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Hi Stewart,
I understand that there is some sentiment attached to the scope and the original mirror set. As Jeff mentioned it will likely be cheaper to buy a new mirror set than get both of those mirrors recoated. Have you used the scope before or had the mirrors tested. Coulters reputation for producing scopes with very poor optics is world renowned. It's likely that the original Coulter mirror set will be very ordinary and to be honest, not worth recoating, if you actually plan to use the scope. In the larger apertures you could get away with an ordinary Coulter mirror, as most of the larger ones 13.1" and 17.5" were mainly used as "light buckets" for DSO's and not as planetary scopes. A 6" scope is more likely to be used on Solar system objects than on DSO's where mirror quality will be a lot more important.
Just something to think about! Not much point spending the time and money rebuilding the scope and getting the optics recoated to end up with a telescope that's a pretty ordinary performer due to the original crappy Coulter Optics. You'll be able to buy a GSO or Synta mirror that's brand new and a much better performer.
The other thing to consider is that a 42mm secondary is a 28% Central Obstruction and probably 1 size bigger than you need for a 6"/F5 tubed newtonian, which will further detract from planetary high power performance.
Cheers
John B
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