We all want to preserve and maintain our expensive astronomical observing gear. Part of realizing that goal would be to keep the optics of that equipment clean. But what appears to be common sense would be wrong. Not only should we not be actively cleaning our optics, we are likely cleaning the wrong optics. And certainly in the wrong way!
In Harold Suiter’s magnum opus Star Testing Astronomical Telescopes, he devotes the final section of Chapter Nine to the effect of dust and scratches on optical performance. Suiter refers to the accumulation of dust and other detritus as contributing to cosmetic errors much like the diffraction effects of spider vanes. There is a generalized scattering of light which has more of an effect on viewing bright objects like planets where low contrast details become washed out. In other words, the appearance of visibly dirty optics has a negligible effect on performance. His guideline is to ignore dirt that covers less than 1/10th of a percent of the optical surface area – say a pinch of salt thrown on the corrector plate of an 8” Schmidt Cassegrain.
In reality you could probably throw an entire teaspoon of salt at that corrector plate and not notice any effect on visual performance because only debris closer to the focal point will make its presence known. That means keeping eyepieces, diagonals, barlows, focal reducers and Newtonian secondary mirrors clean matter more than primary mirrors, corrector plates and refractor objectives. A zeal for cleanliness is detrimental because the cleaning process could cause damage by scratching the antireflective coatings of lenses or scratching the reflective aluminized surface of a mirror.
The best thing to do is ensure your optics are kept covered when not in use. Eyepieces can be stored in individual eyepiece bolt cases. Diagonals and barlows can have both ends stoppered shut with plastic endcaps. It’s the formation of dew that deposits and seals atmospheric contaminants and dust onto refractor lens objectives and corrector plates when it dries. This makes the cleaning process even more difficult so the use of dew shields and anti dew straps when operating the telescope is paramount.
When one ultimately reaches the point where cleaning is required, the first step is to debride the area of solid particles which when wiped become the abrasive element for scratching optical or reflective coatings. Camera stores are a good source for soft bristle brushes and air bulbs with which to blow and swipe those particles away. Compressed cans of air should be avoided for a number of reasons. The cans themselves contain liquefied fluorocarbons (tetrafluorethane) as the propellant and can often be expelled from a fresh can if not held perfectly vertical. In addition to contaminating the optical surface, they sometimes contain antistatic chemical additives, which are sure to leave a residue when the fluorocarbon evaporates. While the propellant does technically have a freezing point below -100oC, it’s unlikely the cans are that heavily pressurized so the propellant likely exists as a liquid closer to its boiling temperature of -27oC. Indeed, I measured a stream of liquid propellant with a laser IR temperature reader at only -38oC. This I believe debunks scenarios of -100oC liquid thermal shock causing micro cracks in both optical coatings and glass elements. The cans should be avoided because there are simpler and better options that do not introduce residue.
Traditionally, optical coatings were singular in layer and were only a few molecules thick. The coating material is either heated resistively or through electron beam bombardment until it vaporizes and passively settles on the optical surface being coated forming a low density, porous and fragile coating that can absorb water. Optical coatings today are laid in multiple layers to provide better light transmission across a wider range of wavelengths and the layers are much denser and better adhering due to processes like ion assisted deposition and ion beam sputtering. These processes accelerate heavy inert ions through a magnetic field into the developing coating layers and physically compress them. The resultant coatings are much more resistant to damage and are likely to be found on the eye lens of a modern eyepiece. Modern diagonals are also likely to feature very tough all dielectric coatings that not only deliver near 100% reflectivity but are strong enough to withstand the repeated wiping action of a cleaning solution. Conversely, care should be taken if cleaning eyepieces, diagonals, refractors and catadioptric telescopes made even late in the previous century. The reflective coating of primary mirrors still remain fragile despite the introduction of overcoating the aluminized or silvered mirror surface with a simple dielectric layer of either silicon monoxide or titanium monoxide. The bombardment with a reactive oxygen ion beam transforms the SiO to a tough quartz layer (SiO2) able to withstand annual cleanings. So why not apply an all dielectric mirror coating like those found in diagonal mirrors given how durable they are? Such coatings require as many as a hundred layers which is thick enough to alter the final figure of the mirror if there is a variation as little as 2% in thickness. An all dielectric mirror coating would also lack a metal bottom layer which means the coatings cannot be simply stripped chemically but must be polished off if the mirror is to be recoated in the future – with further dire consequences to the figure!
