Collimation Cleaning Re-coating Other Repairs

Telescope Repairs and Refurbishing

Whether you have owned and cared for your telescope for some time, or you have gotten a great deal on a telescope that needs some care and attention, you will eventually need to do some repairs or tuning up. If the manufacturer of your telescope is still in business the simplest answer is usually to return it to the manufacturer for the service it needs. However, that is not always possible or necessary. As the body of this page grows, the information and links should help you in making the needed adjustments to put or keep your telescope in top condition.


The most common adjustment that needs to be made to telescopes is to align the mirrors and/or lenses so that they are on the same optical axis. Making the adjustments is called collimating the optics. It sounds much more complicated than it really is, it just takes some patience, practice and perhaps a screwdriver or hex wrench.

The best collimation is done while aiming at a star. If you have an equatorial mount, you can collimate while pointing at almost any bright star, since tracking will be easier. However, if you have an alt-az mount like a Dobsonian you will wnat to pick a star close to the horizon, or perhaps a light several miles away, to make it easier to follow the star as you make adjustments. Alternately, many have now begun to collimate their instruments using a special laser projecting unit that takes the place of the eyepiece. This makes collimation possible regardless of the telescope type or the sky conditions.

Much has already been written about collimating the different types of telescopes, so I will just include links here to other sites with collimation procedures for each telescope type. A link is also included to a laser collimator sales site that has excellent information on how to use a laser to collimate different types of telescopes.

One thing that is often not mentioned in the descriptions is that when turning the adjusting screws on the corrector of an SCT or the mirror of a Newtonian telescope, the star will often move right out of the field of view. It's best then to start your collimation with as low a power eyepiece as you have, and progressively go to higher powers to complete the collimation procedure.

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Clean optics are the best optics. But don't take that to mean you should clean your optics often. All telescope optics, whether mirrors or lenses, have coatings that can be easily damaged or gradually worn away by cleaning. The performance (and value) of a mildly dirty optic with good coatings is far better than clean optics with damaged coatings.

What to do?

  1. Try to keep the optics dry with a dew cap or heater, and if they get wet from dew don't cover them until they become dry!
  2. Store your telescope in a dry place! Temperature isn't that important, but humidity is.
  3. Keep your optics covered except when in use!
  4. When your optics get so dirty you can't stand it any more, clean them.

The first step in cleaning, and often the last, is to remove the dust. But don't just grab the closest thing that looks like a rag and start rubbing. If you have ever looked at dust under a microscope you will better understand what it can do to your optic coatings. Dust usually has sharp, angular corners, so you want to pick it up, not smear it around. One way to do this is by using commercially canned compressed air to blow the dust away. Be careful not to get too close to the optics with the spray wand, so that the occasional slug of liquid propellant doesn't spot the glass. Another way to remove dust is to use a clean camel hair brush such as is sold in camera stores for lenses. Tilt the telescope down so the released dust falls away, and dab very gently with the brush to dislodge the dust that is sticking to the optic. Hopefully you are done. But occasionally the dirt is more persistent.

Refractors, SCTs and Other Catadioptrics: You'll need a liquid lens cleaning agent and a cleaning tool. For the cleaning agent you can use a commercial photographic lens cleaner. As an alternate you can mix 2 cups water, one cup isopropyl alcohol and two drops of dish washing liquid. The dish washing liquid can be antibacterial but is best unscented and without additives "to soften hands." For the cleaning tool you'll want soft clean cotton. You can use the corner of a 100% cotton tea towel if you're sure it hasn't been used for anything else, but don't try to use a regular towel or tee shirt just because it's cotton. The best and safest thing to use, if you can get it, is sterilized surgical cotton. After the dust is removed as per the above procedure, wet and squeeze dry the cotton. Gently dab at the optic from the center out, turning a new section of the cleaning cloth for each dab. Keep a regular pattern to cover the whole surface and either discard the cotton swab when you have used all its clean surfaces. If you are using the mixture described above for your cleaner, you may have the option to thoroughly and frequently wash your cleaning tool instead of discarding it. Afterward use another piece of dry cotton (or the dry end of your tea towel) to touch away any left over droplets.

