I'd not consider a 12.5" mirror too large for a newbie to grind as the process is the same until you hit about 15" or so where subdiameter tools become more useful for the polishing.
Aside from the whole process taking a lot longer than a smaller mirror, the biggest problem that you will have is the actual final polishing and figuring operations. The making of a shorter focal ratio mirror is always a problem as the work takes longer to do. Just pesevere with the process and an excellent mirror will result. I've had people in my mirrormaking class make mirrors with better than 1/10th wave accuracy without problems so have a go at it. The worst is that you will have to go make a smaller mirror to gain some experience in making mirrors if you run into real problems with the larger mirror. Also, there are several excellent sources of help for any of the more minor problems that happen.

• Best f/ratio to do? - mattb - May 30, 2003 - 22:06 UTC
  • F-rations - bobmay - May 31, 2003 - 21:01 UTC
    • f/ratio-I should've been more specific - mattb - May 31, 2003 - 23:55 UTC
      • Re:F/ratio-I Should've Been More Specific - optics - June 1, 2003 - 12:10 UTC
  • collimation difficulty - sellors - June 3, 2003 - 16:29 UTC