The big mirror size makes it a lot easier to set your alignment of the beam. I have tried with 10mm mirrors and after about 3 hours gave it up as a bad joke. You need to be way too precise to do it. You get a bit of leeway with the larger sizes.
Also remember that the gold mirror surfaces are VERY soft and easily scratched so when cleaning them use a WET lens tissue and without any force on the tissue just slowly pull it across the mirror surface. Gold is very malleable and so is also very soft and the coating on the mirrors is vacuum deposited and so is very thin. Remember that at cleaning time. If you keep the dust out then you won't have too many problems.
I have been using Aluminium first surface mirrors from photocopiers for the last few years and they are so much more resistant to scratching than the gold ones. I am going to try them out with the 100 watt laser I have just installed before Xmas and see how the extra power goes with them or if they will melt after a while.
The first gold mirror I cleaned with a lot of alcohol and a cotton swab/Q-Tip scratched the surface and I was only using very light pressure on it for fear of scratching. I won't be using the Q-Tips again as I want the mirrors to last a good few years.
NEVER use Acetone on your laser output coupler as it will erode the special coating on the face of it and may even reduce the output from the tube and stuff it up completely. I go to Chemist shops and ask if they have any Isopropyl Alcohol that is out of date and I get it for about 1/2 price. I never knew pure alcohol had a use-by date. :-) I am still on my first bottle (500ml) after 3 years and have only used about 25mm out of it so it's going to last a long time for me.
That might change when I build my next portable 35 watt laser table.
Hope this helps.
Rich.
I am not completely useless.......I can always serve as a BAD example.