The cleaning described should only be done for the front optical surface. If it is obvious over time that internal elements are dirty, either ignore it, or get someone who knows the optics to clean it for you. This may done by the manufacturer, or by one of the services listed under "Other Repairs". They will also check optical alignment after cleaning, so you get a two for one deal in most cases.

Reflectors:After the cell is removed from the tube or the mirror box remove the dust as described above and put her back. For an occasional thorough cleaning, the mirror can be rinsed and/or washed.

NOTE! The primary mirrors of some modern reflectors are permanently mounted on a wood or particle board cell. DO NOT TRY TO REMOVE THE MIRROR FROM THIS TYPE OF CELL! To clean the mirror with this type of cell you will have to put some heavy plastic (like from a thick garbage bag) around the wood or particle board and carefully tape the edges of the bag to the edges of the mirror. This must be done with substantial tape like mylar packing tape, not cellophane or masking tape. (And don't use duct tape!) This may not keep water completely out but at least the cell won't get completely soaked in the process.

Many primary mirrors are housed in an aluminum cell and can be removed from the cell. If so, CAREFULLY removed the retainers to avoid scratching the mirror. I usually put my mirror in a clean, shallow plastic pan at the telescope before I move the mirror to the sink. (If you think about what could happen if you dropped it or banged it against the side of an enamel sink, you probably will too.) Simply fill the pan (or clean stainless steel sink) with lukewarm water and a couple drops of dish washing liquid. The cleaning tool and method of cleaning is the same as used for the other telescopes above, only you can do the whole operation while the mirror surface is under the water. The water will act as a lubricant during the cleaning process. After the mirror is cleaned, remove it from the plastic tub (or sink), drain, and rinse the mirror with distilled water. This can be purchased by the gallon at the grocery store. After rinsing, place the mirror on edge on a towel in a safe place where it can rest at a steep angle against a vertical surface. The mirror should air dry. I usually put my mirror (on a towel, unless you want to be severely reproved by your wife or mom) against the back of an upholstered chair.

If the telescope is stored properly, the secondary mirrors should not need to be cleaned except to remove some accumulated dust every GREAT once in a while. It's a good thing because they are much harder to remove and replace in most telescopes. This is not usually recommended unless you have some experience with telescopes.

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After many years, or perhaps after an unfortunate accident, your mirror or lens may need recoating. One individual wrote me about leaving the eyepiece cap off his focuser when his reflector was stored in the garage. A mouse got in through the hole and fell to the bottom of the telescope. As you can imagine this worked out to the demise of both the creature and the coating. There are now fewer choices for recoating than in the past, but the links below will take you to the websites of three that still do personal work.

How do I know if I need a new coating?

This might be obvious from many tiny little scratches in the coating due to overzealous cleaning. However coatings can deteriorate over time without it being obvious by just looking at the mirror. To check, if your mirror can be removed from its cell, do so. Look through the mirror backwards at a bright light. If you see a few tiny pinholes there is nothing to worry about, but if you see many pinholes or areas where you can see through the mirror its about time for a recoat.

What kinds of coatings are there?

Reflective coatings fall into categories based on reflectance, durability and cost.

Beral:H.L. Clausing (formerly P.A. Clausing) has been serving the astonomical community for many years. Among the many coatings they supply is one of the least expensive, called "Beral." It is much harder than aluminum and has an average reflectivity of 91%. The cost for stripping and recoating an 8" mirror is incredibly only $35 plus shipping, etc. The only drawback to this coating as compared to a standard aluminum coating is that the life expectancy is reduced, due to not having a protective overcoating. How much less depends on how you care for your mirror, but a Beral coating should easily be good for three or four years.

Standard Aluminum:A standard aluminum coating with the requisite quartz or silicon overcoating (sometimes called "Protected Aluminum") gives you about 90% reflectivity. However, in part because of the multi-step coating process the cost is higher. Typical cost to strip and recoat an 8" mirror is $80-$100.

Enhanced Aluminum:This coating increases reflectivity to about 96%, giving you the most light for your aperture. Naturally, the cost is higher, and can range from $100-$200. If you've got an expensive, high performance mirror however, the additional cost is probably worth the investment.

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General Maintenance and Other Repairs


If you need a major cleaning or a more professional optical alignment and you cannot or don't want to send your telescope to the manufacturer, here are some options to check.

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© 2001-2007 - R. Pollock
Page Revised: February 1, 2007