DIY small scope improvements

How to improve a small telescope

Small telescopes are a great way for many people to get started in astronomy - with 60mm and 70mm scopes now priced in the immediate area of $100 or so, it has never been more affordable to get one.  Unfortunately, in many instances, that means it has never been easier to be quickly disappointed by what can be seen in the sky.  Some aspects of this may be almost entirely out of the control of the telescope owner - light pollution or local light trespass, for example.  But others are the fault of the manufacturer looking to hit a certain price point - and heck, affordability is the reason it got purchased, right?  Well, someone likely cut some corners in the design of that telescope, but that doesn't mean it's unusuable.  It just means putting a little effort into improving it, and fortunately, often that can be accomplished for little or no cost, though some improvement will be more pricey.

Main lens or mirror 

The good news here is, this is likely the one area where the manufacturer did NOT cut costs.  Most small refractors and reflectors have surprisingly adequate optics - but notice I did not say "good" or "great".  Don't be fooled; they are not - and never will be - great.  However, they are perfectly usable, and at this stage, it is likely difficult to notice the subtle differences between a "good" scope" and a "great" one anyway, much less how adequate optics perform.  A few important points:

  • Purple (or better, green) anti-reflective coatings on a refractor lens are best - red, yellow or other colors are more for show than anything
  • A highly polished-appearing surface; scratches can create diffraction and light scatter (more on that below)
  • For reflectors, ideally a parabolic mirror, but if spherical, that's okay (that's discussed later)
  • A longer focal ratio - calculated by dividing the focal length by the aperture in mm, so f/8 or f/10 are usually best for inexpensive scopes

PICTURE ON RIGHT HAS BOTH GREEN AND PURPLE COATINGS (AS SEEN IN REFLECTIONS FROM FLASH), AS DIFFERENT LENS SURFACES HAVE DIFFERENT COATINGS ON THEM.

Note that a little bit of dust on a lens or mirror will not affect performance.  What will actually reduce the performance of a lens or mirror substantially is scratches from cleaning it using the wrong methods!  For all but the worst-looking lenses and mirrors, simply use compressed air to GENTLY blow off loose particles.  A VERY CLEAN fine / soft haired brush may be used to GENTLY remove any other easily removed particles.  In 99% of cases, any dust left WILL NOT affect performance, so don't risk scratching a lens or mirror by trying to remove it.  If the lens or mirror really is that bad, see this site.

Finder 

Small telescopes are likely to have either a very inexpensive 5x24 finderscope that magnifies 5x with a 24mm aperture, or a BB-gun style "Red Dot Finder" (or RDF) that does not magnify the image but lets the user "point" the scope where the red dot appears on the RDF's window.  For less expensive scopes, the RDF style finder is probably easier to use, but is better suited for darker skies.  Unfortunately, most 5x24 finderscopes are not much better, but the key here is to focus as accurately as possible to provide the sharpest image, and align them carefully with the main telescope.

Tripod

Sadly, this is one area where the manufacturer probably cut corners the most, and fixing it can be done in several different ways - some are more involved than others.  New telescopes today are often provided an aluminum tripod with VERY thin metal.  Older ones were often produced from lightweight, open-grained woods.  These is prone to vibration and wobbliness, which when looking at a planet at 150x, also means every vibration from the tripod is magnified 150x!  It doesn't take much movement to cause the image at the eyepiece to dance around.  A few simple steps to improve performance:

  • Hang a milk jug or soda bottle partly filled with sand from the center of the mount, or place it on the tripod's tray to minimize vibrations. 
  • Tighten all wingnuts and screws on the tripod (without overtightening) to minimize flex in the whole structure.
  • Place the tripod on a well-watered (but not wet) field of grass, if possible, as opposed to pavement or concrete

If all else fails, try building a new tripod altogether, or consider purchasing a better one.  Though it it worth mentioning that many objects in the night sky do not require high magnification, and a slightly wobbly tripod may not prove to be too much of a problem.  To determine if a tripod needs replacing after working to improve it, try this test:

  • Set up the telescope, aim it at the Moon or planet, and place a lower power eyepiece in the focuser.
  • While viewing the object, give the telescope a moderate tap - enough to move the tube, but not knock the tripod over!
  • Count how many seconds it takes for the telescope to "settle down" and stop vibrating.
  • 2-3 seconds is acceptable; 4-5 seconds is poor performance, and 6 seconds or more means a different tripod/mount option is probably required

There are replacement wooden tripod legs available; check with that retailer to determine compatibility.

