The glass corrector plate on an SCT will, over time, become dusty and appear to require cleaning. Because it is a piece of optical glass with delicate multicoatings you want to be very, very careful about
There is general agreement that a little bit of dust and smudges on an SCT’s corrector plate do not affect the scope’s transmissivity, and it is safer to leave it alone than to unnecessarily clean the plate and risk scratching it. Conversely, if dust builds up too much and you live in a humid climate, the dust will accelerate dew buildup by providing condensation sites for dew, so observers in humid climates tend to clean their corrector plate (and eyepieces) more often than do observers in more arid climates.
There is general agreement on several basics for cleaning a corrector plate.
First, dust should be removed with a very, very soft camel-hair brush to avoid scratching the corrector. Brushes sold for cleaning good camera lenses, or the larger-size brushes sold for applying good cosmetics, by good department stores selling good womens’ cosmetics, are appropriate brushes to use (yes, I’m sending you to a cosmetics counter to purchase a telescope accessory...).
Alternatively, you can remove dust using a can of “compressed
air”. Now, there is some disagreement about using a can of compressed air to blow dust off a corrector, because such cans are regarded as having the potential to leak drops of
liquid propellant onto the corrector; however, I have never had that problem although I have always been careful to hold the can in such a way that drops can’t fall on the corrector. The
product I like to use is the CO2 Duster Kit shown here on the right, # 114 0208 sold by X-tremeGeek.com for $37. (And yes, if you own a computer, and a telescope with a computer
in it, you are closer to being an extreme geek than you realize <grin>). A second choice is Radio Shack’s “Velocity” dust remover. The X-tremeGeek system uses compressed carbon dioxide cartridges
with no propellant or additives at all, and the Radio Shack “Velocity” doesn’t have most of the junk such as anti-static additives, that are in the cans of ”dust removers” typically sold in office supply stores - be
careful here. Jay Faircloth, an experienced scope user, has written a set of instructions on how to use a can of compressed air to clean optics - click here to download a text file containing his excellent
instructions. One of the most important points is to fire the can at the corrector with several short bursts rather than one long one, to avoid having an liquid emerge from the can.
Second, if you decide to use a liquid cleaner (more on that below) the best tissue to use with a cleaning solution is Kleenex brand unscented tissues - these are softer and less likely to scratch glass than
other tissues; even KimWipes are regarded by optical laboratories as more abrasive than unscented Kleenex tissues.
There are (at least) three schools of thought on the appropriate cleaning solution to use to remove smudges and dust, after as much dust as possible has been removed from the corrector plate:
- Celestron recommends a solution of 60% isopropyl alcohol and 40% distilled water, to which a
couple of drops of liquid dish soap per quart of liquid may be added. Meade’s recommendation is similar: 1/3 isopropyl alcohol (90% or better) and 2/3 distilled water with one drop of
biodegradable liquid dishwashing soap per pint of solution.
- Many observers, including folks working with optical glass professionally, note that plain Windex
-brand glass cleaner (and no other brand) diluted 50% with distilled water, works very well and doesn’t harm the antireflection coatings on the corrector. (I am not making this up, but the
caveat about not using brands other than Windex is serious - other brands, albeit less expensive, typically leave a residue on the glass.)
- Dr. Clay Sherrod of the Arkansas Sky Observatory (ASO) has done a lot of experimenting to
develop what he considers to be the safest method for cleaning telescope optical surfaces. You can find his cleaning method on the ASO web site, or download it here as a word-processing file
(in the wordprocessor-independent rich-text file format):
ASO Cleaning System.rtf
Regardless of the cleaning solution you choose to use, be careful never to apply the solution directly to
the corrector (or eyepieces). Apply the solution to the Kleenex tissue and then clean the corrector, to avoid applying so much liquid that it seeps around the edges of the glass, with possibly detrimental
The process of cleaning eyepieces follows the same instructions as noted above, but there is one additional product you may wish to consider. Many professional photographers use a LensPenTM (endorsed by Nikon and Celestron among others) to clean camera lenses and those camera lenses
typically are several times more expensive than even the highest-priced telescope eyepieces. A LensPen has a soft brush at one end for removing dust, and a soft pad at the other end for subsequently
removing smudges. One major advantage to a LensPen for eyepieces is that it can easily clean the bottom glass surface, which depending on the eyepiece design can be tough to reach with a tissue at
the end of your finger.
Back to Maintenance