I won’t discuss the intricacies of cleaning methods or the different approach to cleaning refractive elements vs reflective surfaces. I will attempt to discuss the efficacy of a number of different commercially available cleaning solutions by testing them with quasi scientific means. All of these formulations rely on some type of water soluble organic solvent that is able to dissolve fingerprint or eyelash oil. They also require reagent grade components meaning chemicals that are at least 95% pure with no contaminants that could leave a residue. Acetone and any number of alcohols are very popular solvents and can be easily found at hardware stores or Canadian Tire but they are all very impure. Likely the best source is from one of the big name pharmacy chains which will often have 97% pure isopropyl alcohol. Water is an important component since it’s the water soluble contaminants in the air that deposit onto optical surfaces after repeated dewing cycles and again the pharmacy is the best source for distilled water.
After a review of the most popular and reputable commercial optical cleaning solutions I’m happy to advocate that you save your money and have some fun making your own. In this era of occupational safety and health legislature, all companies are required to accurately divulge the chemical composition of their products in a material safety data sheet (MSDS). Hence the Zeiss Lens Cleaner is simply 4-6% isopropyl alcohol and >94% water. Baader’s Optical Wonder cleaning fluid is a mixture of ethanol and propanol alcohols. The only lone wolf is an interesting product from Purosol which is advertised as an ecosafe organic plant enzyme extract that breaks the bonds that attach dirt to optical surface and reverses the surface charges to repel new deposits. The product found great success with military optics in the harsh environment of the Gulf Wars and with NASA in the oxygen rich environments of spacecraft because of its lack of flammable ingredients. Indeed it’s MSDS lists … nothing, because it has no hazardous components to disclose! Finally, the Arkansas Sky Observatory is well known to amateur astronomers because of the fine supercharging service that founder Dr. Clay Sherrod offers to owners of Meade Autostar telescopes. Dr Clay also provides an online recipe for an optical cleaning solution that uses distilled water, isopropyl alcohol, Windex and Photoflo. Photoflo is made by Kodak and available from Henry’s chain of camera stores and is used as surfactant to reduce the surface tension of the optical cleaning solution and allow it to more easily wet surfaces and hence clean and evaporate more efficiently (http://www.arksky.org/asoclean.htm).
I purchased some uniform 1 mm thick optical window glass squares with antireflection coatings from Surplus Shed. I subjected these glass samples to the mist of an ultrasonic humidifier filled with a strong solution of magnesium sulfate (Epsom salts) to simulate the mineral deposits that would be left after multiple dewing cycles. The transmission profile across the visible spectrum as measured by a spectrophotometer before and after the glass samples were immersed for five seconds in a cleaning solution would be compared to measure the effectiveness of the cleaning solution.
Don’t ever let your optics get this dirty, glass samples simulating deposit residue left by multiple dewing cycles.
You could buy a used student level spectrophotometer for very little money but who has the room for yet more stuff. In this Instructables link you can see how I designed and built an extremely simple and completely digital Arduino instrument using an RGB light emitting diode: (http://www.instructables.com/id/Arduino-Spectrophotometer/).
All data is expressed as the mean value of six spectrophotometric measurements of each sample and the standard deviation was too low to be worthy of being displayed as error bars for each data point. The drop in the blue spectrum end of light transmission reflects the lower response of the cadmium sulphide photocell to blue wavelengths. The results show that both Purosol and the ASO solution were very effective at removing all deposits very quickly without even physically rubbing the glass surface. Straight alcohol (or any organic solvent) is ineffective at removing water soluble deposits (even after minutes of exposure).
So the ASO solution appears to be as effective as any commercial solution but the use of Windex raises an interesting concern. ASO claims that all Windex is contaminated with insoluble residue that leaves streaks when it evaporates as well as being a potential abrasive. I wanted to prove this Internet factoid for myself and built a Dremel based swinging cup centrifuge which can also be accessed through this Instructables link: http://www.instructables.com/id/Dremel-Centrifuge/).
I centrifuged some Windex samples at about 9000g of force for 10 minutes in my kitchen sink which I figured was more than adequate because this regimen is able to precipitate mitochondrial fractions from cellular extracts. The image below clearly shows some sort of black insoluble residue which is likely contamination from a dirty bottle manufacturing facility rather than a component of the Windex solution itself.
Just to prove that the residue does not come from the brand new centrifuge tubes, I spun some distilled water and no precipitate was present. I also filtered Windex through two sheets of coffee filters and no precipitate was recoverable after centrifugation so this is a simple way to remove the offending material.
So in conclusion do not routinely clean the optics of your telescope. Do often clean the optics of your eyepieces and diagonals with a solution as simple as reagent grade 6% solution of isopropyl alcohol in distilled water. Especially if they are modern 21st century eyepieces and diagonals since they are designed to stand up to repeated and frequent cleaning. Store your equipment in a clean environment and practice anti dew procedures scrupulously. Never use Windex to clean your optics!