Keith B Quattrocchi, MD, PhD                                                                                                                                                                                                            December 28, 2005



A Practical Guide to In-Situ Cleaning of an RCOS 16” Mirror


            DISCLAIMER:  This description of mirror cleaning could lead to PERMANENT DAMAGE to your mirror.  There is NO GUARANTEE implied in this article.  It is NOT a scientific study.  It is a practical guide to what I have been doing over the past year and is based on numerous discussions with other amateur astronomers who face the problem of mirrors coated with dust and/or pollen caked on by dew events. I have put this down on print in the hopes that it will be of assistance to others.   RCOS mirrors, with or without ion milling, are made to exacting specifications and can be easily damaged. They are coated with only a few microns of aluminum which is protected with a few microns of SiO (silicon monoxide). Obviously it is easy to damage these coatings.  This note is NOT intended as a proven guide to how a mirror can be properly cleaned.  For that you can call RCOS or Aries Optics and learn from the experts.  They will likely have you remove the mirror for cleaning.  Usually the mirror is then washed repeatedly (alternating “cleaning solutions” and distilled water, and then “blown” dry). This is an alternative for cleaning and RC-16 in situ (even the experts will clean their optics in situ, if necessary).   


1)  When to clean your mirror:  The short answer, as infrequently as possible.  A little dust on the mirror is harmless and shouldn’t affect the scopes performance.  A simple blast from a “Snow Gun” (CO2 Gun, see below) will often remove dry dust.  Cleaning one’s mirror “right” is a very involved subject.  One great review can be found as part of the National Optical Astronomy Observatory (NOAO) and was written by Gary Poczulp back in 2001 This can be found at: . This site details how their mirrors are monitored and cleaned using a variety of techniques, including cleaning with C02 guns, mild “soap” solutions and various solvents, depending on the contaminants.  Another useful link is the European Southern Observatory  ( ). They note problems using a snow gun (CO2 gun) under conditions of high humidity and discuss their criteria for mirror cleaning. 

             It’s worth noting that the SiO coatings over the aluminum are only a few microns thick, so any cleaning risks injuring them.  Also, some of the cleaning solutions (such as ammonia, which I have used) can react with any exposed aluminum coatings (in theory, and perhaps in practice).  Also, it is VERY easy to scratch the mirror during cleaning (I have done this).  If only the outer silicon coating or aluminum coating itself is scratched, then the problem is gone once you re-coat your mirror.  However, if it’s any deeper you are left with a problem that, to the best of my knowledge, can not be repaired.   

            It all comes down to a tradeoff.  I was repeatedly told by persons much more knowledgeable than myself: 1) Don’t clean your mirror except under the most dire of circumstances and 2)  If you do, remove it before cleaning.  The first comment has largely come form those in low humidity areas such as the southwest (plenty of dust which can be nicely removed with a snow gun, but fewer dew events which can paste dust and pollen to your mirror).  The second comment comes from astronomers with more time than I have.  With a family and full time job and family, I can hardly remove, clean and re-collimate my mirror every time it is caked by a dew/dust event.  In high humidity areas with dust or pollen, there are times when we awaken to a mirror coated with dust/pollen all but glued onto the mirror.  It is only when this happens and becomes thick that I remove it.  I do not actually test for reflectivity, but this would be a good way to make the decision.  It would also be a good way to track the deleterious effects of cleaning on a mirror.  Unfortunately I haven’t reached that point of sophistication. I simply wait until the mirror looks dusky and poorly reflective. 


2) Note on the Snow Gun:  Suffice it to say this is a standard.  They are sold, fully assembled and with great instructions, through New Mexico Skies ( ).  I highly recommend having one.  I have found, however, that they are not very effective in 2 important situations: 1)  when dust/pollen is caked onto the mirror from dew events and 2)  When the humidity is high (>60% is the number I’ve frequently seen quoted). 


 3)  Note on Collodion:  There are several sites which discuss this and many swear by it.  I bought the stuff and found it was NOT workable with the mirror in place.  I tried repeatedly but could not get the technique down to the point that even large parts of the mirror were collodion free (after removing the cheesecloth). Two good sites for the use of collodion are and .     My thought is it’s a great technique for smaller mirrors and those which can be easily removed. 



4)  Cleaning Solutions and Tissues:

            There is a good deal of debate over what to use and what not to use.  Mild diluted detergent and distilled water is the most common “solution” used, and will remove most contaminants from the mirror (again, see the NOAO site for a discussion of more exotic solutions for more difficult contaminants). For us lay people, it would seem reasonable to use diluted detergent/distilled water for most cleaning purposes and isopropyl alcohol for stubborn stains.  There are many who feel strongly that solutions containing ammonia should not ever be used.  The argument is that it can react with the aluminum coatings.  I’ve talked to several experts who disagree with this, noting that there are SiO coatings which protect the aluminum.  They also note, anecdotally, that they use ammonia containing solutions when necessary and have not found it to be a problem.  Most of what I’ve read indicates the best cleaning solutions include:

1)   A mild diluted detergent soap (e.g., 4-6 drops of “Dawn” in a quart of distilled water).

