in place by a plastic strap. So now I can use my modern eyepieces and utilise the full aperture of this telescope.
The f/5 optics when coupled with my 32mm Widefield eyepiece yield about 18× and a beautiful 3·5° field. The views along the milky way are simply superb, and with a 2-inch [OIII] filter attached to this eyepiece I can see the whole ring of the Veil nebula in Cygnus. I can also zoom in using my 16mm Nagler for 37× and a 2·2° field of view for a smaller exit pupil and even better images.
A very Portable Telescope -
I've sold my ball-scope so I can't really leave it here. If you want to see a description of it, then follow this link.
My guidescope -
Yes, I own a commercial SCT. I just couldn't be bothered to make such a small telescope just to use a guidescope. The small SCT does work well for a guidescope - after a bit of modification. I've added 3 screws through the back which hold the primary mirror against the usual flop that so often occurs in this style of telescope.
I use a 12·5 mm orthoscopic eyepiece in conjunction with a 2× Barlow for about 150×. I find this adequate for guiding. I'd like to make another CCD to use as an autoguider, but then I'd have to modify the electronics of the drive system as well. Soon...
I should add that it makes a nice portable telescope for taking away on holidays when there is no room for anything bigger. I mount it on a standard camera tripod but with a small tangent arm slow motion device to help in positioning and tracking. The optical performance isn't brilliant, but it is adequate. I've also used it as a telephoto lens and taken a couple of nice shots with it.
Also, several people have asked how I modified the SCT for the mirror locking screws. It was quite easy, but does require pulling the telescope apart to avoid doing any damage to the optics. This is a picture of the back of the telescope showing the external arrangement, while here is a side view and close-up of one of the screws.
The following description is from memory so details are sketchy at best and may well be wrong in places, too, but it should be good enough to get you going in the right direction. It's quite simple and not dangerous to do as the mirror simply fits back in where it came from and there are no adjustments to do (or even possible).
To dismantle the telescope, the barrel which holds the corrector and secondary needs to be separated from the rear cell. The whole barrel just unscrews from the cell, so is simple to remove and goes back in the right place afterwards. (The cell which holds the corrector is similarly screwed into the barrel.) Next the mirror must be removed from the cell. This involves undoing the central baffle which restrains the mirror. There's a spring involved somewhere here, too, as well as a rubber 'o'-ring. (When focusing the telescope, the mirror is simply pushed up against a spring in the baffle tube which allows movement along it axis.) Having removed the central baffle the mirror is free to be removed. It's then a matter of drilling and tapping 3 holes in suitable places in the cell for 3 suitably long screws. Clean up the cell and replace the mirror and baffle in reverse order.
The screws should either be nylon, or as I did, ordinary screws tipped with blobs of solder. These screws bear directly on the back of the mirror, so make sure you never exert too much pressure on it. The locking is not totally satisfactory, however, as the mirror is held within the central baffle by an 'o'-ring between the mirror surface and a flange on the baffle tube and so it is always a bit of a "spongy" fit. But with careful adjustment of the pressure on the screws it does hold the mirror from moving as the telescope is moved about the sky. You can also use these screws for final positioning of a guide star
A 6-inch f/5 RFT -
This telescope was made almost entirely from spare bits and pieces -its total cost was about A$30. This is mostly because I have an old and well- filled junk box - most people couldn't build it this cheaply. I made the primary mirror (pyrex) on the back of a tool of a friend's mirror project; the secondary was from a long-forgotten project (it has a small chip on one edge, but it doesn't matter for this telescope); while the tube is home-made fibreglass built for a previous f/4 mirror which was sold long ago (the mirror, not the tube), but had to be extended for this longer focal length mirror. The focuser and mirror cells were all made from scrap, but the material to make the mounting had to be bought - which is where most of the money went.
It is designed to be rugged, yet portable. The viewing position was always intended to be seated for greatest comfort and the mount was built with that goal in mind. The f/5 mirror yields better images than did an earlier 6-inch f/4 simply because f/5 is much kinder on eyepieces than f/4. The slight extra length is not a problem and I would definitely recommend never making any visual telescope faster than f/5.
The focuser is of a horizontal-sliding type to ensure the focal surface is as low to the tube as possible. I need only a 34mm minor-axis mirror (22% obstruction) to yield a 12mm fully-illuminated field (almost one degree). The primary mirror cell is one aluminium plate and the collimation bolts are on the outside of the tube, easily accessible from the eyepiece.
There is a more detailed description of its construction and workings here.
I have a pair of 12×50's which I bought when I was first getting started in astronomy. They saw little use due to the difficulty of holding them steady, but I've just made a simple parallelogram- style mounting which makes them fun to use.
The tripod is made of 1×2 inch (20mm × 42mm finished size) timber (Maple) and is adjustable in height from 900mm to 1350mm (although this turned out to be an unnecessary feature). I found the cast tripod top in a swap table at the 1998 Queensland Astrofest for $5 and made everything to suit it. In the centre hole I put a discarded (but serviceable) ball bearing to act as the pivot for the parallelogram uprights. The uprights are aluminium U-channel and hold the two arms 150mm apart. The parallelogram is made from the same timber as the tripod and is 660mm between the central column and the end. I had a suitable weight and steel rod from another telescope which I bolted on to the end of the top arm for an adjustable counterweight. Additional friction is needed in the azimuth direction as the bearing is far too frictionless, but a layer of felt adds the necessary drag.
The binocualars are held on to the parallelogram in a double cradle mounting. A picture is worth a thousand words and should explain most parts of it. Each cradle is 1mm aluminium with a right-angle bend along each side to give it strength. The long arms are 3mm aluminium. Between the altitude cradle and each arm is a piece of teflon. Tightening the lock gives enough friction to counter any imbalance when mounting the binoculars. Nuts are glued to the altitude cradle for the lock bolts.
With this alt-azimuth head I can comfortably stand (or sit) in one place and view large parts of the sky without needing to re-position myself. The motion is smooth but it stays where it is left. I've started to enjoy my binoculars again, and may consider upgrading to something a little larger soon (I made the cradle wide enough to accommodate 80mm binoculars).
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Page last updated 2007/12/15