Fixing Common Telescope Problems


Last updated 2001 January 31

If you have come directly to this page from another Web site, I strongly recommend that you also read my main page on Buying your first telescope in Australia. It contains a lot of more general information about telescopes; and I'm going to assume you have read it. My web server space is limited...


Cleaning telescope optics

This section describes simple maintenance you can do successfully at home if you are careful. If the telescope (or binoculars) are extremely dirty, stained, peeling, mouldy or corroded; you should send them to a specialist for refurbishment (eg: the Binocular and Telescope Service Centre or the Binocular and Telescope Shop).

"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.
You may also need some clean screwdrivers, spanners, etc for minor dismantling.

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:

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:

To clean the mirrors of a reflector:


Optical alignment

[Collimation

- everything misaligned]

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.

[Collimation

- secondary holder aligned]

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.

[Collimation

- secondary mirror aligned]

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.

[Collimation

- everything aligned]

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.


Common optical problems

This section could easily expand into a collection of wierd and wonderful stories so I'm going to restrain myself to a checklist for now :-)

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!


Improving a Department Store Telescope

If you have bought one of these things -- or got it as a gift from some well-meaning relative -- then you will probably have to modify it to avoid disappointment. Ignore the manufacturer's claims of the scope's maximum magnification; read my telescope page for the REAL facts about magnification.

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:


Building an altazimuth or Dobsonian mounting

You can download schematic diagrams (48kB) for a reflector and a refractor mounting. These diagrams include plenty of notes. I deliberately haven't included dimensions, because they are dependent upon your telescope tube size and the materials you have for construction. Use these schematics to help you prepare some actual plans for your telescope.

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....


Back to telescope page
Buying a telescope if you are disabled

URL for this page : http://www.fraserf.id.au/astronomy/scope/fix-it.htm
Copyright © 1996-2005 Fraser Farrell. All rights reserved.

There is intelligent life on Earth, but we are only visitors :-)