Oh yes the day will come when you look through your scope and say I have discovered Dark Matter ! (all over my corrector plate). That's right. Demon Dust, the scourge of the astronomical world and it's sidekick Pollen have descended upon your corrector lens. What to do? What to do? Have No fear People of SCT land. The solution is in hand. How did it get to this stage you say? The biggest contributor to a dirty corrector lens is dew. I have lived in some oh so very humid places where my dew zapper and shield were no match for the dewing forces of Mother Nature.When that dews dries off eventually it tends to "glue" down the dust that was present on the lens during the dewing event. Here are a couple of preventative tips for not ruining your corrector plate. Never put your wet scope in its case and close it The reason is the insidious Nair-do-well, Fungal growth might come a calling. Folks this is a bad thing. Fungus can ruin the coatings on the surface of the Corrector as well as any other optical surface. Do not cap your corrector plate let the morning take care of drying it out or if you have a dew gun (akin to a hairdryer)Take care of the dew in that manner. You could just take it inside and set the scope up pointed down to attract less dust and let it dry out that way. The big thing here is knowing dew is your enemy. One last thing do not wipe the dew off. the impurities will not go away and you will take a chance on scratching the lens and you do not want or need that in your life.
Before cleaning your lens off, you may ask: Just how much dust warrants cleaning your Corrector? That is a splendid question and I see you are on top of your game but the answer is simple or not so simple. Only you will know when enough is enough. Some folks are anal about dust way more than others. A small amount of dust is not going to affect your scope very much at all. The only exception to that rule is if slap on the Solar filter and take a gander at the sun . You might find a few "extra" sunspots that seem to move when the scope moves. I had a child point that out to me last weekend. So in that case you might want somewhat of a pristine lens for your solar work.
Step one in the cleaning process is getting the BIG stuff off. There are two major schools of thought here. The first school of thought is to use canned air to dislodge any particles from the corrector. I do subscribe to this method if all of the safety precautions are used. Number one the Can-O-Air must be for optical surfaces and not for electronics etc... You do not know if the propellant in the electronic cleaner might just "clean" your coatings right off the lens which you might have guessed would not be favorable. With Optical air can in hand and your telescope parallel with the ground (corrector is perpendicular to the floor) Use 2-4 second bursts spraying at an angle to the corrector plate and about 12 inches away from the lens. make sure the can is upright when spraying to insure propellant is not splattered onto the lens This might be all you need to do. In most cases that is all you need to do. Throw the lens cap on it and go get on that Honey "dew" list. A second school of thought is to use a camel hair brush to lightly brush away the offending mote. Yes I have used this method many times if I did not have my Can-O-Air handy and always with success. the third school of thought is using a lens pen to take off the big chunks.
For those that find themselves with stuff that did not come off or a film of impurities from a dew event it is onto step two. For this step you will need Lens cleaning tissue I recommend the Kodak brand I have used them for years with no regrets do not skimp here. Do not use the tissues used to clean eyeglasses also Do not use stuff that is reusable like the soft cleaning cloth. I never know when they need laundered and I never know if the stuff in the cloth all came out in the laundry. There are too many variables here to make me comfortable using them. I do use Kleenex brand white no additives "Plain Jane" tissues They are soft and work very well. They are actually softer than Kimwipes. First thing is to use the tissue with your breath condensation on the lens. Wipe in a linear path and change the area of the tissue every time you make a pass to insure a new section of tissue is doing the wiping. If the breath condensation does not do the trick for the spots smudges etc use of a cleaning fluid is next in the progression. Again Kodak makes a fine cleaning solution or you could make your own and some folks do. (I do) If this is something you want to have a go at then visit Clay Sharrod's site for an RTF version of his cleaning method. Please note Clay does not like compressed air and uses a brush to dust. working in small areas and always following with a dry tissue and always in a straight path will insure a clean corrector with no streaks remember to do this type of cleaning sparingly only when it really needs it You can ensure many years of great observing using these simple methods. Next week we will touch on eyepiece cleaning, and let me tell, you when you have the public take a look through your scope you will have everything from suckers to mascara smearing the eyepiece!!! until then'
Clear skies and great seeing too!