And the mirror figuring goes ever onwards

It’s been over a month since I last wrote about my amateur telescope making project, so I figure I owe an update to anyone who’s following along with this unexpectedly long saga. I’ve still been going to the workshop every week, it’s just that I haven’t been making any measurable progress, so I haven’t had much to write about here. I’ve been stuck in “final” figuring for over two months now; the mirror is all ground and polished, it just isn’t in the correct paraboloidal shape yet.

When last I blogged, I was having trouble getting my mirror paraboloidal. It was stuck somewhere halfway between spherical and paraboloidal. No good. We finally figured out that my pitch-lap was depressed in the middle, so we poured more pitch on top of it and essentially resculpted it to the correct shape. I also noticed that the lid of the plastic container I was storing my pitch-lap in was resting against the middle of my pitch-lap, pressing it downwards for the seven days between each workshop visit. After modifying the pitch-lap holder for height, I finally got my pitch-lap in just the perfect shape, and was able to go to work on my mirror.

So I got really close to the ideal paraboloid (about a 1/3 wavefront error). I had a 0.8 encircled Strehl ratio at one point (as determined by a Foucault test), which isn’t stellar, but it’s serviceable. But that wasn’t good enough for me. After spending so long on this mirror, I wouldn’t settle for mediocrity. So I pressed on, thinking it would only be another week of work to get my mirror into very good shape. Boy, was I wrong.

I ended up over-correcting my mirror past the ideal paraboloid. Even worse, I developed a rolled over edge, which was in hindsight inevitable, because I had a turned-down edge that was growing progressively worse with every week’s session. That’s just how it goes. All of the strokes I was using for parabolization tend to increase the severity of a turned-down edge. If I had finished quickly, the edge wouldn’t have been too much of a problem. But because of the defective lap I was using for so long, I was wearing down the edges for way too long before I had the center to the correct shape, and the edges ended up needing fixing.

So, last Friday at the workshop, I inverted my setup and polished with the pitch-lap on top of my mirror, using very short strokes, for about an hour. Looking at the Ronchi test afterwards was heartbreaking. The diffraction lines curved outwards in the middle (indicating oblateness on that section of the mirror) before reaching an inflection point at around half of the radius of the mirror, then curving again the opposite direction (indicating paraboloidness on that section of the mirror), and then, finally, veering off sharply at the edge, indicating a turned-down edge. I wish I had a picture to show you. It would give any experienced ATMer nightmares.

So, more than two months after I started final polishing, I’m now in worse shape. When I began, I at least had a consistent, near-spherical figure to work with. Now I have something really screwy that will take at least another hour of work to get back to something approximately resembling a paraboloid, before I have to redo the long series of Foucault tests all over again. At least I no longer have a rolled over edge. I just have a tiny turned-down edge, and, crossing my fingers, hopefully I’ll finish final figuring soon this time before the turned-down edge gets too bad again.

Despite all of the roadblocks on the way towards finishing up this telescope, I am having a good time. I choose to look at it from the positive side, and focus on all of the new things I’m learning rather than the goal of finishing which seems perpetually out of reach. I’m not just idly playing with fullnesses of glasses here. I am learning a lot by spending so much time in final figuring. I’ve become good at running and interpreting Ronchi tests, and I feel I would be able to do a Foucault test on my own successfully. I’ve also used a large variety of strokes, learning what kinds of effects each one has, and thus when to use them. I also now know a good bit about creating a pitch-lap and maintaining it over time to ensure it correctly matches the shape of the mirror.

Most people wouldn’t spend as much time in the final figuring stage on their first scope as I have. Final figuring is by far the hardest stage, and it has the most nuanced skills to be learned from it. In comparison, everything else, like beveling the mirror, hogging it out, rough grinding, fine grinding, and polishing, is easy. I found them pretty fool-proof. You’d have to try pretty hard to screw them up. So spending time on them is exactly that: spending time, without learning much in return. In comparison, you learn so much from final figuring. I bet by the time I finally finish this scope I’ll have more experience than someone who finished two mirrors but never ran into any final figuring difficulties. Sure, he’ll have more experience grinding (yawn), but what will he do when he runs into a new problem with figuring he hasn’t run into before? In comparison, I’ve already run into all of them. So if encountering the most problems in my first go at this is my measure of success, then I am doing spectacularly well.

2 Responses to “And the mirror figuring goes ever onwards”

  1. Hans lambermont Says:

    Nice read Cyde ! I’ll paste some irc stuff :
    [20:56:36] i’m unaware of a lot of words he uses to describe what he does, it all sounds like Harry-Potter magic to me
    Good to see you learn and continue. Keep it up !

  2. Cyde Weys Says:

    I apologize if this blog post is dense with lingo. It would just take a lot of effort to redefine all of the terms I’m using in each blog post. If you read the series in order, I do explain most of the terms the first time they come up, so it should make sense. If, after reading the whole series, there are still things you have questions about, please do tell me, and I’ll make sure to elaborate on them in future installments of the series.