A Message About Mirror Coating and Recoating

Mike Lockwood, Carl Zambuto


For the purpose of protecting your investment, whether it be a Zambuto mirror or a Lockwood mirror, or by any other name, we have a recommended list of those to whom we are willing to entrust our mirrors. This does not mean there are not others who can also do the job, but rather these are our top recommendations.

We wish to point out that just because any entity calls themselves a coater, and regardless of how long they may have been in business, this does not guarantee they know how to safely handle your optical surface. Over the years we have come across numerous examples of optical surfaces being damaged, ranging from mild to severe.

This list does not necessarily exclude any other coater, but what we suggest is if you own a Zambuto or Lockwood mirror and wish to have service provided by someone not on this list, we strongly recommend you check with us first. Failing to do so may put your investment at risk.

The following are companies who have serviced Zambuto and/or Lockwood mirrors that we can recommend on an ongoing basis.
*Recently added as a recommended coater for Lockwood Custom Optics.

We will mention here that Zambuto Optical is now coating their own mirrors, and is available to service Zambuto mirrors that require a new coating or need assistance in any fashion. Zambuto Optical Company provides a full service where the optical surface is examined and all aspects of findings are communicated with the customer at each step of the process as needed.

Feel free to check with Mike Lockwood or Carl Zambuto if you have any further questions concerning your LCO or ZOC mirrors.

-Mike Lockwood, Lockwood Custom Optics
-Carl Zambuto, Zambuto Optical Company

More thoughts on coating

Mike Lockwood
A good, smooth mirror after figuring The above message was written by Carl Zambuto and myself, and was prompted by our experiences with mirrors getting damaged by coaters.

This part of the article is written by me alone.

Recently a large mirror that I made was damaged by a coater, so, to drive the point home I decided to include some photos of the damage here.

Bottom line, if you are getting one of my my mirrors recoated, please ask me first BEFORE you send it off for re-coating, and ask Carl first if you are having one of his mirrors recoated.

And by "ask me", I mean email me or call me.  Go old-school and mail me a letter.  I do NOT mean go on Cloudy Nights and ask.  People there tend to promote their "favorite" coater only, often with few reasons to back it up, or perhaps they have a hidden agenda.

The image at right shows what a good, smooth mirror looks like after figuring.

There is a center mark scribed into the surface, and there is a little dirt on the surface, but it looks very smooth over most of the surface.  That is, the transitions from shadow to light are very smooth, and there is little or no texture or roughness seen on the optical surface.

However, the horrific image below provides a particularly memorable example of the damage that nasty chemicals and aluminum splatter can do to a mirror.

Damaged large mirror

Interferogram quantifying damage
Though glass is normally not harmed by mild acids, leaving strong acids on glass for long periods of time can cause damage that will actually change the optical figure, or shape of the mirror.  Caustic solutions can harm glass in a severe manner rapidly, and should not be used on glass.

In the image above, features appear to show a "shoreline" where the chemical action stopped, not quite filling the entire mirror's curve.  The rest of the mirror's surface is filled with irregular damage.  Additionally, molten aluminum somehow splattered on the surface during coating, causing damage in small areas that resemble bubbles in the surface.

An interferogram of the same mirror seen above is shown here at right.  The "wiggles" in the fringes help to quantify the damage done on the optical surface.  In some places it is as much as 1/2 wave on the wavefront, and it appears in a very irregular manner.  The damage has even caused a frightening sort of "face" to appear in the center of the mirror!

The roughness results in large slope errors that throw light well outside of the desired image size for a star, bloating the star image and destroying the ability of the telescope to detect faint objects and produce high-resolution images.  We tested the mirror, in the damaged condition shown here, and that is exactly what the images showed.  Instead of tight points, stars were fuzzy blobs at best.

Fortunately insurance paid for the rework of the mirror.  After that, the images were superb and are now yielding valuable scientific data.

The images below show before and after images of some type of damage to the optical surface of a different mirror, using a Ronchi grating for testing.  Obviously the damaged optical surface is on the right.

Before and after images of another damaged mirror

Again, the images produced by the telescope after the damage was done were noticeably inferior to the original, smooth-figured mirror shown on the left.

The initial coating of a mirror is the least dangerous - the coater is working with clean, freshly polished glass, which just needs a thorough cleaning.  Nasty chemicals should not be required.  Most of the time just about any coater will get this first coating right.

However, when sending a mirror in for recoating, there is no telling what might be used to strip the coating, and this is a very important detail.  (LCO uses Ferric Chloride or a Green River solution for safe coating stripping.)  This is yet another reason that coatings should not be allowed to degrade too long, since this makes them even more difficult to strip, which may encourage some to get out the really nasty chemicals.

When something goes wrong during the coating run, the coating will need to be stripped and re-done.  This happens to the best coaters, but they know how to safely strip the bad coating and clean the mirror so that it is ready to receive a fresh coating that will adhere tightly to the glass.

Opticians, or people who have done optical work, generally know how to treat an optical surface without damaging it.  Many other experienced coating houses do, too, but clearly not all of them.

Again, inclusion in the above list of recommended coaters does not mean that those are the only coaters who know what they are doing.  It means that those are the coaters who we work with regularly, have pricing that is reasonable for amateur telescope optics, and who adhere to stripping and cleaning prodedures that we are comfortable with.

Be careful, and if you have questions, contact me.