Fixing Common Telescope Problems

• Cleaning telescope optics
  
  • cleaning eyepieces
    
  • cleaning refractors/SCT's
    
  • cleaning reflectors
    
• Common optical problems
  
• Optical alignment
  
• Improving a Department Store Telescope
  
• Building a Dobsonian mounting

"The optics" are the various mirrors, lenses, prisms or eyepieces which are used to form the image. These are all precision components which need to be looked after with care. Protecting the optics from airborne debris will make a big difference to their longevity. Use covers on the ends of the telescope tube, and return eyepieces to their storage containers when not in use.

However, you should never cover the optics if they have dew, ice or condensation on them. Moisture plus confined air encourages fungal growth. Place the telescope indoors (or use a hair dryer on LOW heat) until it is dry - and don't touch the wet optics.

Telescope optics will collect dirt and pollutants every time they are exposed to air, and will eventually look grotty. But when should you clean them?

First of all, be lazy. Think of something else to do instead of cleaning your telescope. Many professional telescopes are only cleaned once or twice a year, because it does take quite a lot of accumulated debris to noticeably affect performance. So don't panic over a few bits of lint on your primary mirror. If you are seeing smeared-looking or blurry images; first check your eyepieces for fingerprints, sweat, dew, mascara or other deposits. Your eyepieces are the components most likely to get grotty in normal use.

Here's what your cleaning kit should contain:

1. A few litres of distilled or deionised water, plus a small spray container.
   
2. Isopropyl alcohol (available from pharmacies and chemical suppliers) or medical grade methylated spirits. Ordinary methylated spirits from your supermarket will leave a residue when it dries.
   
3. Plenty of sterile cotton wool (available from pharmacies).
   
4. Lanolin-free dishwashing detergent. Anything advertised as "kind to hands" or "softens your skin while you wash" is likely to contain lanolin. Lanolin does horrible things to optical surfaces.
   
5. A blower brush or lens brush (available from camera shops) for dust removal.
   
6. A very clean kitchen sink with draining board.
   

Dantronix are now selling a product called "OptiClean Polymer" which claims to remove dirt and grot down to the molecular level -- without harming the optics. I haven't tried it so I won't comment on its effectiveness.

NEVER, EVER, USE HOUSEHOLD WINDOW OR MIRROR CLEANERS ON TELESCOPE OPTICS. These products often contain ammonia and/or sodium hydroxide which are deadly to astronomical mirrors and most lens coatings. Similarly, towels and paper tissues should be kept away from optics, because most of them will cause scratches.

You should only need to clean an eyepiece's outer lens surface - the glass nearest to your eye. This is the surface most likely to be dirty or stained in normal use. Never dismantle eyepieces unless you know exactly what you are doing. The internal surfaces should never need cleaning, unless moisture or liquid has got past the outer lens. If this has happened, get the eyepiece cleaned by a specialist.

To clean the outer lens surface of an eyepiece:

• Blow off any loose dust using the blower brush.

• Mix equal volumes of isopropyl alcohol and distilled water to make up about 1/2 cup of "stain remover." If using methylated spirits, mix 1 volume of spirits with 2 volumes of distilled water.

• Soak a fresh ball of cotton wool in this mixture, and dab (don't wipe) the lens surface with the cotton wool ball. If the ball becomes stained, discard it and get a fresh ball.

• When the lens surface looks clean, spray the surface with distilled water to remove the alcohol.

• Place the eyepiece on its side to dry naturally.

Refractors and Schmidt-Cassegrains both get dirty at the top end (for cleaning purposes, Maksutov = Schmidt-Cassegrain). The refractor's objective lens (the big one at the top of the tube) and the Schmidt-Cassegrain's corrector plate (the window-like covering at the top end) can be cleaned as follows:

• If possible, lay the telescope on the draining board so that the corrector plate (or objective lens) is vertical, and facing the sink. Alternatively, turn the telescope on its mounting so that it is aimed slightly downwards.

