Equipment & Maintenance
Telescope Tune-Ups
Look through an astronomy magazine and you'll find an abundance of telescope advertising — not to mention articles describing objects for a telescope to explore. But there usually isn't much to tell you what to do when something isn't working right. It can be disconcerting to discover that your primary mirror has a dust layer thick enough to write your name in, or that the inside of the tube has taken on permanent arachnid residents. Maybe the mount has developed a wobble too.
Most telescopes are surprisingly tough. Pyrex mirrors are quite resistant to shock damage. A novice just needs to devote a little care and patience to become proficient at handling most maintenance issues. A surprising amount can be done with simple tools and common sense.
Collimation
Collimation is the alignment of a telescope's optical elements so that they share a common axis. It matters most for reflectors (Newtonians and Dobsonians) and Schmidt-Cassegrains, which have multiple mirrors that can drift out of alignment. A miscollimated scope produces stars that look like tiny comets — a bright core with a flare to one side — and fine planetary detail is impossible.
Newtonian / Dobsonian Collimation
A Newtonian reflector has two mirrors: a large primary at the bottom of the tube and a small flat secondary diagonal near the top. Collimation is a two-step process:
- Center the secondary mirror under the focuser and orient it so the primary's reflection appears centered within it.
- Adjust the primary mirror's tilt so the reflected image of the secondary (and your own eye at the focuser) appears centered in the primary's reflection.
A collimation cap (a $10 plug with a central hole) or a laser collimator makes this process much faster and more precise. Many experienced observers can collimate a Newtonian in under 5 minutes with practice.
A quick on-sky check: defocus a bright star until you see concentric rings of light (diffraction rings). If the dark shadow of the secondary is centered in the rings, you're collimated. If it's off to one side, go back to the adjusters.
SCT Collimation
Schmidt-Cassegrains are collimated by adjusting three screws on the secondary mirror housing at the front of the tube. A slight turn of one screw at a time — while watching a defocused star — is the method. SCTs hold collimation well and many owners rarely need to adjust them, but a hard bump to the front can knock them out.
Refractors
Quality refractors are typically collimated at the factory and rarely need adjustment. If a refractor appears miscollimated, do not attempt to adjust it yourself — the lens cells require precision tooling. Contact the manufacturer.
Cleaning Optics
A dirty mirror or lens reduces contrast and can cause internal reflections, but overcleaning is a far more common problem than undercleaning. Dust on the primary mirror of a reflector has a surprisingly small effect on image quality — a dusty mirror is better than a scratched one. Clean only when the dust or contaminants are clearly degrading your views.
Cleaning Mirrors (Reflectors)
- Remove the mirror carefully. Most primaries are held by a retaining ring or clips. Note the orientation before removing.
- Blow off loose dust first with a rubber bulb blower or clean compressed air (held upright to avoid propellant). Never use breath — moisture and saliva damage coatings.
- Wash gently in a large container of room-temperature water with a few drops of dish soap. Use cotton balls — never paper towels — with gentle straight strokes. Do not scrub.
- Rinse thoroughly with clean water, then a final rinse with distilled water or isopropyl alcohol to prevent water spots.
- Air dry face-up in a dust-free area. Never wipe dry.
Cleaning Eyepieces and Lenses
Eyepiece lenses and refractor objectives should be cleaned with lens-cleaning solution and proper lens tissue or microfiber cloths designed for optics. Use a circular motion from center outward, with very light pressure. A blower brush removes loose particles before any wet cleaning.
The inside surfaces of eyepieces should never be touched. If internal dust becomes an issue, return the eyepiece to the manufacturer.
Mount Maintenance
Altitude-Azimuth (Alt-Az) and Dobsonian Mounts
Dobsonian mounts use Teflon pads riding on laminate or Ebony Star surfaces. If the motion feels rough or sticky, check for grit between the surfaces — even a single grain of sand can cause a rough feel. Clean the bearing surfaces with a damp cloth, dry thoroughly, and the motion should return to smooth. Some owners add a thin layer of furniture wax to the laminate for extra smoothness.
Equatorial Mounts
Equatorial mounts have worm gears and clutches that occasionally need lubrication. If your mount has a periodic error that seems worse than before, or the clutches are slipping, consult your manual for lubrication points. Most modern EQ mounts use grease that lasts for years under normal use.
Check that all locking knobs are working smoothly and that the polar axis is not developing slop. If the mount feels sloppy in the polar axis, the bearings may need adjustment — a job for our tune-up night.
For computerized (GoTo) mounts, keep the firmware updated and recalibrate your alignment model periodically. A poorly aligned model causes targets to land outside the field of view even with perfect mechanics.
Common Problems and Fixes
| Symptom | Likely Cause | Fix |
|---|---|---|
| Stars look like comets or have a flare | Miscollimation | Collimate the optics (see above) |
| Images are soft at all magnifications | Dirty optics or bad seeing | Wait for better seeing; clean if optics are visibly dirty |
| Image shakes and won't settle | Mount vibration; focuser too stiff | Tighten mount bolts; use a vibration suppression pad |
| Eyepiece fogs up | Dew forming on glass | Use a dew heater on the eyepiece; or warm it gently |
| GoTo mount misses targets | Poor polar alignment or alignment model | Re-polar-align; redo 2- or 3-star alignment |
| Focusing is very sensitive — one half-turn goes too far | High-ratio focuser needed | Add a focus motor or a low-profile Crayford focuser |
Getting Help
HVA members have collectively hundreds of years of telescope experience and a genuine love of helping people get the most out of their equipment. Bring your scope to any club meeting and someone will take a look. You can also email hvabod@groups.io with questions.
For in-depth reading, Sky & Telescope and the Cloudy Nights forums (cloudynights.com) have thousands of articles and threads on every aspect of telescope maintenance and repair.