16 April 2005

I cleaned my mirror today. I used the technique described in Astronomy Hacks: Tips & Tools for Observing the Night Sky. It is essentially the same thing You read on every web site that talks about cleaning mirrors.

1)     Prepare your work area. Using the kitchen sink is traditional, but if you clean your mirror there make absolutely sure the sink and surrounding surfaces are completely clean. Even one particle of abrasive cleanser grit can scratch the mirror. Pad the bottom of one side of the sink with a frequently-washed folded towel or use a RubberMaid tub to provide a soft resting surface for the mirror. Fill that side of the sink to a depth of three or four inches with lukewarm water, adding a good squirt of Dawn or similar clear dishwashing liquid. If your mirror is particularly filthy, add a pint of isopropanol for every gallon of water.

If I was going to use the kitchen sink I would first have to wash the dishes that are in it so I opted for the bathroom sink. I only have a 4.5" mirror so don't need all the space in a sink anyway. I placed a face cloth in the bottom of the sink. It is so well washed that most of the terry tufts are gone. And it hasn't been used to wash me since it last came from the laundry.

2)     Remove the primary mirror cell from your scope and carry the mirror in its cell to your work area. The mirror cell may be difficult to remove. In extreme cases, you may even need to use a hammer to free the cell from the tube. (Use a rubber mallet or a scrap piece of lumber to protect the tube and cell.) Because the primary mirror may come loose suddenly, it's a good idea to have a pillow or other soft landing spot under the mirror cell in case you drop it. Also, if you are removing the mirror cell form a Dobsonian scope, remember that the tube will nosedive as soon as you pull the mirror cell out. We generally support the front end of the tube with a chair, stool, or similar object, or have a helper support the front end of the tube until the primary mirror cell is removed.

I had removed my cell and mirror before I sent the scope off to have the bar cut. I just never got around to cleaning the mirror. I should have since it had fingerprints around the edge where I had taken it out the first time and I don't want them eating into the finish. Anyway, I performed the disassembly on my bed so there was no way the mirror was going to hit anything hard when it came out. Unlike most scopes, my mirror falls silvered side first out of the front end of the tube (the rear coffee can) and is hard to get out without touching the surface somewhere. Hence the fingerprints around the perimeter.

3)     Remove the primary mirror from its cell. If it is not already marked, it's a good idea to make an index mark on the side of the mirror and on the mirror cell so that you can later reinstall the mirror in the original orientation. When you handle the mirror itself, wear rubber gloves, which both prevent getting fingerprints on the mirror and give you a much better grip on the mirror, particularly once it's wet and slippery. Avoid touching the surface of the mirror, handle it only by the edges as much as possible.

My mirror is attached to an aluminum plate with double stick tape. That is my cell. Unless the tape dissolves during cleaning I don't think I am going to try to take it off the plate yet although I think centering it on the plate would probably help collimation. The first time I took the mirror out of the tube I found that it would only go back in in one particular orientation so I immediately took it out again and marked it so it would be easier the next time around.

4)     Run the tap water until it is lukewarm. Hold the primary mirror at an angle and rinse the surface thoroughly with tap water. The goals are, first, to rinse off and grit or other abrasive particles, and, second, to remove as much as possible of the water spotting, film, and other grunge without touching the surface of the mirror. Don't hurry the process. We generally spend five minutes or more rinsing our 10" primary mirror. Make sure the water stream touches every part of the surface. The stream of water from the tap is a gentle, safe way to dislodge grunge without touching the mirror.

Tip     If you are concerned about thermal shock, make sure your mirror has equilibrated to room temperature and rinse it in room temperature water. Many mirrors use Pyrex glass, which is very resistant to thermal shock. But even plate-glass mirrors are not at risk if temperature differences are small. We wouldn't bring a mirror in from a cold outdoor storage place and run hot water over it before it had a chance to warm up to room temperature, but we can't imaging that running lukewarm water over a room-temperature mirror could cause a problem.

I held the mirror under the tap for a while then set it on the towel at an angle and let it rinse itself while I got the soapy water bath ready. I could see little Vs pointing away from the source of the water stream and don't know if they are crud or just air bubbles from the water attached to the mirror and disturbing the flow across it. Either way they appear which ever way I turn the mirror. I thought my mirror would be at room temperature since it has been inside for months. But I had been storing it on top of my refrigerator toward the back partially under a cabinet so it was probably warmer than room temperature because of all the heat coming off the refrigerator cooling coils. It didn't crack though. I got a big aluminum mixing bowl that will hold over a gallon of water and washed that with Dawn, I just happen to have that for the rare occasions when I actually cook something here. Then I squirted some Dawn in the bottom of the bowl and headed for the bathroom

5)     After the mirror is thoroughly rinsed, lower it gently (face up) into the sudsy water on the other side of the sink. Swish it around gently in the sudsy water for a minute or so, and then allow it to rest on the towel or other soft surface at the bottom of the sink. Then allow the mirror to soak for at least 10 minutes or so. If it's particularly dirty, it won't hurt to let it soak for an hour.