• Cover any electronic components so that they cannot get wet.

• Blow off any loose dust using the blower brush. For refractors, also blow out any loose dust inside the dewcap (the short "tube extension" in front of the objective).

• Spray the corrector plate (or objective) with distilled water to wash away any remaining dust, and allow it to dry.

• If there are fingerprints or other stains, mix up about 1 cup of "stain remover". To see any stains on a refractor objective, shine a torch into the tube from the eyepiece end.

• Soak a fresh ball of cotton wool in this mixture, and dab (don't wipe) the stain with the cotton wool ball. If the ball becomes dirty or stained, discard it and get a fresh ball.

• When the corrector plate (or objective) looks clean, spray the surface with distilled water to remove the alcohol.

• Leave the corrector plate (or objective) in the vertical position to dry.

• For Schmidt-Cassegrains; if any liquid has got inside the corrector plate, leave the telescope's front cover off, and allow to dry naturally. NEVER remove the corrector plate - or the primary mirror - unless you know exactly what you are doing. The optical components of commercial Schmidt-Cassegrains are assembled - and tested - in a specific relationship to each other, at the factory.
  Similarly, if liquid has got inside a refractor's objective lens, allow it to dry out naturally.

• If internal stains on a corrector plate or objective lens are evident, then the telescope optics should be cleaned by a specialist, or returned to the manufacturer.

To clean the mirrors of a reflector:

• You will need to remove the primary mirror (the big one at the bottom of the tube) from the telescope. Most reflectors use a mirror cell, attached to the end of the tube, to support the primary mirror. All good mirror cells have large and obvious screws (or bolts) on the back side, to adjust the tilt of the primary (see alignment below). You do NOT need to undo these -- look instead for screws or bolts at the sides or edges of the mirror cell. In general:
  • Reflectors less than 150mm aperture use three screws (or bolts) through the sides of the tube into the mirror cell.
    
  • Reflectors over 250mm aperture with cylindrical tubes use six or more bolts through the sides of the tube into the mirror cell.
    
  • The mirror cell in many square-tubed reflectors (eg: some Dobsonians) is built into the panel which closes the bottom end of the tube. Remove this panel.
    

• If possible, the telescope should be aimed upwards when removing the mirror cell. As you remove the cell, the telescope tube will suddenly become top-heavy; so take precautions against it moving.

• It is advisable to remove the primary mirror from its cell before washing, because this will keep liquids out of the cell assembly. However, you can begin cleaning before mirror removal. Place the cell (with mirror) on a clean surface (such as the kitchen sink draining board), with the mirror facing upwards. Then use the blower brush to blow off any loose dust.

• You will see some restraining device(s) around the edge of the mirror's upper surface. Typically, these are small padded metal plates which are held in place by screws attaching to the mirror cell. Some mirror cells may use a padded ring, or spring-steel "fingers". Carefully remove the restraining device(s) from the mirror. A magnetic screwdriver is helpful!

• Some large Dobsonian mirrors are restrained by a sling or girdle which is hung between two strong supports on the side of the mirror cell. The sling supports the weight of the mirror when the scope is aimed low in the sky. Be sure to replace the mirror and cell "right way up" when you're finished.

• You should now be able to lift the primary mirror out of the mirror cell. Small mirrors may be resting directly on the padded metal surface of the cell. Larger mirrors are supported underneath by an arrangement of balanced metal plates -- be careful not to disturb these, because their positioning is important to your mirror's optical performance. Good mirror cells will feature alignment studs and holes which "automatically" reposition these plates when the mirror is replaced.

• Place the mirror (facing upwards) in the kitchen sink, then fill the sink with warm (NOT hot) water until the water is about 10mm deep over the mirror. Running the water across the mirror's surface will help loosen the dust.

• Add 2 or 3 drops of lanolin-free dishwashing liquid.

