Is is feasible to have an acromat recoated or would it simply be cost prohibitive?
Walt
Walt
Re-Coating an Acromat
Started by Walter, 12/22/2006 12:55AM
Posted 12/22/2006 12:55AM
Opening Post
Posted 12/22/2006 02:05AM
| Edited 12/22/2006 02:06AM
#1
From all the past threads it appears that most coatings can not be removed, they must be polished off and that requires re figuring the objective and then re coating. This is cost prohibitive unless it is a rare collectors item I would think.
I have heard that some early coatings were removable, but this is for old scopes not anything current or newer.
I have heard that some early coatings were removable, but this is for old scopes not anything current or newer.
[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue
Amateur Imager
Posted 12/22/2006 06:29AM
#2
Walter,
Typically, achromats are single coated with MgF2, which increases the transmission by 3% per surface. So, even if the coating looks terrible, if the wear and tear is only on the outer exposed surface, the transmission loss is insignificant.
A more important problem is the phase shift difference between rays that pass through 1/4 wave thick coating and those that don't, where it's missing. You don't really want an extra 1/4 wave error added to any fine lens. For that reason, if a lot of the coating is gone, say 25%, (which would look outrageously horrible under a bright light) best-resolution has been slightly compromised.
The up side is, if that much coating came off due to natural causes, it can't have been that good in the first place, and it probably will not require abrasives to remove the rest.
Ron
Typically, achromats are single coated with MgF2, which increases the transmission by 3% per surface. So, even if the coating looks terrible, if the wear and tear is only on the outer exposed surface, the transmission loss is insignificant.
A more important problem is the phase shift difference between rays that pass through 1/4 wave thick coating and those that don't, where it's missing. You don't really want an extra 1/4 wave error added to any fine lens. For that reason, if a lot of the coating is gone, say 25%, (which would look outrageously horrible under a bright light) best-resolution has been slightly compromised.
The up side is, if that much coating came off due to natural causes, it can't have been that good in the first place, and it probably will not require abrasives to remove the rest.
Ron
Posted 12/22/2006 07:56AM
| Edited 12/22/2006 08:00AM
#3
Perhaps contact these folks
http://www.majestic-coatings.com/
I have in 2 cases been able to improve or
remove degraded coatings with ammonia-based
lens cleaners like Kodak Lens Cleaner and
Residual Oil Remover. They removed
the degraded and cloudy parts of the coating,
leaving either clean glass or traces of good
coating. The method was same as lens cleaning,
just use them on a cotton ball, but with slight
pressure and many repetitions. Watch progress
carefully and stop when there is no more
improvement.
In the one case where I had a lens coating
professionally polished off and re-coated
the result was not entirely satisfactory.
The result looks good cosmetically, but on
an optical bench with knife edge test
you can see the surface is fairly rough, and
in the field planetary contrast is slightly
impared. I think they polished until the
old coating was gone, but it really should
have been fully re-figured after that. I
presume the glass and coating polished off at
different rates, so the net result had lots
of random hills and valleys.
(It was not Majestic, but a different company.)
http://www.majestic-coatings.com/
I have in 2 cases been able to improve or
remove degraded coatings with ammonia-based
lens cleaners like Kodak Lens Cleaner and
Residual Oil Remover. They removed
the degraded and cloudy parts of the coating,
leaving either clean glass or traces of good
coating. The method was same as lens cleaning,
just use them on a cotton ball, but with slight
pressure and many repetitions. Watch progress
carefully and stop when there is no more
improvement.
In the one case where I had a lens coating
professionally polished off and re-coated
the result was not entirely satisfactory.
The result looks good cosmetically, but on
an optical bench with knife edge test
you can see the surface is fairly rough, and
in the field planetary contrast is slightly
impared. I think they polished until the
old coating was gone, but it really should
have been fully re-figured after that. I
presume the glass and coating polished off at
different rates, so the net result had lots
of random hills and valleys.
(It was not Majestic, but a different company.)
Funding Member
