Archive for the 'Telescope Making' Category

Progress on my telescope’s mount

Wednesday, March 12th, 2008

Despite the lack of any progress updates here in awhile, I actually have been slowly working on my telescope. I’ve merely been slowing down because my attention has been diverted by other things (like amateur radio and other miscellaneous projects I haven’t bothered blogging about). So here’s an update on my latest progress. I haven’t touched the mirror since the last update, but I’ve made a bit of progress on the mount. Enjoy the pictures.

PVC clamped onto cradle

The cradle (wooden box) has internal dimensions of just a hair over 10.5″, so the telescope tube can slide in it. It’s 14″ long. Here, I have a section of 10″ PVC pipe clamped to it in preparation for tracing its outline. The PVC section will be what rotates the cradle up and down, providing altitude control.

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Mirror-making breakthrough!

Monday, January 14th, 2008

When last I regaled you with tales of my continuing telescope-making adventures, I had spent four hours working back from an oblate sphere to a paraboloid. Little did I know how much I accomplished in that session. This past Friday, I only had to do about ten minutes worth of polishing before another Foucault test revealed that I might be done! Yes, that’s right, after more than four months of working on this cursed piece of glass, it could be good to go! I say “could” because the Foucault test revealed a pretty marginal 1/3.7 wavefront error, but most of that error came from an anomalous reading on the last zone on the very edge. Taking that reading out, my mirror was a respectable 1/5 – 1/6 wavefront error. Next Friday we’ll definitely do some more testing to determine more accurately what the situation is.

But the ultimate test is how well the mirror performs under actual use. If you pop it in a tube and it resolves clear images of what you’re looking at, all of the Foucault readings in the world are meaningless — the mirror is done! An un-aluminized mirror (aka glass) reflects about 4% of the light hitting it, making it more than good enough for viewing brighter stars and the Moon. So, I’ll try out the mirror as it is now, and if I like what I see, I’ll get it aluminized. If not, I’ll go back to figuring. But to test out how good it is, I actually need a tube to put it in.

Thus, I’ve started construction of the rest of my telescope. I went to the hardware store and bought most of the simple items I need: 3/4″ plywood, paint, screws, springs, silicone, and other stuff. I also put in online orders for all of the specialty parts that can’t be obtained in a hardware store: the focuser, diagonal secondary mirror, curved-vane spider, and eyepieces. I’m still missing a few things here and there, like angled aluminum to hold the mirror in place and teflon for the Dobsonian mount, but I’ll get it all eventually.

So it’s finally starting to look like I will actually finish this project. I will admit, it’s taken a bit longer than I thought to get where I am now, but I’m happy nonetheless. Figuring the mirror is the only truly precision part of the endeavor. Everything else is just basic handyman-type construction work: cutting wood to shape, screwing and bolting it together, painting, etc. That won’t pose a problem. Thanks to my dad and shop class in high school, I’m pretty comfortable around the various power tools I’ll need to use to finish this project.

And, just to tease you, here’s a picture of my tube after a day of painting. The tube is 55″ long and 10″ in internal diameter. I’ve applied one coat of primer on the inside and two coats of primer and one coat of topcoat on the outside. Still to go, a couple coats of flat black on the inside and another coat of topcoat and two coats of glossy acrylic on the outside. But even with just the one coat of topcoat, you can see now neat it looks. I chose a very unique type of paint for the topcoat. I didn’t originally have steampunk in mind when making this scope, but it looks like that’s where I’ll end up!