• Gently swirl a wet ball of fresh cotton wool over the mirror. If the ball becomes stained, discard it and get a fresh ball. Repeat this process until the mirror is clean. If the mirror is really filthy you may need to refill the sink and continue this step. Then empty the sink.

• If there are fingerprints or other stains, mix up about 1 cup of "stain remover". Soak a fresh ball of cotton wool in this mixture, and dab (don't wipe) the stain with the cotton wool ball. If the ball becomes stained, discard it and get a fresh ball.

• Mirrors used near the sea, or sources of industrial pollution, will tarnish within a few years. Tarnish -- typically a brown or dirty yellow patina on the mirror -- CAN'T be removed by this cleaning procedure. If tarnishing is severe then your mirror needs to be re-aluminised. This is a task for a specialist.

• Rinse the mirror with warm tap water, then spray with distilled water.

• Stand the mirror on edge and allow it to dry naturally. Place some folded towels or other padding around it, in case it falls over!

• Replace the mirror in its cell, replace the restraining device(s), then refit the mirror cell to the telescope.

• The secondary mirror (the small one at the top of the tube) is usually glued onto a cylindrical holder of some kind, which is typically attached to the spider with a central bolt. The spider is another cylinder held in the tube's center with a 3 or 4-vaned support structure. The other 3 (or 4) smaller bolts around the edge of the spider are used to adjust the tilt of the secondary mirror.

• To remove the secondary mirror, undo the central bolt until the mirror holder is free. Do NOT try to remove the mirror from its holder, and don't accidentally drop it down the tube!

• The secondary mirror can be cleaned using the procedure for eyepieces (see above). Be aware that liquid may get between the mirror and its holder, so be sure everything IS dry before replacing the mirror holder on the spider.

• Your optics should now be clean, but you will now need to realign them. Read the next section!

If you have an ordinary reflector, you can adjust or collimate the optics yourself using simple tools. First of all, remove the eyepiece and look down the eyepiece holder into the telescope. You should be able to see the secondary mirror, a reflection of the primary mirror in the secondary mirror, and a (reflected) reflection of the secondary mirror holder and spider in the primary mirror.

If your telescope is properly aligned, all of these components should be concentric with the central axis of the eyepiece holder; but you probably aren't that lucky and instead you may see something resembling this first diagram, or one of the diagrams below. If things are really bad all you may see is a view of the inside of the tube in the secondary mirror. But don't panic; even this can be cured ;-) Note that if you see something resembling one of the later diagrams, then skip to that part of the procedure! For ease of identification I have colour-coded the components in these diagrams.

Throughout the collimation process, it is important to do your checks while looking down the central axis of the eyepiece holder. Various "collimating aids" are commercially available, but if you have the standard 32mm (1.25 inch) eyepiece holder then an empty 35mm film canister can be used instead. Drill a 3mm hole through the exact centre of the canister's lid, and cut a 20 to 25mm hole through the centre of the canister's base. Don't just cut off the canister base -- many canisters lose rigidity if this is done.

Then insert the canister (base first) into the eyepiece holder, wind the focuser all the way out, and look through the 3mm hole in the lid. The telescope should be pointed towards the daytime sky (not the Sun!) or similar bright, diffuse light source. You should be able to see the bottom edge of the eyepiece holder and the secondary mirror hardware.

The first step in collimation is to get the secondary mirror holder into the right position. For reflectors, this is such that the secondary mirror is concentric with the bottom edge of the eyepiece holder. Don't worry about the mirror's tilt (if any) just yet; we fix this in a moment.

Secondary mirror holders are held in place by the spider -- typically this consists of 3 or 4 thin arms (or metal strips) radiating from the mirror holder to the telescope tube. These arms often pass through slots in the tube and are held in place by nuts.
Loosen the nuts to allow the spider assembly (with secondary mirror) to be slid along the tube slots so that the secondary mirror reaches the right position. Occasionally there may be no slots, in which case you will have to do this adjustment simultaneously with the secondary mirror alignment described next.

