AMATEUR TELESCOPE MAKING
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.

 

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Monday
Feb142011

figuring IIIb: second figure xp file

The plot below is after a couple of sessions of strokes that are a variant (i.e. shorter) of Texereau's "Classic Method" stroke III, as suggested by Jay Drew. Below that, a sequence of Ronchi images taken, left to right, from inside focus, through focus at the middle where one Ronchi band acts like a Foucault knife edge, to outside of focus. Still considerable center and edge correction to come, but looks like progress to me.

 

Sunday
Feb132011

figuring IIIa: figure xp’s visualization

I have to say this isn't what I was visualizing looking at the foucault images, especially that edge. Although I guess it just means "higher than it ought to be" rather than "raised."

Saturday
Feb122011

figuring III: a set of zone measurements

The images above show the nulls (or as close as I could come to a null) for each of 5 zones on the 6" F4  mirror I'm working on. For a larger image, you can click on the image above (you'll need to click your “back” button to return here). The difference in knife edge position between the zone 1 null and the zone 2 null is small; the photos don't show the changes in the shadows well but I could repeatedly come to the same result by eye, watching the shadow appear to move across the surface from left to right or right to left depending on whether I had the KE inside or outside the null position. I also note that zones 2 and 3 have visible transitions within them.

The measurements are shown in the chart below (averages of 3 readings for each). My tester uses a dial indicator; I "zero" the dial at the null for zone 1 before taking the other readings. As I noted in the update to the previous post, there seem to be differing opinions about whether one should aim for a value calculated for the average of the zone radius, or the radius that divides equal areas of the zone.

The dark splotches in the images are some sort of camera artifact, maybe from dust in the optical system. The camera, a low-end and elderly Cannon point-and-shoot model, is just sitting on some blocks stacked up to position it approximately in front of the knife edge. Moving the camera causes the splotches to move at half the rate that the image of the mirror on the preview screen moves. It’s hard to hold the camera still enough for the 1/6 second exposures I'm using—that’s why some of the images are blurred.

In all the images the knife edge enters from the left.

I now realize that it’s all too easy to choose a knife edge position that generates a nice "donut" shadowgram, but that a subjective interpretation of such an image can be very deceiving. Below are two images, on the left showing an apparent donut, and on the right at the closest thing to a real zone 1 null that I could produce. Hmmm.

Monday
Feb072011

figuring II: the first quantitative measurement

If I understand correctly, for a Foucault tester with a light source that moves with the knife edge, the difference in knife edge position for a null at the center of a paraboloidal mirror and a null at the edge of the mirror is given by the square of the mirror radius divided by twice the mirror’s radius of curvature. I take this mirror's radius to be 2.95" (there's a chamfer on the edge) and its measured radius of curvature is 49.5". The resulting desired KE movement is then .088".

I measured this using a Couder Mask, and came up with .037". So, although the mirror’s figure looks fairly smooth and the desired "donut" shadowgram is roughly approximated, I have some distance to go before it becomes a paraboloid. This is also evident qualitatively I think in the most recent Ronchigram (previous post); the lines are not evenly curved.

Monday
Feb072011

figuring I

I’m working on figuring a 6-inch F4.125 mirror. This is my second (adult) mirror, and it’s a “fast” one, which makes the accuracy of its parabolic shape both more critical and harder to attain than the less sharply curved paraboloid of a longer-focus, "slow" mirror. The advantages of a fast mirror are a short, easily manageable optical tube assembly, and a wider field of view. I want to end up with a small, easily portable telescope that’s good for looking at wide swathes of sky. However, such fast mirrors have other disadvantages besides the difficulty of figuring the mirror.  One is the increased prominence of coma, an optical aberration that smears images outward radially toward the edges of the field. This can be more or less fixed with rather expensive correcting lenses. Another is that when the telescope is to be a Newtonian type, a larger diagonal mirror will be needed, which will block more of the light entering the system and decrease contrast and brightness. This is mitigated by keeping the path from the diagonal to the focus as short as possible by using a low profile focuser mounted as close to the diagonal as feasible. All telescope designs are tradeoffs.

Below is a sequence of Foucault shadowgrams of the mirror. The latter 4 are accompanied by Ronchigrams.

I don't have photos of the tests done before this sequence, but I was told by the Stellafane crew that I had a “pretty good sphere”—the starting point for parabolizing. The first session of the traditional parabolizing stroke produced the first image. To my untrained eye, it looked scary—it seemed to lower the entire central three quarters or four fifths of the surface (leaving a distinctly raised edge) and to dig a very sharp hole in the central zone. (In all these images the knife edge enters from the left, creating the illusion of glancing light striking the surface from the right.)

Following are my contemporaneous notes on these images:

1. (12/27/10) Worked about 10 minutes in W parabolizing stroke, trying to keep mirror (on top) center traveling to about 1” from edge of tool. Kept strokes very slow, rotating mirror and moving around barrel after each to side to side transit.

2. (1/1/11) Did 10 more minutes of W stroke, slightly shorter strokes, also rotating mirror more frequently and moving around barrel faster. My reaction to this shadowgram is still WTF? The knife edge enters from the left.

3. (1/4/11) 15 minutes similar to second session. I don’t know what to make of this—it seems to be moving back toward a sphere. All of these photos were taken at the point nearest average focus—nearest approach to “winking out” without directionality to shadow movement as knife enters the light cone. I haven’t attempted any quantitative measurements yet, but this seemed to “wink out” more abruptly than the others    

4. (1/5/11) 20 minutes, and I think I drifted back into somewhat longer strokes. The central depression seems to be spreading.

5. (1/9/11) This is after (Stellafane) mirror class. The outer zone is  spherical; the inner area is depressed. I was told to work tool on top for 15 minutes or so, using a chordal stroke with about 3/4” of overhang, followed by mirror on top for 5 minutes with regular parabolizing stroke as above.  Then it was suggested that I should...

6. ...repeat the above sequence, using a little more overhang on the chordal stroke. These photos are after only about 7 or 8 minutes of TOT chordal and 5 of MOT W stroke. I think the difference in shadow contrast in the foucoult photo is due more to my (lack of) photo skill than any real change in the mirror figure, although the transition between the spherical part and the rest seems softer.

7. (2/1/11) This is the result of 15 minutes of a fairly long w strokes. 

8. (2/6/11) These images made at home after a fairly long working session at Saturday’s mirror class (5 or 6 sessions, each “twice around the barrel,” with testing and advice in between from Stellafane friends Jay Drew and Rick Hunter. The tests show that the mirror, from about #5 on, has been spherical in its outer zone with considerable correction in the middle. You can see this in the Ronchigrams; the lines are straight near the edges and curved in the central area. The correction gradually spreads outward in images 6, 7 and 8. But you can also see that there is a small area in the very center that's spherical (looks flat in Foucault shadowgram). Strokes used, at different times are standard W and a very short W, with strokes only a quarter or so of the mirror diameter.