Mount 

Back in the 1950's through the 1970's, most amateur telescope makers placed the telescopes they sold on mounts that were adequate for the weight/mass of the telescope.  (The same cannot be said of the tripods, but see above for that.)  Own a telescope from that era?  Congratulations!  It is likely a decent instrument, needing just a little work on the tripod to keep it from being wobbly, and perhaps a little cleaning of old grease / relubrication to return it to useful performance.

Newer telescopes from about the 1980's on are a different matter.  The optics remain adequate, but in many cases the mounts are not.  Often a 5 or 6 or 8 pound telescope is place on a mount that should probably not be holding more than about 2 or 3 pounds, and that is unfortunate.  But there are ways to fis this:

  1. Build a wood alt-az mount for a small refractor
  2. Build a wood Dobsonian style mount for a small reflector
  3. Purchase a new mount that is adequate for the weight of the telescope (look for used equipment here or here)

But there are other ways to improve a mount.  Simply taking them apart (assuming some mechanical aptitude), cleaning, regreasing and reassembling can go a long way towards improvements.  Tightening slopping adjustments where gear teeth mesh can help.  Tightening screws, nuts and other connections can make a world of difference!  Just keep in mind that there is no subsitute for an adequately massive mount for a given telescope size.  It is sort of like trying to put an automobile on bicycle wheels/tires, and expecting it to perform well.

Eyepieces

The most likely scenario with most small telescopes is the following: The long focal length eyepiece (a 20mm or 25mm one) is probably useful, and worth saving while the short one (perhaps a 4mm or 6mm) is probably junk and will make a not-very-good paperweight.  However, there is good news: A decent eyepiece that will vastly improve higher magnification views can very likely be used in any future telescopes purchased.  Alternatively, if purchased on the used market (at Astromart or the CloudyNights.com classifieds, for example) then it should be possible to re-sell it for the purchase price.  But there are some things of which to be aware: Eyepiece barrel diameter, eyepiece lens design, and anti-reflective coatings.  Learn all about beginner eyepieces in this video.

Focuser

This is a sometimes-overlooked section of small telescopes.  Old, small scopes have metal ones - which may have decaying felt in them.  Newer ones are likely plastic, and produced with poor tolerances.  The fastest, and quickest, way to make a simple adjustment to try and improve focuser performance is to look under the focuser.  There may be a plate with two or four screws in it.  This often is pressed up against the axle that holds the pinion gear, that engages the rack gear on the focuser tube.  By tightening these screws, the performance of the focuser MAY be able to be improved.  It also might just make the focuser knobs harder to turn.  if that's the case, back off the tightening of the screws until it moves smoothly.

For the more mechanically inclined, remove these screws and the plate.  Check out the grease on the pinion and rack.  Is it sticky/gooey?  If so, try cleaning it with a solvent like denatured alcohol, or a plastic-safe one like "Goo-Gone" or "Goof-Off."  Then replace the grease with a tiny bit of automotive or white lithium grease.  

If the focuser tube is "sloppy" in the focuser frame, try replacing the felt (in older telescopes) or removing the felt or plastic strips and replacing them by layering up 3 mil thick / 3/8" wide strips of Teflon/PTFE tape (McMaster Carr carries this).  Layer the tape until the tube fits snugly - but still slides freeling - inside the focuser mechanism.  Replace the plate and screws over the focuser knob/axle, then tighten until the entire mechanism moves smoothly without binding or flopping around.