Some small reflectors use a single arm to support the secondary mirror holder. This design works well, provided the support arm is able to be adjusted within the tube, and is strongly built. My 150mm reflector's support arm is a 20mm by 5mm thick aluminium strip, bent into an L shape and held to the tube by bolts passing through slots in the plate.

Now take a look at the secondary mirror holder. A few simply consist of a metal (or plastic) block solidly fused to the spider assembly. These have to be adjusted by moving the entire spider assembly (a real nuisance). Some compound the problem by using a single plastic arm, instead of a spider, for support. Plastic tends to deform in warm climates.
Better holders are held onto a spider by a relatively large central bolt, surrounded by 3 smaller screws (or bolts). These work antagonistically on the holder's base. The big bolt "pulls", the screws all "push", and together they hold the secondary mirror holder firmly in place. The tilt of the secondary mirror is adjusted using these screws. If you have a non-adjustable spider, then the position of the secondary mirror is also controlled by these screws and the bolt.

To save a lot of time, I suggest that you loosen the central bolt a couple of turns (but don't let it fall out) and hold the secondary mirror holder against the adjusting screws by hand -- but don't touch the surface of the mirror! Grip the holder's sides instead. Rotate the holder if necessary to achieve correct secondary mirror positioning. Use your other hand (with the screwdriver) to turn the screws as needed; then retighten the central bolt. The secondary mirror is correctly positioned when the reflection of the primary mirror is concentric with the edge of the secondary mirror AND concentric with the eyepiece holder.

The final step in collimation is to get the reflected image of the secondary holder concentric with the image of the primary mirror. To do this you adjust the tilt of the primary mirror using some large and obvious bolts (or screws) on the bottom of the telescope tube. Don't confuse these with the fasteners holding the mirror cell in the tube!

Many commercial reflectors use three antagonistic pairs of bolts spaced 120 degrees apart. In each pair, one bolt "pushes" against the mirror cell, and the other bolt "pulls" upon the mirror cell. These can get very exasperating because the bolts must be adjusted a little bit at a time, while you check and re-check the view through the telescope. An assistant may be helpful.

Some reflectors replace the "push" bolts with strong springs (between the mirror cell and the back of the tube); through which the "pull" bolts pass. The springs are partly compressed by the tension on the bolt, thereby forming a rigid support which is also easy to adjust. If you are building your own telescope then I recommend this design. The valve springs from old engines are ideal, and readily available from car wreckers.

When your telescope is properly collimated, the view should resemble this final diagram. However, short-focus (f/5 or smaller) reflectors should have the image of the primary mirror displaced slightly down the tube ie: deliberately off-centre towards the primary.

You will get good views of stars at this stage, but for perfect images you will need to test star images on a perfect night. Or you can use a laser collimator installed in the eyepiece holder. Only slight adjustments of the primary mirror's tilt should be necessary.

For safety reasons, a laser collimator should not be used until the telescope is close to perfect adjustment. Shining a laser down a misaligned telescope can be hazardous to you and any bystanders.

If you have a Schmidt-Cassegrain you can collimate them by adjusting the tilt of the secondary mirror and (sometimes) the primary mirror. There will be three collimation screws on the secondary mirror holder (the "disc" in the middle of the corrector plate), possibly hidden under a plastic cover. There may also be a fourth screw or bolt in the centre of the secondary mirror holder -- DON'T TOUCH this one on a Schmidt-Cassegrain!

Aim the scope at the daytime sky or other bright, diffuse light source; and replace the eyepiece diagonal (if any) with your 35mm film canister or collimating tool. The view will be similar to the reflector diagrams above, except that you won't see the secondary mirror holder.

