On the way to Flat – Bumps

I would call them speed bumps, but speed and this process are oxymoronic.  From the last post out of Steve’s optical shop you may recall that our interferometry images are analyzed by the Open Fringe software and produce a 3-D image of the surface that is corrected for the desired curve. That means that when we get our perfect parabolic shape, the image will be flat. The interferograms are so sensitive, and the software is so flexible, that we will never see a flat surface, but when we say “flat”, we mean flat-within-a-reasonable-limit. Here’s what we had on June 5.

bumpy surface 6-05

From the blue low spots to the high red hills, the FFT measurement of the 40-inch mirror surface from the interferogram is about 8 wavelengths of light high.

This looks more like the foothills of the Sierras than the wide, flat Central Valley tomato fields. And at this stage of our work, you can clearly see that the mirror is not symmetrical (!).

No Symmetry = Astigmatism.

Among other things. But never mind, we know we’re not at our destination, so we should just enjoy the scenery on our way there. We’ll take a few snapshots to remind us of the fun we’ve had on this trip and use them for slideshows, later, to bore the neighbors and relatives.

The 3-D surface is compelling enough to get my attention, but even more interesting in some ways is a graph that shows how a vertical profile of the mirror appears in sixteen different directions. Picture a sheet of something very flat (a plane) passing vertically through the mirror from north to south, as you stand above it. Imagine it making the left half of the mirror disappear and then put your eye at the mirror surface level to look at the revealed edge of this intersection of plane and mirror. And then imagine Sixteen surface profiles from June 5, 2013 that your eye can see variations in height of a few wavelengths of red (laser) light, each one about 635 nanometers. Now repeat that process sixteen times in sixteen different, evenly spaced directions.

This is what we see on that ride. In the center of the mirror, the lines are closer together, which tells us that the surface heights are similar as we take that vertical cut through the mirror in the sixteen different directions. As we get further out toward the edge of the mirror, we see the effect of the Sierra foothills in the 3-D surface image. In some directions the hills are high, much higher than the central valley. In other directions, the hills are really lower than the central hill (an alluvial plain?).

It doesn’t take much to imagine that this would not produce a wonderful image at the eyepiece.

But this is just sightseeing and we have more to our story of what we did on our summer vacation. Steve, Larry and Mark have been hitting glass all summer. Since that day in early June, they have been pushing the smaller tools (the smallest is 12 inches) on the higher hills and using the bigger tools (the biggest is 26 inches) to occasionally smooth out the roughness created by the local figuring work. They work a routine of glass pushing and figure testing with the interferometer, with same day analysis and reporting. There have been fourteen of these sessions since June 5, and there is progress.

Notice the central hill has flattened considerably, and that all directions are now nearly in sync.

Notice the central hill has flattened considerably, and that all directions are now nearly in sync.

You can see that by August 5, 2013, the change is dramatic. The central hill has been the subject of small tool polishing, as have the many hills around the outer zone of the mirror. That work has resulted in significant smoothing of the surface. The profile lines show the story even more clearly – the variations of height across the different directions on the mirror surface are gone. The same curve and shape is present across the entire mirror. This means that we are successfully removing astigmatism from the figure. Steve says we still must be vigilant to keep it out of the figure as we continue toward our destination. Below you can see the 3-D surface rendering from Open Fringe, to compare to the June 5 version, as well as a synthesized foucault shadowgram, if you are used to seeing a mirror figure in that form. This synthesized mirror surface is also a product of the Open Fringe software. Note that the spike in the upper right of the 3-D image is just an artifact of the masking tape we place on the mirror at that spot to ensure we have images registered correctly in the interferograms.

Surface 8-05
Simulated Foucault 8-05

Our quest continues on this optical road trip. We will continue to see the mirror surface flatten until we come out into the Valley of the Best Possible Figure and park the RV for a short break before we begin the next leg of the journey.

NEXT TIME: Yes, I know we have designed the mount for a curved secondary mirror, but wouldn’t a 1-meter folded Newtonian be wonderful?!

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