So I figure at least half the world-wide knowhow on caring for front surface mirrors must be represented in the collective knowledge of this forum's members (the other half is probably on Photonlexicon). I have an 8 inch parabolic mirror that I took out of an old reflecting telescope. It's f/3 or f/4 which would be perfect for my smallish table. The problem is that besides being dirty it has a plastic disk right smack dab in the middle of the mirror :wall: presumably as an aide to collimation of the telescope. I cannot be sure that the mirror has a SiO overcoat (though I hope it does ) so I need to clean it without touching the surface. I am looking for suggestions on how to get rid of the disk. My thought was to clean the mirror first, then to place a liberal amount of acetone on the mirror with the hopes that the acetone will dissolve the glue that's holding down the disk without hurting the mirror. Hopefully, if I am successful, the mirror surface under the disk will still be usable. I'm posting before going through this exercise so that someone smarter and more experienced than I am can stop me before I do irreperable damage.
hi Ron, well with the disc in the middle its basically useless, so it has got to come off, most good mirror coatings have a SiO coating on them any way. i would puddle acetone in the middle untill it works it loose then run the whole thing under warm to eventually being hot water then introduce dish soap onto the surface,using clean fingers evenly coat the mirrors surface with the soap, evenly coating while still running under hot water, continue running hot water until all the soap is removed' then quickly remove the mirror from the running water and blow off the water using compressed can air while the mirror is still warm from the hot water. The trick is to coat the mirror with the soap without actualy touching the mirror with your fingers always have the thick soap acting as a barrier to keep the mirror's surface free from fingers contact. good luck!
i'd recommend acetone too, to free the glue, but I'd watch to be sure it doesn't dissolve the plastic and deposit that on the miror too. After the glue it loose, pull the plastic off and finish cleaning the mirror. I'd also recommend your local amateur telescope making community for info on local coaters if you need to do that. Stellafane? CloudyNights ATM section? Good luck!
Check the wiki. Also if you look in the old forum posts I got advice and removed the disk from my mirror. It was pretty easy but my mirror is very slightly hazed where the ring was. I have since returned the mirror to telescope use and put a hole reinforcer there.
O.K., I went to the Wiki and it put the fear of God in me! I decided on plan B. I found an 8 inch f/4 parabolic mirror on eBAY that was selling for $80 new. Supposedly, it is correct to 1/4 wave. I suspect it might dissappoint a real astronomer particularly since the f/4 would probably give significant spherical abberration, but I bet it would make a better collimating mirror than what I would end up with after trying to clean and acetone-treat my own mirror. For the small investment I figure it's worth a try and I can always go back and try to apply the techniques that I learned here on my own mirror later. Dave, Bob, Colin thanks again for your suggestions. This forum is a GREAT resource.
(the other half is probably on Photonlexicon). I have an 8 inch parabolic mirror that I took out of an old reflecting telescope. It's f/3 or f/4 which would be perfect for my smallish table. The problem is that besides being dirty it has a plastic disk right smack dab in the middle of the mirror :wall: presumably as an aide to collimation of the telescope. I cannot be sure that the mirror has a SiO overcoat (though I hope it does 
) so I need to clean it without touching the surface. I am looking for suggestions on how to get rid of the disk. My thought was to clean the mirror first, then to place a liberal amount of acetone on the mirror with the hopes that the acetone will dissolve the glue that's holding down the disk without hurting the mirror. Hopefully, if I am successful, the mirror surface under the disk will still be usable. I'm posting before going through this exercise so that someone smarter and more experienced than I am can stop me before I do irreperable damage.