You don't need to worry about centering the secondary mirror or its holder, so skip to the secondary mirror tilting adjustments. This is a very sensitive adjustment on a Schmidt-Cassegrain so proceed carefully -- you should not need to do more than 2 turns on any screw. If a screw appears to "stick" it has reached the end of its thread; try adjusting the other two screws instead. DON'T TOUCH the centre screw/bolt (if there is one) or the secondary mirror may fall off inside the tube. You also have to avoid touching the corrector plate accidentally! Adjust the collimation screws until the reflection of the primary mirror is concentric with the edge of the secondary mirror AND concentric with the eyepiece holder.

Do not adjust the primary mirror without the manufacturer's explicit instructions, unless you know exactly what to do. Designs vary, even among telescopes from the same manufacturer; so what you think are the primary mirror's collimation screws may in fact be the screws that hold the mirror cell in place! Commercial Schmidt-Cassegrains are focused by moving the primary mirror; so there's a lot of hardware in there.

If you have a refractor or a Maksutov; do NOT attempt to do any optical alignment (or disassembly) at home, unless you are an optical expert and have the appropriate equipment. These telescopes are pre-aligned by the manufacturer and the relative orientations of the optical components are NOT random. In fact, better-quality specimens will have individually matched and tested components.

Before you send away one of these telescopes for expert attention, check that its "misalignment" is not in fact a faulty eyepiece or diagonal.

• I can't see anything!
  • Is it a clear night? Telescopes can't see through clouds you dimwit... ;-)

  • Have you removed the telescope's protective covers? I saw someone take nearly 100 photos of the October 1976 total solar eclipse through his telescope before he noticed that its front cover was still on. Totality ended about five seconds later....

  • Is the telescope properly collimated? A 120mm reflector was GIVEN to me as a "useless broken piece of junk" by someone who had tried unsuccessfully to use it for five years. The only fault was a badly misaligned secondary mirror -- which I fixed in 2 minutes. My children now use this telescope.

  • Is the telescope aimed at something visible? Some parts of the sky are devoid of bright objects. Try looking at the Moon (if it's up).

  • And finally, are YOU dark-adapted? Don't expect to see much if you have just come outside from a brightly-lit room, or if you are badly afflicted by light pollution. For more information on light pollution and what you can do to stop it, visit the International Dark-Sky Association (IDA) home page.

    Outdoor lighting in Australia is controlled by three Standards. AS1158 is for public lighting, AS2560 is for outdoor sports lighting, and AS4282 is for all other outdoor lighting.

• I can't find anything or it keeps moving out of view. First of all, telescopes -- even at low magnification -- view only a SMALL piece of the sky. At high magnification you view a TINY piece of the sky. Try looking along the telescope tube at your target to get aimed approximately. If you have no experience aiming a telescope, practice with the Moon first, using your LOWEST magnification! When you can find the Moon easily -- and you are accustomed to your telescope's movements -- you can then try the harder task of finding specific stars and planets.

  Secondly, all experienced observers find targets using a low magnification; then they centre the target in their view, THEN they change to a higher magnification. All "computer controlled" telescopes expect you to find some "known" stars as part of their initialization procedure - read the instructions. And don't expect a finder scope to be properly aligned (or to stay aligned).

  Unless your telescope has a tracking motor that can counteract the Earth's rotation, anything in the view will soon move out of the view, and new objects will come into view instead. At high magnifications this can happen in a few seconds, but it takes longer at lower magnifications. You can use this movement to find astronomical east and west in your field of view -- objects come in from the east and "move" to the west if the telescope is kept motionless.

  However you should check that the telescope itself doesn't consistently move up (or down) whenever you aim at something and let go of the telescope. This is a symptom of poor tube balance or a mechanically inferior mounting. Department Store Telescopes are often afflicted with both of these problems.

• Everything's blurry or everything's just a big blob.
  • Are you focused? Stars should focus down to POINTS of light, not discs!

  • Are the optics clean? Check the eyepieces first.

  • You may have a defective eyepiece. Try a known good eyepiece.

