Cleaning Large Mirrors in Situ

[glend (Glen)]

After spending last week at the Lake Chaffey dark site (with the heat and winds) my primary mirror was looking fairly dusty and needed cleaning. I've cleaned 12" mirrors out of the scope before but that's not as easy with a 16" mirror (especially as the GSO ones are Sikaflexed into the carrier ring in three spots (something I think is a good idea simply because it prevents mirror shift in the carrier, and prevents people stuffing up the placement and clamps and pinching the mirror on reinstall). The Sikaflex doesn't prevent cleaning, you just need to accept that the mirror will have to either be cleaned in the carrier out of the bottom tube section, or cleaned in situ.

It seemed to me that there is less chance of damage if it is simply cleaned where it is.

My approach was:

1. Prop the bottom tube section up with a strong timber under the rear steel ring to give the mirror a good tilt.
2. Get a powerful hair dryer (with cold air only) and blow off as much of the debris and dust as possible - without touching the surface obviously.
3. Place a shallow flexible baking tray (the BBQ ones are ideal) under the rear of the scope tube where the water will drain out.
4. Spray (using a clean garden or Preval aero sprayer) tap (or distilled) water over the mirror surface. I prefer to start with spraying as the pressure helps work things loose on the surface.
5. Pour distilled water over the tilted surface as a final rinse. Repeat this once more.
6. Pay attention to how much is going into the drain trays and make sure you switch them over if they become near full. Some may find duct taping a small tarp or sheet of plastic useful to avoid water going into the base (depends on your base design obviously).
7. Allow to drain, most of the water will drain out of carrier ring, and settle in the bottom of the tube where it joins the rim, and then drain into the container due to the gap between the tube and rim.
8. Once mostly drained, use a dish towel to absorb as much of the remaining water from the rear of the mirror and carrier as possible (there won't be much if you were careful when pouring the water of the surface. Wipe around the inside of the bottom of the tube (behind the mirror carrier) and clean up any remaining at the bottom of the tube. On the GSOs you can get an absorbant towel into the gap around the bottom rim and this helps get as much as possible. Don't touch the mirror surface with the towel.
9. Remove the prop and let the tube return to normal vertical position and then re-prop - this should encourage any remaining held dropslets to run off and collect at the bottom tube corner - repeat the towel process described in #8 above.
10. Allow the mirror and tube to air dry. There should have been no water on the tube above the mirror surface. You can spot grab any pesky drops on the mirror surface by touching the top of the drop with a paper towel to wick it up but this may not be needed.

Don't blow dry the mirror as this will just stir up dust in the air and it may resettle on the surface of the mirror. Better to let it dry naturally.

Photos are attached.

This probably is not a solution for some people, especially those that swear you should never clean a mirror, or those that think rust is going to destroy their scope while they sleep (as long as you dry it out properly that will not be a problem as all the parts are painted or plated). Also note that I did not touch the mirror surface with anything except water.

PS: Don't worry about your mirror fan, it is protected from the water by the way it is mounted behind the mirror on the carrier, and it should not get wet unless your using a garden hose.

[Satchmo]

My 2c ....Your process wont cut through the tenacious layer of grease ( a combination of air pollution and eucalyptus oil ) that deposits on mirrors in Australian coastal areas. For that you need a stage of detergent soak and some mechanical action with submerged bunch of cotton wool.

This grease layer if not removed causes lack of contrast and halos around bright stars. Without this step the `cleaning' is really illusory- good for removing loose dust at best . If you can breathe on your mirror and see a blotchy patchy breath pattern then you have a grease layer - when you breathe on it and th ebreathe pattern is smooth and unifrom then the mirror is clean .

Cleaning a silicon dioxide or quartz overcoated mirror is no more delicate than a window in your house and should not be pampered within an inch of its life at the expense of actually getting it clean and getting lovely high contrast images

[glend (Glen)]

Yes Mark I accept that can be the case, and I have done that in the past on my older scope. However, this scope is only six months old, always shrouded, and capped top and bottom when stored in my garage. I checked for haze before I started, but this was done to remove the dust blown into it at the windy dark site.

All readers should heed Mark's advice and include a detergent step if required, and there are a number of mirror cleaning threads, articles, and videos on utube that cover the detergent use.

[AG Hybrid (Adrian)]

Originally Posted by Satchmo

My 2c ....Your process wont cut through the tenacious layer of grease ( a combination of air pollution and eucalyptus oil ) that deposits on mirrors in Australian coastal areas. For that you need a stage of detergent soak and some mechanical action with submerged bunch of cotton wool.

This grease layer if not removed causes lack of contrast and halos around bright stars. Without this step the `cleaning' is really illusory- good for removing loose dust at best . If you can breathe on your mirror and see a blotchy patchy breath pattern then you have a grease layer - when you breathe on it and th ebreathe pattern is smooth and unifrom then the mirror is clean .

Cleaning a silicon dioxide or quartz overcoated mirror is no more delicate than a window in your house and should not be pampered within an inch of its life at the expense of actually getting it clean and getting lovely high contrast images

Dammit Mark. Now I have this niggling feel to clean my mirror. That being said when I cleaned my mirror 3 months ago I used detergent and cotton buds. So maybe I don't have to clean it after all.

To keep the large particles off my mirror. I usually give the mirror a good (but quick) blast with compressed air. Keep its looking good.

On a side note. Glen. Consider flocking your lower OTA. So shiny in there around your mirror. Although your cleaning technique might not work very well.

[sn1987a (Barry)]

I've been using those 5lt bottles of distilled water. Mark one with an x and add 1 drop of detergent. Then fill up a dedicated spray bottle also marked x. Warm sunny day on the back lawn tilt scope down to comfortable angle and pack a few towels over the electrics and a bit of cardboard or plastic sheet to drain most of the water onto the grass. Good squirt all over with x bottle paying attention to dirty areas. Then rinse all over another squirt bottle of distilled water only. Finally a few splashes straight from the distilled bottle to final rinse. Let dry last few drops wick off with pointy paper towel.

[Pinwheel (Doug)]

The only issues I see is "Rust" developing on metal parts that get water trapped in bolt threads etc. Also I don't recommend anyone using dish washing detergents because it contains caustic soda to eat thru animal fats. I suggest you investigate in a NON wax car wash detergent. I used to work for Selley's and learned all about detergents & whats in them. Some are Acid, some are Neutral PH & some are Alkaline. Hope this helps.

[pdalek (Patrick)]

I have had good results using a dual action airbrush, fed from a diaphragm compressor ( but any source of oil-free air would do).
This type of airbrush allows you to control the air flow and vary the ratio of liquid to air.
Work from the centre out.

[astro_nutt]

Here's a few more ideas you could try!
Hold the mirror vertical and use a end of a wooden dowel, gently tap the back of the mirror to help loosen any heavy particles as well as dust. Use a vacuum cleaner to collect.
Line the inside of tube with plastic covered cardboard to protect the matt paintwork.
A bellows type foot pump, (for air mattresses, etc available from K Mart for about $6.00), provides oil-free air.
If possible, heat the mirror up by gradually using warmer water to around 40C. This should soften any grease or oil before adding detergent.
Cheers!

[sn1987a (Barry)]

Last time I washed.

[sn1987a (Barry)]

Rear view

[marc4darkskies (Marcus)]

Here's a process for those with truss Cassegrains ...

http://www.ourastrogallery.com/mirror.htm

Most of the newer scopes allow you to unscrew the primary baffle to make it easier.

Cheers, Marcus