Adventures in Mirror Cleaning

The general advice on cleaning mirrors is don't.  That is good advice.  There comes a point where time, dusty wind, or a unsupervised child will force you to clean the mirrors.

This page specifically talks about my experience as a first time user using Collodion USP.  This is sold in tubes by Mavidon.  You can buy larger amounts for big projects. The web site gives very complete instructions on its use and safety.

Some Warnings that the Instructions Also Cover.


There is a definite learning curve in using Collodion.  I will present 4 examples that show what to do, what can go wrong, and how to make things right.

8" Main Mirror

Secondary Mirror

18" Primary Mirror

15" Primary on a very hot day

Case 1 - 8" Main Mirror

dirty 8" mirror

I created a dam around the full circumference of the mirror using masking tape.  The picture above shows this partially in place.  It is very important that the seal be tight.  The Collodion is very thin and will leak if given a chance.

As the Collodion drys it will pull away from the surface. Don't rush this process.  After a bit it will look like this

during cleaning

While this picture does not show it very well I did not put Collodion on the very center of the mirror.  That way the center dot was preserved.  This meant the very center was not cleaned, but the center is lost anyway to the shadow of the secondary.

The Collodion drys to a hard film.  When it is completely dry it remove it with tweezers.  Carefully blow off flakes or use a corner of some tape.

clean primary

Quite a difference!  I probably should have puffed the mirror since even in this short time it accumulated some dust.  Note the contrast between the center which was not cleaned and the rest of the mirror.

Case2 - Secondary

dirty secondary

The secondary was a bit harder since you can not lay it flat.  I clamped it to a miter box which made the surface level.  I taped around the edges as best I could using 3M blue tape.  The shape meant that on the right side above the tape was very close to the surface of the mirror.

During my first attempt at this mirror I rushed the drying process and / or did not shake the tube.  The result was

Not quite clean

That was discouraging.  I tried cleaning the residue with some USP Isopropyl  Alcohol.  That helped, but did not remove everything.

Before Try 2 I shook the tube to mix the Collodion.  I pored it on rather thickly not by intent, but because the mirror was not initially clamped level.  The Collodion was allowed to dry for about 3 hours at 70º F (21º C).  This time the dried Collodion came off cleanly.  The result is not perfect, but way better than at the start.

clean secondary

Case 3 - 18" Primary Mirror

This was the mirror from an astronomy club telescope.  It was simply dirty through use.  I did the cleaning on a March date which means cool, but not cold temps (probably 70sº F or 20sº C).

The mirror definitely needed cleaning.  It was center doted with a reinforced circle.

As you can see I built a dam around the edge of the mirror.

I used the larger size this time.  I poured the Collodion on starting from the edges trying to preserve the center dot as I had done on the 8".  Unfortunately since this mirror was larger the curve was deeper.  It flowed into the center.

I ran out!

What I should have done at this point was to tip the mirror to cover the gaps.

This time I used cheesecloth.  Once the Collodion was on I laid some precut lengths on the mirror and gently pushed it into the drying Collodion with gloved fingers

When the Collodion dried it was embedded in the cheese cloth.  I just lifted the cloth off the surface and I was done.

Not perfect, but much better.  You can see the spots that did not get Collodion. 

The center dot was history.  You could faintly see where it was, but I wanted to remeasure.

What I did was to create a template using a CAD/CAM program. I then printed this using 3 overlapping legal size sheets. By combining 3 layers each rotated 90º I could cover 1/2 of the mirror as shown at the left. This is an example of putting the center dot on a 10" mirror.

Using the template not only tells me where the center dot is, but also protects my nice clean mirror.

Other authors use rulers to measure the location.  That will work, but it seemed like more effort and faced a number of measuring uncertainties.  Bear in mind for the larger mirrors a template sized to the diameter of the mirror will actually be too small since the mirror surface is larger due the the curve.  Just make sure that you have allowed a uniform amount of space at the edges.

Case 4 - 15" Mirror on a day that was too hot

Call it global warming or bad luck.  My final example was a 15" done on a day in triple digits F (> 37ºC). 

This mirror was covered by the red dust from the former site of the northern california summer star party.

This mirror was glued to its mirror cell.  I did not want to take the telescope appart.

Everything was proceeding as with the 18" scope.  This time I swirrled the mirror to make sure the entire surface was covered.

The first hint of a problem.  A lot of Collodion was stuck to the mirror.

When this happened before on the secondary I diagnosed the problem as not shaking the Collodion.  In this case I believe the Collodion did not dry properly due to the heat.

An air puffer would not remove the residue.

I was out of Collodion.  Faced with either not using the scope for several weeks or trying to wash the surface I chose the latter.

I tilted the scope at angle so any water would run off (about 30º).  I then pour about a gallon (4 liters) of distilled water over the surface.

The result exeeded by wildest expectations!

The mirror was cleaner than any other mirror in this series.  I used my puffer to encourage any lingering drops of water to leave.  In the high heat and low humidity the mirror dried quickly.

I did not use alcohol to remove water spots.  Alcohol might have attacked some of the parts of the scope.  I did not observe any signficant water spotting.

Lessons Learned

Is It Better Than using Water?

Good question.  I have never used water to clean a mirror mostly because my optics are mounted using silicone glue.

What I can say

But I would still not make cleaning mirrors something I would do very often. 

rjh 3/11/09b