• Bright objects have rings or spikes or coloured haloes.
  • A faint blue (or violet) halo is normal for refractors and simply indicates that blue light is not being focused to exactly the same point as the other colours. You can see this phenomenon best by looking at Venus or the bright edge of the Moon. You should NEVER see this phenomenon in a reflector or Schmidt-Cassegrain and it will be very difficult to detect in the best refractors.

  • Tiny faint rings around bright stars are normal -- these are known as Airy Rings and are an example of diffraction. Diffraction is a fundamental optical phenomenon caused by the wave-like nature of light, and you cannot stop it happening. This image shows a highly magnified view through a refractor.

    Other telescope types show similar (but usually brighter) Airy Rings, which in reflectors may be accompanied by diffraction spikes radiating from the star. The spikes are caused by the spider arms. Each arm generates TWO diffraction spikes 180 degrees apart, so 3 arms will generate six spikes and 4 arms will generate eight spikes. However with 4 arms exactly 90 degrees apart, four of the spikes will overlap the other four spikes and be "hidden" from view. Some reflectors may use curved spider arms to eliminate the spikes, by spreading the diffracted light around the image.

    Diffraction also causes the rings (and spikes) around bright stars on astronomical photos.

  • Multicoloured haloes (or any halo) may be caused by a poor-quality eyepiece or objective lens, dirty or greasy optics, or high cloud. Contact lens wearers may see pearly-looking haloes around bright objects if their contact lenses are not clean.

• Bright objects have "ghosts"
  • Your eyepiece may not have anti-reflection coatings on its internal surfaces. Get a better eyepiece.

  • Some junk telescopes suffer from internal reflections inside their tubes, because the manufacturer didn't bother to paint the inside of the tube black.

• Everything's upside down and/or back-to-front!
  • Upside down (ie: field rotation) is NORMAL for an astronomical telescope. If this bothers you (or you bought the scope for birdwatching) then you can get erecting prisms for many telescopes. Or buy the equivalent "terrestrial" or "spotting" scope model. However by doing this you will lose some light (to absorption) within the additional optical components.

    If your view is upside-down -- or sideways -- when compared to a standard starchart; don't worry, just rotate the chart to match what you're seeing. The real difficulty is with back-to-front views....

  • Back-to-front (a.k.a. reversed or "mirror image") is also normal for telescopes with an ODD number of reflections; such as Schmidt-Cassegrains (primary mirror + secondary mirror + eyepiece diagonal) and most refractors (eyepiece diagonal). This can be very confusing for beginners when they try to match the eyepiece view to a (non-back-to-front) standard star chart. Many computer star atlases can print charts any way you want.

• Stars near the edge of the field have tiny "tails" or stars change shape when I focus.

  The tiny tails are caused by coma, which becomes increasingly obvious at shorter focal ratios (f/4 or below). Coma correcting accessories are available (but they are expensive).

  The shape change vs focus is most likely caused by astigmatism -- one or more optical components are not perfectly symmetrical. You will see the image's direction of elongation change as you move from one side of "focus" to the other. The offending component can be identified by rotating it about its centre, whereupon the astigmatic image will rotate too. In practice, the eyepiece is the most easily tested component -- just spin it while it's in the eyepiece holder.

  A mild astigmatism may be caused by excessive pressure on the optical components. If it's a reflector, check that mirror retaining devices are not pressing hard upon the mirrors, and that the primary mirror cell is evenly supporting the mirror's weight. If astigmatism is severe then the offending component(s) should be replaced.

  If a new telescope is astigmatic, you should return it and demand a refund.

• The whole field doesn't come into focus at the same instant or there seems to be a "ring" of best focus.
  Your telescope mirror has spherical aberration -- the same problem that crippled the Hubble Space Telescope. The only real cure is replacment of the optics. If this is a mirror you have made, you will have to return to your "optical figuring" stage. If a new telescope has spherical aberration, you should return it and demand a refund.

