Cleaning mirror on my 18" dob

[OzEclipse]

I have owned reflectors for 45 years and cleaned mirrors many times, mainly my 6" f7. That mirror is easily removed from the telescope by loosening three knurls screws on the cell and retracting the rear section of the cell with mirror safely inside.

I have now owned my 18" dob for about 7 or 8 years. Although i park the dob indoors in a clean location as dust free as a house can be, the mirror has been slowly accumulating dust and has needed a clean for a few years now. As a champion of procrastination, I have put it off for several years. One of the main things holding me back was that the mirror weighs the best part of 15kg and you have to bend down and lift it up through the rocker box. For someone with chronic lower back issues, there really is no safe way for me to do that. Some time back I devised a way to wash the mirror in-situ without soaking the interior of the rocker box. I also wanted to have some help incase an part required two people. My friend Phill @scribbly is here for 3 day stay. So he was on hand to help if required but I wanted to see if i could do it alone. It was easily a one person job.

Today I cleaned the mirror using this modified method. I tipped the fully assembled scope to a low altitude so that the open rear of the mirror box cleared the rocker box exposing the mirror. I passed a 6x4 foot poly tarp through the side so that it was under the now almost vertical mirror. I also placed a thin sheet of. perspex under the tarp so that water draining off the mirror would just drain out the back and not pool in pockets formed by the weight of the water.

What you will need:
One clean spray bottle - mix about 5 - 10 drops detergent into 500ml water
3 old dishwashing detergent bottles that were rinsed very thoroughly .
- one with tap water
- one with rain water,
- one with demin water.

You can skip the rain water and just use demin. If I had more demin, I would have done that. Living out in the country far from pollution and acid rains, rain water is very clean.

1. I mixed some dilute dishwashing detergent into a spray bottle and sprayed the mirror surface thoroughly until soapy water was draining down te surface onto the tarp. I checked the drainage and made sure it was draining and not running into the rocker box. Working well

2. Next squirt tap water to rinse off the soapy water

3. repeat steps 1&2

4. squirt rinse with rain water

5. squirt rinse with demin water twice

6. Leave the scope so the mirror can drain to beads of water then use the edge of a piece of dry kitchen absorbent towel to wick up the remaining droplets.

The result is pretty good with much of the dust and dirt gone. A thin hazy adhered grime remains. I should have applied a gentle swish of a piece of kitchen towel soaked in soapy water to dislodge the most adhered grime from the mirror but I forgot. Today is very Now that i know how easy this is, I will "rinse and repeat" again very soon but add a gentle mechanical swish wash action to the surface to dislodge the most stubborn layer of grime.

My friend Phill @scribbly is here for 3 day stay. We washed his 10 inch ultra light ultra portable dob using much the same method though with the dob disassembled and the mirror still in the rocker box. His scope is a Canadian ultralight 10" Genstar made by Dwight Hansen in Edmonton with mirror by Barry Arnold. Only 5 or 6 of these scopes were ever made. However, the mirror is glued to the cell with silicone sealant hence us doing the wash in situ. Phil held the mirror box with mirror surface vertical and kept a finger on the mirror edge for safety while I washed.

Using much the same method, we washed his mirror. In this case, I did give his mirror a gentle mechanical wash with a soapy kitchen towel which dislodged the hazy surface grime and left a beautiful clean finish.

My hands were both full and wet and I didn't take any photos. I will set up an intervalometer and photograph the process next time I repeat this process, hopefully soon, to include a gentle mechanical wash to remove the hazy grime.

My rocker box is all constructed from marine ply so a bit of water won't harm the mechanical structure. However, the 10000 step shaft encoders for the NEXUS navigation system is down there and I didn't want any water ingress to the encoder circuits or mechanics.


Joe

[Richard]

I would think that a 18 inch mirror would be much more than 15kg ?, but have never had one only 12
If you cant pick up 15kg then you have a serious back problem

[OzEclipse]

@Richard
It’s a thin mirror, 30mm thick with a 27 point floating cell.

I’ve estimated the weight from the thickness,diameter and density of glass. I can lift more than 15kg if it is a safe lift. I can lift and carry the 18kg head of my equatorial mount easily. The way this has to be lifted due to the design is the worst way to lift any weight - bend at the waist reach down into a deep well in the mirror box and lift up from the waist. No way to bend knees and squat to lift.

This has solved the problem.

Joe

[Richard]

Ok 30mm thick for a 460mm mirror is remarkable , I thought that it would be around 50mm , but yes I get it, is the way to get it out lots of strain on lower back so not a good idea

[scribbly]

Thanks Joe for helping clean my mirror too; I think the scope to be from around 2003 circa and I think this may have been the first cleaning of the mirror (the previous owner had not done so in the years he owned it).

[NGC 1365]

Hi Joe, my Suchting 18 inch mirror is 40mm thick and weighs a little more than yours. I clean mine every year as it gets quite a bit of dust on it from regular use. Thanks for sharing your cleaning method.
Ivan

[OzEclipse]

Hi Joe, my Suchting 18 inch mirror is 40mm thick and weighs a little more than yours. I clean mine every year as it gets quite a bit of dust on it from regular use. Thanks for sharing your cleaning method.
Ivan

Hi Ivan,
The workability of the method depends upon the design of the rockerbox and mirror mount.

I hope to document it with images next time make it a little clearer.

Joe

[NGC 1365]

Joe, to make the removal and re installation of my mirror easier, I got rid of the sling support and installed wiffle tree edge supports. Although it does not help with the actual weight issue, it makes frequent removal easier as the wiffle tree supports don't shift position and collimation is retained. Removing and re installing a sling was always the main problem for me. I could wash it in situ, similar to your option, that would definitely solve the weight issue.