Re Grind an old mirror

[adept22]

Hi all. I have managed to pickup an OLD 10" Mirror thats a F 3.5 Mirror, its coating is shocking and i could recoat it but im not really interested in a F3.5. Is it possible to RE-Grind that mirror to a higher F ratio say f5-6 to save having to get another blank? and then getting a coating put on?

[koputai (Jason)]

You might be better off selling it to someone who wants a 3.5, and buying one in f5-6 for yourself.
Of course, if you're keen..........

Cheers,
Jason.

[Blue Skies (Jacquie)]

I can't see why not. But I'll let the more experienced mirror grinders answer this one - I might learn something, too!

[cristian abarca]

You can regrind the mirror to what ever F ratio you want. Just keep in mind that it will take a bit of effort and if you are up to it you'll find plenty of people here to guide you through it. Have a look here to see what is involved. I haven't quite finished it yet but I will soon. http://cmabar.googlepages.com/250mm%C2%A0newtonian

Regards Cristian

[Rod]

Cristian is right. You can re-grind it to any focal length you like but I tend to agree with Jason. There are probably a few people out there looking for an F3.5. If the figure happens to be good then it would be a shame to destroy it.

Could you strip the coating and have someone test it for you? You can remove the coating with ferric chloride (the stuff you etch circuit boards with). You soak the mirror coating in the chemical overnight.

A simple ronchi test will tell you if there are any gross errors (rolled edge etc).

Rod.

[Roger Davis]

Or if you want, you could use caustic soda (no aluminium in it, so not DRANO) in a stainless sink or plastic tub, just be sure not to overdo the concentration. AND WEAR GLOVES. Wash thoroughly when finished.
Testing is important for peace of your mind and others! Find someone to help if you don't know how.