A quick note about using this Web page: All of the images on this page are thumbnails that represent much larger images. If you click on any image, you will see a new window that shows the full-size image for you. After viewing the image, you can just close that window, and you will return to the full Web page. All of the videos included here will play directly from YouTube, although they are displayed in a fairly small window. The usual YouTube controls should work. This page is fairly long, and you can scroll up and down through the content with your mouse. Due to the large amount of image and video content, it may take a little while for the images to display on your screen.
About the videos here: There are quite a few videos available below that describe the grinding and figuring sessions. If you click on a video, it will play in place, in the default size you see. But all of the videos were shot in higher definition and those are available for you. That will give you a larger format, and you can see more detail. To get the higher-res videos, just click on the "YouTube" in the bottom right corner when you are playing a video. That will transfer you directly into YouTube, where you will have the full set of controls that are normally available. When you are finished with a video on YouTube, just close the window and you will return to this page. Thanks, and enjoy the presentation! -Gordon Waite
Generating the 12.5" F4.4 Mirror Blank
Left: Prepping for generation on our Grizzly 45" radial Drill Press with sine table.
Center: A 7" segmented diamond cup used for generating.
Right: Using the Smart AngleCube to set the angle of the sine table. The actual angle setting for this mirror was 1.8 degrees.
Far Left: Paused during generation to measure the radius at various points on the blank. Sine table was then adjusted to produce a more perfect generation.
Center Left: Gordon showing the 12.5" mirror after generation and #60 smoothing is complete.
Center Right: Using a rotary tool and diamond wheel to bevel the mirror blank.
Far Right: Checking the finished bevel with the mirror on the turntable.
Grinding the 12.5" Mirror
Far Left: Using a rotary tool and a diamond bit to route small channels between the tiles on the dental plaster grinding tool.
Left Center: Rough grinding on the fixed-post machine. Turntable runs at 60 rpm. There is 20 pounds of weight on the back.
Right Center: Radius check after #80 SiC abrasive is finished. Radius is around 107 inches.
Far Right: Radius check after #120 SiC abrasive is finished. Radius approaching the 110-inch target.
Above is a short (but loud) video of rough grinding this mirror on the fixed-post machine. The mirror has 20 pounds of added weight, and the turntable is running at 60 rpm.
Making Polishers for the Fixed Post Polishing Machine
Far Left: Aluminum substrates used to make pitch polishers. All are 1" thick with a 7/8" hole for the pusher on the polishing machine.
Left Center: Pitch is melted in an aluminum pan. Polisher will be poured over the cleaned grinding tool.
Right Center: After pouring the rough polishing tool.
Mating the flat part of the polisher with the molded curve for the top.
Far Left: Laying out the facets with a single-edge razor blade.
Center Left: Razor blade is used to carve out the facets, and cut shallow lines on them for micro-faceting.
Center Right: Smaller polishers for figuring: 5.5", 4.5" and 3.5" diameters on aluminum substrates.
Far Right: Cold pressing the three small polishers simultaneously on the mirror.
Looking at the Mirror After Rough Polishing
The mirror was polished using the 9.3" polisher on the fixed-post polishing machine running at 60 rpm. The overhang was set to approximately 1.5" (15% of the 9.3" polisher diameter) and brushed with a brass brush after every 45 minutes of polishing. All polishers were made using #64 Gugolz pitch. The polisher did not need to be trimmed at all during the rough polishing session. Polishing continued until the mirror passed the simple laser test.
Left: Ronchi after rough polishing was finished. (133-line Ronchi grating)
Right: Foucault after rough polishing was finished. (Knife edge coming in from the right side)
The Ronchi and Foucault images show the typical situation after rough polishing is finished. The Ronchi lines are reasonably parallel over a large part of the mirror, except for the center 25%. So the mirror is generally fairly spherical, except there is a pronounced hill at the center of the mirror. The edge of the mirror has a bit longer radius than the rest of the mirror, but less than a quarter wave or so. This can also be seen as the flat area around the outer edge on the Foucault image. The Foucault image makes clear a series of concentric rings present on the mirror. These are typical after fixed-post polishing, as polishers leave zones based on whatever facet geometry is present. No effort is made to eliminate these before rough polishing, as these defects are shallow, and quite easy to remove after rough polishing is finished. At this point, there is no real benefit in taking Foucault numbers. See the following three videos for the first steps in figuring this 12.5" F4.4 mirror.
Figuring Session #1
After Figuring Session #1
After session #1, you can see that the mirror has improved considerably. The central hill is much reduced. And the concentric rings on the mirror are gone. The edge of the mirror isn't yet perfect, but it will improve as parabolization continues. The following two videos show the techniques to be used in session #2.
Figuring Session #2
After Figuring Session #2
After session #2, Once again, the mirror is looking much better. The Ronchi exhibits the curved lines that indicate parabolization in increasing. The defect at the center is only faintly visible now, and the edge shows good improvement. The following video shows the techniques to be used in session #3.
Figuring Session #3
After Figuring Session #3
After session #3, After this session, the mirror again shows improvement. The central hill has been planed down, and there is only a slight remnant left. The edge is better, and the mirror continues to increase correction. The following video shows the techniques to be used in session #4.
Figuring Session #4
After Figuring Session #4
After session #4, the 12.5" mirror is showing very good progress. According to the FigureXP report, the mirror is nearly 1/2-wave at this point. The Ronchi test is showing very much smoother lines, and a good edge. The Foucault image shows a little roughness from using the small polishers, but not too much of a problem. There are also two faint sleeks near the center of the mirror that should disappear in the next session. The following two videos show the techniques to be used in session #5.
Figuring Session #5
After Figuring Session #5
After session #5, FigureXP reports the mirror as a bit better than 1/4-wave, with a Strehl ratio of 0.84. This would be about the final quality for a normal commercial mirror. The Ronchi now shows full correction, although the image here (and the Foucault image) both suffer a bit from air currents in the testing room. We will continue on with another session to improve this mirror to a higher standard. The following two videos show the techniques to be used in session #6.
Figuring Session #6
After Figuring Session #6
After session #6, this mirror is finished. FigureXP gives the mirror a Peak-to-Valley WaveFront error of 1/18th-wave, with a Strehl ratio of 0.99. I should note here that FigureXP is always optimistic with the ratings, particularly the Strehl ratio. I would never claim that this mirror has an actual P-V rating so high. When our customers have had our mirrors independently tested via interferometry, mirrors with similar Strehl ratios usually test at 0.96 or 0.97 Strehl. Again, we would never claim that a mirror actually has a Strehl of 0.99. The Millies-Lacroix Analysis produced by FigureXP also shows a good mirror. All five test points fall within the target "tornado" diagram. The Ronchi bands are very smooth on this mirror, and the edge is good. All traces of the central defect are now gone. The Foucalt image shows a smooth mirror with little surface structure. This indicates that during use this mirror will produce well defined, contrasty images.
Additional Foucault Images of the Finished 12.5" F4.4 Mirror
A note about the diffraction ring. This mirror has a good diffraction ring all around the mirror. If you click on the top left image, the diffraction ring is visible on the left side. And if you click on the bottom right image, you can (barely) see the diffraction ring on the right. These diffraction effects are a bit difficult to photographic, but are more obvious visually.