Cleaning a Primary Mirror

After a summer/fall of frequent observation and two major rebuilds of the furniture for my 16 inch Lightbridge optics, there was a fair amount of earthy dust and even more saw dust coating my primary mirror.

I had a good long conversation with the owner of a 24 inch StarMaster scope at the Texas Star Party about cleaning mirrors. Before forgetting all his advice, I decided to do a full baptism of my mirror last weekend.

I removed the mirror cell from the base box like I have a number of times in my building project. Taking it out is never a problem. (Putting it back in is not quite so simple. It takes a steady hand, carefully placing the mirror cell on three springs without knocking the springs over. )

Next, I removed the three mirror clamps from the periphery of the cell. That does not release the mirror, because there are four places on the periphery where black RTV calk was used to further hold the mirror in the cell. I took a sharp knife and cut through each bead of calk next to the mirror. (Marking the mirror to know what rotation angle it was in the cell is a really good idea.)

I then was able to slide the mirror out of the main cell casting. It came out with the rear-of-the-cell spider pieces attached. There are three independent supports that make up this “spider”. Each support consists of two three cornered plates on a balance beam. Each three corner plate had one sticky pad attached to the mirror with double sided paper tape. After fiddling with these tape pieces unsuccessfully for a few minutes, I just pulled the triangles off the mirror, leaving a paper spot on the mirror in 6 places.

I then took the mirror to the back porch, placed it mostly vertical against the house, and hosed the mirror off. That took most of the contamination off the mirror, I am quite sure. But I wanted to go a little further.

I filled up a roll under the bed Tupperware container about half way. This was a new container I bought for this purpose and hosed out. These containers have a bottom which is just a little more than 16 inches by about 27 inches. I put a couple drops of dish washing detergent in the water and used the hose to spread that out.

I put the mirror in the water bath with about an inch of water over the top of the mirror.

I had previously obtained some cotton balls that were 100 percent cotton. There was no way to verify that they did not have any softener in them, like surgical cotton. But they were called 100 percent cotton and did not list any other ingredients.

I used a cotton ball and gently ran it from the center of the mirror to the edge. I threw that cotton ball away and did the same with another ball just a half inch or so from the first one. I continued in this manner around the mirror – one pass of each new cotton ball from the center to the edge. My intent was to have the smallest risk of collecting a scratching grain and scratching the mirror over and over with it.

After this cleansing, I took the mirror out of the bath. I put it vertically next to the house again, and rinsed it with a quart or so of distilled water.

Almost all the water drained off the mirror leaving it without drops. There were a few drops and I collected them with the corner of a piece of toilet paper.

After the mirror had finished drying, I saw that I had missed a few places on the mirror where I had evidently not used a cotton ball. There were some small streaks of dusty appearance between my radial strokes - but there were not many. Evidently, using the cotton balls had removed a layer of dust that had remained adherent during the spray cleaning and the immersion.

After the mirror dried, I was able to put the rear support beams with triangular plates in place, and then to slide the mirror back into the cell so that the cut surfaces of the RTV calk exactly matched. I put the three mirror clamps back on. (Remember not to do this tightly.) Then The mirror cell was bolted back in the bottom box of the scope.

It was all pretty simple and the mirror was obviously much cleaner after the operation. Even just a spraying with a hose would clean the mirror a lot the next time round.

I don’t think I would do this more than every year or so, but it is nice to have a clean mirror again.