Cleaning Newtonian Mirrors.

I’ve noticed that one issue that seems to give folk concern about investing in a good Newtonian pertains to having to clean the optics every now and again. I’ve never really understood this mindset though. Having had my closed-tube 8-inch Newtonian for about 18 months now, and having clocked up a few hundred hours of observations with it, I felt it was time to give the mirrors a cleaning. Here’s how I do it:

The mirrors are removed from the tube.

Two fairly grimy mirrors

Two fairly grimey mirrors.

















First I make sure that all the loose dust and debris has been blown off using an air brush. Next, I run some cold tap water into a sink and add a drop or two of washing up liquid. The water we use here is very soft; indeed we are graced with some of the softest water in the British Isles, which also makes drinking tea especially pleasant! If your local water source is hard, I’d definitely recommend using de-ionised/distilled water.

Starting with the secondary mirror, I dip my fingers into the water and apply some of it onto the mirror surface with my finger tips, gently cleaning it using vertical strokes. Did you know that your finger tips are softer than any man-made cloth and are thus ideal for cleaning delicate surfaces like telescope mirrors?

Finger-tip cleaning of the mirror.

Finger-tip cleaning of the mirror.

















Next, the mirror reflective surface is rinsed under some cold, running tap water.

Rinse the secondary with some cold tap water.

Rinse the secondary with some cold tap water.

















The procedure is repeated for the primary mirror;

Gentle massaging of the mirror using the finger tips.

Gentle massaging of the mirror using the finger tips.

















Rinsing the primary mirror using cold tap water

Rinsing the primary mirror using cold tap water.

The mirrors are then supported on their sides to allow them to drain excess water, and then left to dry in a warm, kitchen environment. Stubborn water droplets nucleating on the mirrors are removed using some absorbent tissue.

Washed and drying out in the kitchen.

Washed and drying out in the kitchen.

















Finally, the mirrors are placed back in the telescope tube, making sure not to over-tighten the screws which hold the primary in place inside its cell. All that remains then is to accurately align the optical train, as described previously.

There we are! Not so difficult after all; and all done in about 40 minutes! The soft water doesn’t show up any significant spots after cleaning unlike hard water sources and now the optics are as clean as the day they were produced.

With a busy season of optical testing and planetary observing ahead, I know that my 8-inch will be operating as well as it possibly can. And that’s surely good to know!


I feel a nice, hot cuppa is in order!

De Fideli.

6 thoughts on “Cleaning Newtonian Mirrors.

  1. Hi Neil-having followed all of your posts ; this is the first time that I learnt that “Octavius” has been further modified to be a closed tube.
    I no longer have the 8″ F7 Dob made by David Lukehurst of Nottingham but i clearly remember -almost 20 yrs ago -whenI went to collect my Newt that he was finishing a closed tube for another customer and told me that the flat glass “cover” cost about the same as a quality parabloic mirror!
    Really interested to know the benefits.
    Regards, and thanks again for the high quality “stuff” your give out for free. Clear annd steady skies, rich.

  2. Good morning Rich,

    No, my 8″ f/6 doesn’t have a closed tube. Where did you get that idea? Why would I be washing the mirrors if it were a closed tube system?
    Nor have I any intention of closing off the tube. That measure would probably take the ‘scope longer to acclimate.

    Best wishes,


  3. Really sorry Neil to have got this wrong. Did not understand anyway. My comment was only sparked off by the second sentence at the top of this blog under the heading ” Cleaning Newtonian Mirrors”.
    Good info about just using your fingertips for when I do get a 10″ Flextube Dob. Best Wishes. rich.

  4. No worries Rich. I confuse myself all the time lol.

    That 10 inch Dob sounds dandy though. I’m sure you’ll get a big kick out of it.

    With best wishes,


  5. I hope that it will be my last scope purchase for sometime. What I get from your extensive exprience is such a scope can still be “grab and go” and the easiest to keep in top condition. Many regards, rich..

  6. Hello Rich,

    Big ‘scopes can ideed be used in grab and go format, especially if you do a bit of planning ahead. We’re lucky here in the UK in that there are never large temperature variations as day turns to night.
    About buying telescopes; I know what you mean. We live in an extravagant age. Luckily you don’t have to spend a great deal of money to get a very good telescope, as my ongoing work on reflectors reveals.

    Good night,


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