Telescope tube, 3 coats paint

Week ∞ of amateur telescope making

Thursday, January 10th, 2008

Last Friday at the telescope making workshop I made positive progress (though I’ve said that many times before). At the end of the session before that, I was left with a raised hill in the center of my mirror following a series of short strokes to correct my rolled over edge. So during this past session I focused on fixing the raised hill and trying to get back to paraboloidal. I basically did the same W-shaped parabolizing stroke for hour hours, with intermittent Ronchi tests, and the result was very promising. Take a look:

Mirror comparison

The photograph on the left shows my mirror at the beginning of the session. The central hill should be immediately obvious. Also note that the rest of the mirror doesn’t have the appropriate paraboloidal curve. The middle photograph shows my mirror at the end of the session. Notice how the central hill has been worked out, and the curvature has gotten better. This definitely warrants a Foucault test to see how close it is to the ideal curve. The computer-generated image on the right is what a Ronchi test on my mirror should look like if it was an ideal paraboloid. The number of lines isn’t really relevant here; what you should be looking at is the shape of the lines. And the central photograph makes the mirror looks a little bit scratchy; it’s not. I just failed to completely wipe all of the water droplets off of mirror after washing it.

So, I made an enormous amount of progress in just this one session (when I first started figuring, that amount of progress took a month). I’m getting better at this. I just haven’t arrived at my destination yet. Tomorrow is another night at the workshop. Hopefully it goes well!

And the mirror figuring goes ever onwards

Wednesday, January 2nd, 2008

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.

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Slow and steady wins the telescope-making race

Saturday, November 24th, 2007

A Ronchi test on my mirror as of 2007-11-16
The amateur telescope making (ATM) workshop wasn’t open yesterday on account of Thanksgiving, but I still haven’t written up a progress report yet on last week’s workshop, so I’ll go ahead and do that now. But first, allow me to talk about the others in the workshop, because there are some fascinating people there without whose help I would not be able to go on.

The workshop has been getting really busy lately. Guy, the guy who runs the workshop, says this always happens around autumn. I believe it. Last week I counted a record sixteen people at the workshop (it had been as low as four a month prior). Many people there are just starting on their projects, while others are returning from long absences to finish theirs up. A father is making a 8″ scope with his young son. A young man in the Armed Forces is making an 8″ scope, and last week his high school aged (?) friend joined him, making his own 6″ scope. There are also several older people making scopes, one of whom is quickly catching up with me.

Then there are the veterans, the people who’ve done it all before, multiple times, and just show up to hang out with friends or make steady progress on their projects in the company of others. One man has at least ten different mirrors and lenses under various stages of construction. He’s also built a mirror-matic at home that he uses for rapidly hogging out and polishing mirrors (it can be done faster than a human with a lot less effort).

Another man, an electrical engineer, is currently working on the automatic grinder in the workshop, which was built and then donated by a previous person at the workshop who gave up on their mirror. The automatic grinder needs a lot of work, but it is salvageable. He also likes singing and playing piano, and he’ll occasionally entertain the rest of us by playing on the piano in the workshop (it doubles as a music instruction room; the piano sits right next to the vaccum chamber).

And then there’s Bill, who seems to find a sense of zen in telescope-making. He polishes really slowly, trying to create the perfect mirror with no imperfections at all. But he doesn’t even keep the end products of his work. He donates them to Guy, who either sells them to raise funds for the workshop or uses them in telescopes for a weekend program he teaches at for inner-city DC youth. Guy recently built three brightly-colored telescopes that he uses to get the kids interested in science. As far as I know, the mirrors were all donated by Bill. Bill has been trading blanks with Guy to build more mirrors out of, and he’s also taken on a few mirror projects abandoned by people who stopped showing up at the workshop long ago, leaving their projects unfinished.

So that’s the story of who shows up to the workshop. It’s a really great group of people, with a nice mixture of ages. Everyone is there because they want to be, not because they have to be. It’s thus a much different atmosphere than, say, school or work.

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A triumphant telescope-making turnabout

Thursday, November 15th, 2007

Two weeks ago, I found the zen in telescope making. I learned to appreciate the journey more than the destination, because I was clearly getting farther from the destination throughout that night. But this past Friday was an utter triumph. Forget the zen of telescope making; bring on the ecstasy of telescope making!