2)   Organic solvent.  Two popular ones are Isopropyl alcohol (91% usually available at Wal-Mart), or Edmund’s Liquid Cleaner ( available at ). 
The Edmund’s product seems to leave much less residue. 

3)  Ammonia solution (original “Windex” formula frequently sited).  If using any ammonia containing product be sure NOT to leave it on the mirror for any significant length of time (back to the issue of potential damage to the aluminum coating). 

            There is reason to use ammonia sparingly, as any exposed surface of the mirror’s aluminum coating will be potentially damaged by ammonia.  If detergent or dilute ammonia are used, it is critical that these agents not dry on the mirror.   They need to be immediately rinsed clear with distilled water.  If isopropyl alcohol is used, then distilled water is not usually considered an essential last step, but it may still be a wise one (most isopropyl alcohol solutions contain some contaminants).  In fact, many use isopropyl alcohol after dew events where the main damage is water streaking without any significant contaminants.  This seems reasonable (again, every cleaning will damage the mirror to some degree).

            Areas which won’t come clean with diluted detergent can be spot cleaned with xylene (see NOAO site above) or cleaning with isopropyl alcohol. 

            Again, one key point is that when using ammonia or any soap containing solution, never let it dry on the mirror.  It should be rinsed off with distilled water. 

            For cleaning solutions (see Figure 1) I have used diluted “Dawn”,  Tech Spec Lens Cleaner (fancy form of ammonia) or plain old Windex (notice it’s the “original” formula, as I don’t want anything unexpected in the solution).  The Windex bottle on the left contains distilled water. I didn’t include an image of the Windex bottle with diluted detergent.  The distilled water can be obtained from a grocery store (be SURE it’s DISTILLED, such as “Poland Springs” water, and not just “pure”).  Tech Spec Lens Cleaner can be obtained from Edmund Scientific Company.  

            What tissues to use is another subject.  One popular tissue are Kaydry EX-L Delicate task Wipes, available through Markertek ( ).  These tissues are unusually soft and tend not to leave a static force.  Kimwipes are also popular, and useful for blotting (some feel they are too harsh to use when wiping over the surface).  As will be described below Johnson and Johnson sterile cotton balls are also absorbent and soft,


Figure 1


6)  Time to Clean the Mirror:


            I’ll briefly describe how I’ve been able to clean my mirror in situ.  You’ll notice this only applies to the primary mirror.  Then secondary can be cleaned as well, but there is a real risk of taking the central felt mark off.  Be very careful not to do this (if you do, the solution is usually sending back to RCOS for re-marking). 

            Figure 2 is a bit blurry, but shows dust and spotting on the lens.  The image doesn’t do justice to how bad the mirror looked.  Again, I only clean the lens when the problem is significant. 


Figure 2


            The trick is to first spray the lens with diluted detergent, doing this thoroughly and then following this with distilled water, without any drying occurring prior to completing the rinsing.  This can be difficult.  You have to squeeze into the truss of the RC-16 with the spray bottle (and later with Kaydry or softer tissue).  I do not know if this could be done with a 12” scope and I doubt it could be done with a closed tube.  But at 5’8” I KNOW I can get in, without putting pressure on the truss, and clean the RC-16. 

            First I stuff paper towels under the mirror (Figure 3), least the water flood the scope.  I often add more or change it during the process (it gets soaked).  The RC is pointed just slightly down, so no water will fall behind the mirror.  Also, I will first use a blow dryer on the mirror when the conditions are extremely cold (not enough to crack the mirror, just enough to discourage condensation during the process of cleaning). 


Figure 3



Next I repeatedly spray the mirror with distilled water, to hopefully tease off some of the particulates.  Then I spray the mirror with the mild diluted detergent.  I do this for 3-5 minutes.  My goal is to rinse off any dust or pollen, and give it enough time to dissolve.  Figure 4 illustrates how things will look at this point. 


Figure 4

Next I begin to rinse with distilled water (using a Windex bottle filled with distilled water).  Spray this repeatedly until the mirror is clear, with only distilled water covering it.  I consider this step to be key, especially if you’ve chosen to clean the mirror with  Windex (but certainly important with mild detergent as well).  With isopropyl alcohol or the Edmund Liquid Lens Cleaning agent, washing with distilled water is apparently optional. 


Figure 5


Next the mirror is blotted dry.  What many will tell you is to BLOT the mirror with Kimwipes or similar high grade paper and to NEVER wipe the mirror.  I’d recommend an even softer tissue, there as many on the market.  Some of the softer tissues leave a bit more lint behind, but this is easy to remove with either air or a snow gun.  Blotting or gently running the paper over the mirror with only the papers own weight as its’ force on the mirror, will usually work, and is the right way to dry the mirror (or one can blow the mirror dry with compressed air).  I find soft tissues can be used with minimal “wiping” lightly when necessary, but this is done at real risk to the coatings and could even scratch the mirror.  I always try and use only the weight of the tissue itself (what I usually do) or blotting only (which I find doesn’t always do the job). 

            Finally the mirror is clean again (Figure 6 and 7).  Hopefully most of the coatings are still present.  I often use the snow gun at the very end, to remove any remaining dust or tissue remnants. 




Figure 6






Figure 7