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How to Clean a Refractor Objective Lens

Before: Six yearsí worth of dew spots and dust on a TSA102 Objective. After: like new.

If youíre thinking about cleaning your objective, the general advice would be Ďdonítí! Your lens can get a lot of crud on it before it degrades the view. However, there might be times when you do need or want to clean it: if itís very dirty, or if you suspect harmful contaminants - mould or sap for example - that could degrade the coatings (or even etch the glass).

I recently bought a TSA102 that was in pristine condition after six years (a real credit to its owner), but the lens had never been cleaned and it showed. There was no effect whatever on the image, but I couldnít know if the dirt was just dust and dew-spots or whether something more harmful like sap-spots lurked. I decided to clean it Ė something I have done on a number of occasions before with other scopes Ė and share my method. The technique I use is derived from various sources, but particularly from Roland Christen.

A Note on ED Glass and Fluorite

Fluorite is soft and so are many ED glasses. In a triplet they tend to be sandwiched away in the middle of two harder elements, but Fraunhofer ED and fluorite (the Takahashi FS series) doublets have the ED element at the front (though other fluorite doublets have it at the rear). I have successfully used this procedure on an FS objective that was very dirty, however I have seen an FS-128 ruined by rough cleaning (no, not by me)!

Take extra care with a Fluorite or ED front-surface doublet; ask yourself if cleaning is really necessary.

As usual, the disclaimer that you follow these instructions at your own risk (but itís a procedure that is virtually risk-free if you do it carefully with new cloths).


You will need the following:

         Lint free optical wipes Ė plenty of them.

         A good water-free cleaning fluid. I highly recommend Baader Optical Wonder.

         A new, unopened, quality micro-fibre lens cloth.

         A lens brush.

I recommend buying good quality, branded stuff. I always use a brand new micro-fibre cloth, just to be sure. How much is that lens worth?


1.      Clean the environment thoroughly and vacuum to lower dust levels. Let the dust settle again before starting work. Wearing an overall might be a good idea (though I confess I didnít).

2.      Set-up the OTA somewhere clean, free of clutter and supported so it doesnít roll around.

3.      Ensure you have good, relaxed access to the lens.

4.      Last thing, wash your hands thoroughly and dry them on something low-lint (I used kitchen roll).

2&3: Support the OTA with easy access to the lens


Step 1: Brush off the dust

Before you actually clean the lens, get as much dust as you can off to minimise risk of scratching. Do this by gently sweeping the dust off with a lens brush. I also use a vacuum held nearby (about ten centimetres) to suck away the dust I brush off, but make sure not to touch the lens with it!

Brush the dust off before cleaning.

Step 2: Clean the surface

1.      Double-up an optical wipe and spray a little cleaning fluid on it. Donít spray the lens!

2.      Gently clean a very small area with a wipe-dab action Ė donít rub Ė then discard the wipe.

3.      Repeat over the surface of the lens. I used about six wipes for a 4Ē objective.

4.      Let the fluid almost dry.

1: Spray the optical wipe, not the lens.

2: Gently dab-wipe to clean.

3 & 4: Use plenty of wipes, then let the fluid almost dry off.

Step 3: Final buff-up

1.      Gently mist the surface with your breath (donít spit on it!).

2.      Unwrap that new, quality micro-fibre lens cloth.

3.      Carefully, with small, gentle strokes and light pressure, buff off the mist to leave a shiny lens.

1: Mist the surface with your breath.

2 & 3: Gently buff with freshly unwrapped lens cloth.


Stubborn Spots

On this lens, the basic procedure, with the gentlest intervention, cleaned the lens perfectly. If you have remaining spots one method you can use (recommended by Roland) is to rub the spot gently with a dab of saliva on a clean fingertip. This may sound dubious, but the enzymes in saliva help to break down organic residues and your skin is a very soft cleanser (if itís clean). You should then leave it a minute or two for the enzymes to act before cleaning and buffing.

And Finally Ö

Examination from the focuser end with an oblique light source revealed a small smear which is invisible from the front Ė Iím not bothering with it. There was absolutely no trace of scratching, but this isnít a procedure I recommend you should do more than a few times in the life of a scope.