This past Friday saw me doing really short strokes for about an hour trying to fix the turned-down edge, but with little success. At this point, we collectively said “Screw it.” The turned-down edge only takes up a small fraction of an inch on a mirror that is 8″ in diameter. It’s not a big loss. After I’m completely done with figuring, I’ll simply bevel off that edge, so that all of the remaining mirror surface is correctly figured.

Once I gave up on the somewhat unrealistic goal of a perfectly figured mirror from edge-to-edge (it’s very hard to do; many ATMers simply don’t bother), I met with success after success. I switched to a wide W-shaped parabolizing stroke, doing five minute sessions interspersed with frequent Ronchi tests. I made sure not to apply too much pressure. After each five minute session, the lines in the Ronchi test were slightly more outwardly curved, thus indicating I was getting closer to a perfect paraboloid. And along the way, the abnormal hole in the center of my mirror vanished. I had the same nice curve across the entire surface of the mirror.

By the end of Friday’s session, I was so close to the correct paraboloid shape that a Ronchi test wasn’t cutting it anymore. So at the start of the workshop tomorrow, we’ll do a Foucault test, and then I’ll know where to go from there. Optimistically, I could be done with figuring by the end of the workshop tomorrow. Then, the final step with the mirror will be aluminizing it; after that, the only things left to do are — everything else! But the primary mirror is by far the hardest part, and takes more time than the rest of the scope total.

I’m so excited. All of my hard work is about ready to pay off. With any luck, I’ll be looking up at the heavens before the end of the year.

The zen of telescope making

Wednesday, November 7th, 2007

A strange thing happened last Friday at the telescope making workshop. I actually made negative progress. Yet I’m not annoyed by it at all. I think I’ve discovered the zen of telescope making. Forget the destination and just learn to embrace the process — there are all sorts of worthy things to be learned by experimenting and messing up.

Two Fridays ago I started final figuring on my mirror. My mirror was oblate (the opposite side of parabolic from spherical), so I had a bit of work to do to correct it. I experimented with a figuring stroke that was very heavy on the edges and light in the center. It put me past spherical and towards parabolic, but unfortunately, it also created a depression in the center of the mirror, a defect that can be hard to work out.

So this past Friday I tried some different figuring strokes. I did fifteen minutes of the standard one-third-over-center stroke, the same stroke I had been using for many weeks prior to get to spherical. The central hole widened and became less steep, but did not disappear. So I tried using the star method, where a paper star is put between the pitch lap and mirror and then a weight is placed on top. This is supposed to depress the center of the pitch lap, so that it hopefully won’t wear away the center as much. It didn’t seem to help so much. So I tried a classical W-shape parabolizing stroke modified to spend more time in the center and less time on the edges. My mirror just got more funky, and the hole remained a problem.

At this point I was frustrated. I foolishly decided to try and force things by polishing really hard, like I had done back during initial polishing. I also used a longer stroke, because the shorter stroke I previously used during initial polishing led to my initial oblate shape. Those of you who’ve polished mirrors before know exactly what happened — when I looked at a Ronchi test, I noticed I had developed a turned-down edge. And the central hole was still there. Thus, my mirror was in worse shape at the end of last Friday than at the beginning.

I noticed the seasoned telescope-making veterans at the workshop being very encouraging and complimentary with me. I also remember being repeatedly warned about how figuring was the hardest stage of making the mirror; there’s a whole filing cabinet in the workshop full of the mirrors of people who’ve abandoned their projects. The majority of them quit during final figuring. It’s the first stage where the sense of progress can easily disappear. You’re no longer moving onto smaller and smaller grits each week, and a minor mistake can quickly reverse weeks of prior progress. So the vets, trying to prevent another person from disappearing, never again to be seen in the workshop (and another mirror being added to the filing cabinet), were being extra nice. But I couldn’t help feeling that they were underestimating me.