Harold Suiter's book Star Testing Astronomical Telescopes is an excellent reference for diagnosing and fixing all sorts of optical problems.

So you still want to read about some wierd telescope problems? Let's see...a large spider constructed its egg sac upon the secondary mirror of one of my telescopes. One of my friends found her cat asleep inside her telescope tube one night. And I know of one amateur observatory that was wrecked by a mouse plague. The mice chewed through just about every non-metallic object (including the insulation on electrical wiring), and mouse urine proved to be devastatingly corrosive to optical and electronic components. Yuck!

The typical manufacturer ruins these scopes by supplying cheap dodgey eyepieces, and putting it on a flimsy wobbly mounting. However, a surprisingly large number of Department Store Telescopes do contain some reasonably good primary/secondary mirrors, or a decent achromatic objective lens. Some "telescopes" are best used to support tomato bushes; but many others can be resurrected by doing a few things that the cost-cutting manufacturer didn't.

The first upgrade is some decent eyepieces. If your scope can accept 32mm (1.25 inch) diameter eyepieces, then you have a huge selection of aftermarket eyepieces to choose from. If you're stuck with 25mm (0.96 inch) diameter eyepieces, then your choice is more limited -- but you can certainly get better eyepieces than the ones supplied with the telescope!

Even the traditional Orthoscopic or Plossl-type eyepieces will noticeably improve the typical Department Store Telescope. These eyepieces are available in both 25mm and 32mm diameters from most dealers and will probably cost you $Aust70-130 each. You can try more exotic (and expensive) eyepiece designs but you're unlikely to get a correspondingly better image.

The second upgrade is a steady mounting. One quick fix is to suspend a few kilograms underneath the telescope, between the legs of the tripod. This lowers the scope's center of gravity (improving stability) and holds the legs more firmly onto the ground (reducing vibration). A couple of bricks is sufficient. Don't overdo this remedy. I saw one owner try this with a concrete paving slab, which promptly collapsed the mounting and bent the tripod legs!

Even a stable tripod is unhelpful if the parts connecting it to the telescope tube are badly designed (or built). For example, some tubes are held by a single bolt passing through a flange on the underside of the tube. This is inherently wobbly, because the tube is supported at a single point which isn't at its centre of gravity. It's also exasperating to aim at anything. Other sources of wobbles are the thin-walled tubing used for construction, and the lack of control over mechanical backlash.

Another solution is to build an entirely new mounting. Reflectors can be converted to a Dobsonian mounting. A modified mini-Dobsonian can be used to support a refractor. If you're good with tools you can try building a German equatorial mounting for your telescope, using stronger and larger components than the original. For example, galvanized water pipe fittings have been used for decades in home-made telescope mountings.

Some general rules for mounting design include:

• The telescope tube must be balanced between its points of support.
  
• All rotating sub-assemblies must be balanced around their axes of motion.
  
• All moving, swinging, rotating components should be able to be clamped into position easily. Alternatively, sufficient friction should be built into the mounting to prevent the telescope moving suddenly in a breeze.
  
• The telescope should be difficult to knock over.
  
• You should be able to look through the eyepiece no matter where the telescope is pointed.
  

Note that these mountings are intended for visual observations only. They are not suitable for the precise tracking needed for astrophotography or long-exposure CCD images. However you can purchase computerised finding devices, which can tell you where the telescope is pointed and also tell you where to move the scope to locate a given object.

Mel Bartels has an excellent description of How To Motorise A Dobsonian; as well as links to amateur telescope-making sites.

Many books and magazines specifically about telescope making have been published; and amateur astronomy magazines frequently have articles on this subject. Thousands of amateurs have homemade telescopes and/or mountings, so don't be at all embarrassed about taking your masterpiece to a viewing night. If your mounting is strong, durable, doesn't wobble, is immune to breezes, holds the telescope safely, and allows it to move smoothly and controllably; then it's a good mounting. Even if it needs a coat of paint....