Mike Lockwood's Foucault Testing Hall of Shame

Well, after lots of mirror testing I have seen some mirrors that just stood out in terms of the problems they showed on the test stand.  The most interesting of the worst of them are shown here, in no particular order, and sparing no optician.  The first two mirrors' images were taken by me with my digital camera through my small 3x finderscope that is used on my slitless Foucault tester with a blue LED light source.  The scope consists of a ~30mm achromatic lens with ~80mm focal length.  Eypiece in the scope is a 26mm (~3X), which gives nice sharp images.  It is refocused for mirrors at different distances from the tester.

Mirror 1:  Here are some images of a 6" F/8 mirror (coated).  It was in a telescope that was donated to our local club.  It has the absolute worst case of turned-down-edge (TDE) that I have ever seen.  The outer 1/8"-3/16" of the mirror had a radius of curvature more than two inches longer than the rest of the mirror!

6in mirror with TDE6in6in6in

Mirror 2:  This is a 10" F/6.8 mirror (coated), also from a donated telescope.  It has a very unique form of surface roughness, a very regular pattern that resembles chicken skin, a golf ball, or puffy patches of clouds, depending on how you look at it.  The way I look at it, it is just plain bad, the mirror has over three times the correction it should have, a mild turned edge, and there are hints of astigmatism in the third image.

Dimpled mirrorDimpled mirrorStill dimpley

Mirror 3:  This is another 6" F/8 mirror (coated), made recently by Coulter Optical.  As you can see, it resembles a bullseye more than a parabola, but it did actually meet the specification of being a 1/8-wave mirror on the surface!  This is machine work gone wrong, with zones carved in due to a repeating pattern.  Image was taken with a friend's (John Stone) digital camera positioned directly behind the knife edge.

Coulter 6" F/8 mirror, coated

Mirror 4:  This was a 12" F/6 mirror from Cave (from the seventies, I believe).  The figure was not that bad, but the surface roughness made me decide to refigure it.  It is now and excellent performer.  Image also taken with a camera directly behind the knife edge.

Cave 12.5" F/6 before refiguring

Mirror 5:  Here is arguably the worst of all.  Below you can see a composite of 12 images taken of a 32" F/4 mirror.  (I can't say anything about the source of the mirror so don't ask.)

The mirror was rotated 30 degrees between photos.  Astigmatism can be seen in most images, and its form changes systematically with the rotation of mirror.  A layer of carpet supported the bottom of the mirror on a curved support.  There is some central zoning, too, and other problems that I won't mention.

The main point is this is what astigmatism looks like with the Foucault testIn some orientations, it is not so obvious (120 deg.), but rotate the mirror a little bit and there it is.  Pure astigmatism goes from invisible to most visible in only 45 degrees of rotation, so be thorough and careful.  The moral is, rotate the mirror on the test stand and see if the shadows change.

By the way, we could not measure the difference of correction in a statistically significant manner for this mirror.  That is, the corrections were different within certain zones for the two diameters, 90 degrees apart, which were our best estimate of the major and minor axes.  However, there was not an obvious difference in overall correction - it merely indicated some local zoning in the neighborhood of 1/8th wave.  However, this mirror clearly has astigmatism in the amount of many of waves.  (If anyone cares to estimate it, please do and send a note to the ATM list!!)

Composite of Foucault images

To top it all off, a ronchi photo is shown below.  Given the Foucault images, it is easy to guess what it would look like - an S in the middle of the mirror.  (It's a bird, it's a plane - no it's SUPERastigmatism!)

No, I don't know the distance from COC of the photo.  Grating was probably 133 lines per inch.

Ronchi image

Hall of Fame

Well, I had intended to put this up sooner, but taking images that accurately depict what your eye sees when doing the Foucault test is not easy.  The images below are my best so far, but there is still much room for improvement.  The vertical bands which may appear to be surface roughness are NOT on the mirror, and their position moves as the small scope (which I use on my Foucault tester) is rotated.  My digital camera is positioned at the eyepiece and records the images through the scope.  Visually, the mirrors look exceptionally smooth.
10" reference sphere
First mirror is my 10" F/12.5 reference sphere, pictured at right.  The mirror was uncoated when this images was taken.  Images in this section were taken with my digital camera and 3X scope behind the knife edge.  The back of the mirror was fine ground, so no spurious reflections can foul up the photos.

The pattern vertical "lines" or texture is NOT ON THE MIRROR SURFACE - it is due to something in the small 3x scope that my digital camera takes images with, and seems to be caused by small amounts of dirt on the eyepiece of this small scope.  The vertical pattern MOVES WITH THE ROTATING EYEPIECE, proving it is an artifact of the imaging, not the mirror.  To the eye, the surface is REALLY smooth, and the difference in gray level is MUCH less apparent than it is in the photo.

The small brighter spots are light reflection off defects in the glass (bubbles, etc.) which are well below the surface.  Note the bright diffraction ring all around the mirror.

At the right a slight darkening can be seen an inch or two inside the edge.  (Again, to the eye this difference in brightness is not nearly as apparent.) This is a slight error where the surface turns upward slightly for ~1.5", and it has a slightly shorter ROC than the other portions of the mirror.  The difference in null positions between the null for it and the nulls of the outer zone and the central area of the mirror is 0.005" for my moving source tester, which corresponds to a wavefront error of ~1/100th wave on the wavefront, according to FigureXP.

The darkening at the upper left, especially near the 10 o'clock position is due to an AIR CURRENT.  It is very difficult to get air still enough to record an uncorrupted image of a mirror that approaches a 1/100th wave (wavefront error) sphere!

13.1" F/4.5 x 7/8" thick Pyrex mirror
The next mirror, pictured at left, is of my (coated) 13.1" mirror.  It is Pyrex, 7/8" thick at the edge, and is gently supported at the bottom edge by a conforming curved block lined with pile-type weatherstripping.  Again, the back is fine ground.

As above, the vertical texture is due to something in the imaging train, and is not present on the surface.  This mirror is exceptionally smooth to the eye.  Again, note the diffraction ring at the left (some of it was lost when I resized the image).

It was interesting to watch the air currents move, particularly in the nulled area where the darker regions join at approximately the 60% zone directly above the center of the mirror.  Many photos were taken to try to get an image with quiet air, and this is the best, though it is not perfect.  The air currents slightly darken the region mentioned before, which to the eye matches the region of the 60% zone below the center.

There are also traces of air currents (more visible on other photos) just below the center of the image, in the form of a slight bulge.  If viewed at the wrong moment, the air currents can give the impression of slight astigmatism by making the light-dark transition region (running vertically along the center of the mirror) tilt slightly, as it does in this image.  In reality, though, the transition is perfectly straight, which is not indicative of astigmatism.  It should be noted that in this image, the knife edge was not perfectly vertical, and makes the transition line appear to tilt slightly to the left at the top of the image.  This makes no difference at all.

In reality, this mirror is beautiful to the eye when viewed on the test stand, and it is typical of what my parabolic mirrors look like when I am finished with them - there are no visible zones, surface texture, or hints of astigmatism.

If any of you on the ATM list can give me some insight into the cause of the vertical artifacts, please post a message.

Back to my home page....