I’ve learned many things about figuring a telescope mirror, far more through my failures than if I had simply met with immediate success. I’ve learned an important lesson about patience, and how it never pays to try to force things. I still have a good bit of work ahead of me. But I’m content in the knowledge that I have enough patience to see this through, so I’m not too bothered when an entire week or two are wasted. I’ll be finished eventually. That’s why the veterans of the workshop shouldn’t worry about me. I plan to use my telescope for decades, so what does it matter if it takes another month to complete it? The longer I keep working on it, the more I learn about the process, even if it isn’t immediately reflected with progress. Therein lies the zen of telescope making.

Really wishing my telescope was finished right about now

Sunday, October 28th, 2007

I’ve been working on a telescope for the past two months. I’m in the final figuring stages right now, which entails a repetitive cycle of a few minutes of light polishing followed by a Ronchi or Foucault test to see how the figure has adjusted. I’m aiming for a perfect paraboloid. I was oblate coming into figuring (my previous stroke was too short), but in the past two weeks, I managed to get it to spherical, then onto nearly paraboloidal. My only problem is that the center of my mirror is a little too low right now, so I need to work that. We did make a new, softer pitch lap (by adding a bit of turpentine to the mix), and that has helped immensely.

I’m really wishing my telescope was finished now, though. In case you hadn’t heard, Comet 17P/Holmes recently experienced an eruption/outgassing, brightening from a dismally dim magnitude 17 to magnitude 2.8, which is easily visible with the naked eye. I went out to take a look at it earlier tonight — it was amazing. It appears to be the third brightest object in the constellation Perseus, but just looking at it, you can instantly tell that it’s not a star because it has a fuzzy, rather than pointlike, appearance.

Unfortunately, the only optics I have available to me right now are a set of 8×30 binoculars (that’s 8X magnification with 30mm objective lenses). They’re not much better than the naked eye. Yes, they do allow me to see slightly fainter objects, but the comet doesn’t exactly present that problem. It’s easily visible to the naked eye. And the binoculars are really old. They have some sort of collimation problem, such that even though they are 8X zoom, I really can’t see objects any better looking through them. So you see why I’m looking forward to finishing up my 8″ telescope. It’ll be too late to see this comet by the time I’m done, but I should be ready for whatever the next unpredictable bright night sky object happens to be. That’s the beauty of astronomy: it’s so totally unpredictable. You just have to be ready to take in the sights.

If you haven’t gone outside to look at this comet yet, I really do recommend it. You can see it with the naked eye even in polluted urban environments. Just go outside after dusk and look towards the northeast. You don’t even need to be familiar with what the constellation Perseus looks like. Just look for the only fuzzy bright object in the sky and you can’t miss it. And you don’t want to miss it. Night-sky-gazing can be a near-religious experience.

Finishing up the polishing of my telescope mirror

Sunday, October 14th, 2007

On Friday I continued working on my telescope project. Unfortunately, I didn’t have as much time to work on it as I did in previous weeks, because I had an after-work event to attend. But I did reach another milestone: I finished polishing my glass. I know I finished polishing because I couldn’t see any pits in the surface of the glass at 100X magnification. And now, some pictures.

Telescope 96

This is what my pitch lap looked like after cleaning out the grooves between the pitch segments using a razor blade.

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Fourth week of amateur telescope making

Monday, October 8th, 2007

On Friday I continued working on the 8″ f/6.1 telescope that I’m making by hand. I finished up grinding last week and went onto polishing this week. I took a bunch of photographs to document the process.

Polishing requires the use of a pitch lap to polish against. The pitch laps at the workshop are made from a ceramic plate roughly the same size as the glass covered in squares of pitch resin. The reason it’s in squares is because it needs to adapt to the shape of the glass, and if it’s all one big segment, it won’t be able to change shape nearly as easily.

Telescope 46

We started off by making a new pitch lap. This picture shows the process of removing hardened pitch off an old pitch lap using a hammer and chisel in order to reuse the ceramic.

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