I put the bowl into the sink and lowered the mirror into it then stood there lifting it up and down through the water for several minutes and then set it on the bottom of the bowl. Back to the kitchen I go and get my gallon of distilled water. Into this pure water that I spent weeks searching for, I put a single drop of Dawn (see step 9). I shook it up to distribute the soap through the gallon of water.

6)     Swish the mirror one last time for a minute or so, and them carefully lift it from the sudsy water and rinse it thoroughly under tap water set to about the current temperature of the sudsy water. Examine the surface of the mirror carefully. Unless the mirror was truly filthy, its probably now as clean as it needs to be.

I swished and I rinsed and the mirror looks worse than when I started. It looks like there was a film or something beneath the dirt what wasn't visible but now is in contrast to the areas of clean glass.

7)     If any streaks or smudges remain on the mirror, decide whether to continue cleaning the mirror or declare the job done. Mirror streaks and smudges have almost no effect on image quality, so it's usually best and always safer to accept whatever mirror problems remain. If in doubt, play it safe by skipping the following step. Otherwise, proceed to the next step.

Tip     The center of the mirror, that portion shaded by the secondary mirror, does not contribute to the image you see in the eyepiece. Any spots or other contamination near the center of the primary mirror (including, of course, the center spot) can safely be ignored.

I think this mirror needs more cleaning but want to know what it next. I don't remember if I blogged it but I used Hack #33 to center spot my mirror with a permanent marker. The mark is not being washed away.

8)     Resubmerge the mirror in the sudsy water. Holding the mirror at a slight angle under the surface of the water, draw a pad of sterile surgical cotton or a cotton ball across the surface of the mirror from one side to the other. Do not press down on the cotton3usually the weight of the cotton itself is sufficient3to remove any remaining file. Use each piece of cotton only once; replace the cotton for each swipe.

The mirror has been soaking for nearly an hour now with occasional agitation and it does not look any better. There are places where the water sheets off it when it is held at an angle. And there are places where it looks like the mirror is covered with sand. I think I will get the cotton treatment eventually, but first I am going to rinse it good with tap water and see what it looks like. If it still looks like it has crud on it I will subject it to heavy duty cleaning (for a mirror anyway). Well it looks pretty good after rinsing it off a long while again. I looked at it with a magnifying glass under olbique light and couldn't see anything on the surface or any sign of a film so I am gong to declare the job done.

9)     Once the mirror is clean and thoroughly rinsed with tap water, the next step is the final rinse. Hold the mirror nearly vertical, resting on a soft pad, and flood it with at least a gallon of distilled water to which you have added a drop or two of surfacant to reduce the surface tension of the water and prevent beading. We use Kodak Photo-Flo surfacant, which many astronomers prefer, but a drop or two of clear dishwashing liquid such as Dawn works just as well and leaves no residual film.

My mirror is small enough that I can hold it in one hand so that is what I did while I poured the adulterated distilled water over it. About half way through the gallon I stopped and turned the mirror around 180° .Everything was going great until I started to run out of water. As long as I had a lot of water and it was pouring out of the jug with force it sheeted up and glowed right off the mirror. But when there wasn't that much force it didn't flow off the lower edge of the mirror. I only used one drop of surfacant, maybe it would have worked better with two.

10)     Hold the mirror vertically and allow the final rinse to drain completely. If the mirror is clean, the rinse water will "sheet" off the mirror, leaving few or no droplets n the mirror surface. You can remove whatever droplets remain by using the corner of a paper bowl to absorb them without touching the mirror's surface.

I had some major water to blot up at the bottom of the mirror. My now toilet tissue says it is dust free so I used a square of that.

11     When the mirror surface appears pristine and dry, place the mirror face up on a folded towel or other soft surface and allow it to try thoroughly. If you're concerned about dust, cover the surface of the mirror with sheets of facial tissue. Once the mirror is completely dry, put it back in the mirror cell and reinstall it in the telescope.

I got the spots dried up and the sheet of water appears to have dried without spotting the mirror. I lined the plastic sandwich box I had been storing the mirror in with a clean face cloth and moved it back to the top of the refrigerator where it is not likely to be damaged and the warm air might help to dry it. I am sure there is water between the back of the mirror and the front of the mounting plate as well as in the screw holes. I don't want to put it back together wet and have the screws further rust. So I will let it dry until tomorrow night and maybe into Thursday. I will have to collimate it once it is reassembled so will have to finish building my colimation tool (Hack #37).