I made my first mirror and telescope in my teens, and then did other things for close to fifty years. Now I'm pushing glass again. This is just a convenient place for me to keep track of the process. For a wealth of information about amateur optics and telescope making, visit the Stellafane web site.




new lap

The photo below shows my new lap, poured from straight Gugholz 64. My lack of experience caused me to let it cool a bit too much before I attempted to channel it, so I was in an awful hurry to get that done before the pitch got too hard. Hence, perhaps, the uneven facet sizes, although I hope that's not a problem. The lap is also thicker than necessary, and the channels only go down about half the pitch thickness. I may deepen them before I attempt again to parabolize.

A couple of sessions of 1/3 center-over-center strokes have not made a noticeable difference in the ring zones visible in the photos in the last post. They have, however, produced some overall roughness. I guess, barring contrary advice, I'll just go ahead and do some more of the same. The same stroke, with constant around-the-barrel walking and mirror rotation, produced a nice smooth sphere prior to my first attempt to parabolize.


negative progress, positive learning

So, above is the FigureXP graph of the surface error of my 6" F4 mirror when I put it aside to fiddle with my tester (see post below). Stellafane Mirror Class was last Saturday, so I took the mirror with me in this condition. Dave Kelley, one of the resident testing and figuring gurus, took a look at it. His beautifully machined aluminum testing apparatus is of course vastly more precise and sophisticated than my makeshift plywood rig. For starters, his test bench includes an exqusite (better than 1/20 wave PV, I've been told) 16" flat for autocollimation testing. More importantly, he has a professional career’s worth of experience interpreting what he sees at the knife edge, and an apparently intuitive knowledge of what to do strokewise in response.

Through the day and with frequent testing, Dave had me work with various strokes and various amounts of suppression of the middle of the lap—which was accomplished by pressing with a square of paper between the mirror and the lap and later by actually shaving away at the lap in the middle with a razor blade. I should have made better notes; I don't remember precisely all the variations in strokes used. At first I was completely mystified because I assumed that the goal was to move the mirror toward a paraboloid. But after a while I understood that Dave wanted me to go back to a sphere and start over with the parabolizing.

I still wasn't sure why he was having me do what I was doing. When I got home it occurred to me to have Figure XP display the graph again, but this time in reference to a sphere rather than a paraboloid. This is easy—the conic constant of a paraboloid is -1; that of a sphere is 0, and this is something you enter when setting up the file, so all I had to go was go back to the setup page and change that one field. Below is the result, showing the mirror’s departure from sphericity. It's easy to see why he wanted me to work the outer zones but not the middle, and I think the strokes he prescribed were chosen to avoid turning down the edge.

The result at the end of the day was very near a sphere, but with annular rings of error showing in the foucault test, as you can see below. I can see them a little in the Ronchi test, and more so in the Foucault, but they're pretty small errors.

The kicker, though, was that Dave thinks many of my problems are due to the lap, which he deems too hard and insufficiently off-center. So my next project is to make a new lap. I hope to find time to do this well before the next (and last for this year) class, so I can work the mirror a bit before then, first to try to smooth out those rings, and then to start parabolizing.

Note: I had to exaggerate the contrast in photoshop to make those rings visible onscreen. They were somewhat more obvious to my eye at the KE. Also, those black blotches are artifacts of dust in the camera optics.

And a final note: At the meeting after the class, I once again was voted into (associate) membership in the Springfield Telescope Makers. I can't begin to express how great it feels to be once more, after so many years away from the venerable group, welcomed into the fold. To any members who happen upon this site: THANK YOU VERY MUCH.


i can always find a way to distract myself from the main project

Over the course of my recent mirror-making efforts, one thing has continually bothered me: the difficulty of getting my eye into a good position to see the Foucault shadows well. This may be partly due to my age, as pupils tend to grow smaller with the passing decades, and partly due to the fact that I have a prominent brow and it gets in the way. Early on I built a Stellafane-style tester, and then rebuilt the knife-edge/LED assembly twice trying to arrange it so I could get my eye close enough to the knife edge to see the light cone from the mirror. I got it so it could work for me by abandoning the front panel on the Stellafane plans in favor of a narrow stalk that holds the light source and knife edge. But it was never comfortable and having my eye that close to the razor blade always made me nervous, so I'm attempting yet another revision to try to improve the situation.

My thought was that if I could place a small telescope right behind the knife edge, I could look at the shadows with my eye at an eyepiece safely distant from the blade, and more comfortably positioned. The telescope, I quickly discovered, would need to have a very fast focal ratio to have a wide enough field to accommodate the whole mirror. Pawing through the junk drawer, I came up with an old f2.8 camera lens for the objective. The eyepiece came from one of two identical Bushnell binoculars, both mechanically broken in exactly the same way, given to me by two friends who know of my interest in optics. I also used a prism from the binocs to allow the eyepiece to be vertical. Some more scavenging produced a heavy cardboard tube and a bit of a pvc plumbing fixture, which went together as you see in the picture below. The prizm is attached to the back barrel of the camera lens with Gorilla Tape, one of the most useful products on the planet. The telescope yields a magnification of about 2X, which is fine, but magnification isn't really the point. It also reverses the image, a fact I have to keep in mind in testing.

The whole assembly is held to a vertical plywood stanchion with a u-bolt. I re-purposed the back plate that came with the u-bolt to make the KE assembly, drilling a hole in it to hold the LED and attaching the KE with a binder clip. This assembly hangs off a piece of threaded rod glued into a hole in the plywood stanchion. The oversized holes in the plate make it possible to adjust its position somewhat. Here's the whole tester, with the new assembly in place, and below that, a close-up of the KE/LED holder. 

It works, more or less. Alignment is tricky, mostly because I'm a lousy craftsman and nothing is square. Just getting the telescope/KE optical axis aligned with the motion of the tester is not that easy, although I've almost got that worked out. The telescope optics do reduce the contrast considerably, which may be a problem when judging nulls. And I'm not sure I can find a way to photograph the shadowgram. But at least I can see the whole width of the mirror, stably and in reasonable comfort. We'll see how it works out in actual testing, which I haven't attempted yet.



neither improvement nor disaster 

Latest plot and ronchi sequence below. There really isn't a clear null for the center zone—there’s too much change in the shadows across it. I don’t have a lot of faith in the meaningfulness of this plot for the central area. There’s very little difference in my perceived nulls for zone 1 and zone 2. I put it at .005" but I'm not feeling too confident about it. I guess my next step is to cut out a 7-zone mask, but on this little 6" mirror the outer openings are going to be pretty small.


garbage in, garbage out

One minor problem with programs like Figure XP is that once you've typed the data in, you can get seduced by the pretty graph and never look at the data themselves again. So if you make a transcription error, you might not notice it right away.

I did just that on the plot seen in the previous post. Here's the corrected plot, with the knife edge displacements properly entered.

The good news is that the figure is actually closer to ideal than the previous, erroneous plot. It looks ugly but note the scale at the left: the errors are actually smaller. The bad news I've already done a round of work on the mirror based on what I saw before.

As soon as I get the measurements and output another plot, I'll post it.