03 Thursday Jan 2019
Posted in astronomy, astrophysics, flat, optical flat, Optics, Telescope Making
Tags
figuring, flats, Gordon Waite, machine, optical, Optics, parabolizing, Polishing, Telescope Making, testing optics, Waite Research, youtube
Gordon Waite is a commercial telescope maker who has made a number of very useful YouTube videos on his grinding, polishing, parabolizing, and testing procedures. I thought some of my readers might be interested in viewing them. The link is here, or else you can copy and paste this:
10 Saturday Nov 2018
Posted in astronomy, Hopewell Observatorry, Optics, Safety, Telescope Making
Tags
Bob Bolster, George Ritchey, Grinding, Hopewell Observatory, matching Ronchi, Mel Bartels, Polishing, Ronchi, ronchigram, Telescope Making, testing
I have been wrestling with this mirror for YEARS. It’s not been easy at all. The blank is only about twice the diameter of an 8″ mirror, but the project is easily 10 times as hard as doing an 8-incher. (Yes, it’s the one in the photo heading this blog!)
Recently I’ve been trying to figure it using a polishing/grinding machine fabricated by the late Bob Bolster (who modeled his after the machine that George Ritchey invented for the celebrated 60″ mirror at Mount Wilson over a century ago). That’s been a learning exercise, as I had to learn by trial and error what the machine can and cannot do, and what strokes produce what effects. The texts and videos I have seen on figuring such a large mirror with a machine have not really been very helpful, so it’s mostly been trial and error.
My best results right now seem to come from using an 8″ pitch tool on a metal backing, with a 15 pound lead weight, employing long, somewhat-oval strokes approximately tangential to the 50% zone. The edge of the tool goes about 5 cm over the edge of the blank.
This little movie shows the best ronchigrams I have ever produced with this mirror, after nearly 6 hours of near-continuous work and testing. Take a look:
And compare that to how it used to look back in September:
Also compare that to the theoretically perfect computed ronchigrams from Mel Bartels’ website:
Part of the reason this mirror has taken so long is that after grinding and polishing by hand some years ago, I finally did a proper check for strain, and discovered that it had some pretty serious strain. I ended up shipping it out to someone in Taos, New Mexico who annealed it – but that changed the figure of the mirror so much that I had to go back to fine grinding (all the way back to 120 or 220 grit, I think), and then re-polishing, all by hand. I tried to do all of that, and figuring of the mirror, at one of the Delmarva Mirror Making Marathons. It was just too much for my back — along with digging drainage ditches at Hopewell Observatory, I ended up in a serious amount of pain and required serious physical therapy (but fortunately, no crutches), so this project went back into storage for a long, long time.
Recently I’ve tried more work by hand and by machine. Unfortunately, when I do work by hand, it seems that almost no matter how carefully I polish, I cause astigmatism (which I am defining as the mirror simply not being a figure of rotation) which I can see at the testing stand as Ronchi lines that are not symmetrical around a horizontal line of reflection. (If a Ronchi grating produces lines that look a bit line the capital letters N, S, o Z, you have astigmatism quite badly. If astigmatism is there, those dreaded curves show up best when your grating is very close to the center of curvature (or center of confusion) of the central zone.
Using this machine means controlling or guessing at a LOT of variables:
Here is a sketch of how this works
08 Saturday Sep 2018
Posted in astronomy, flat, Hopewell Observatorry, optical flat, Optics, Telescope Making
Tags
Astro Bananas, cassegrain, couder, double pass auto collimation, ealing, foucault, Hopewell Observatory, matching Ronchi, Mel Bartels, Ronchi, ronchigram
The other regulars and I at the DC ATM group at the CCCC have been trying to test a 12 inch Cassegrain mirror and telescope manufactured nearly 50 years ago by a company called Ealing and currently owned by the Hopewell Observatory, of which I am a member. It hasn’t been easy. I discussed this earlier on Cloudy Nights.
Reports from several people, including Gary Hand and the late Bob Bolster, indicated that the optics on this mirror weren’t good at all. Apparently the folks at the University of Maryland’s observatory were sufficiently unhappy with it that they either sold it or gave it to National Capital Astronomers, a local astronomy club, who in turn gave it or sold it to Hopewell Observatory.
With a plain-vanilla Ronchi test, we could see that the mirror was very smooth and continuous, with no turned edge, astigmatism, or bad zones. With the Foucault/Couder zonal test (aka “Foucault” test) , I got results indicating that it was seriously overcorrected: some sort of hyperboloid, rather than the standard paraboloid characteristic of classical Cassegrain telescopes, which have a parabolic primary mirror and a hyperbolic secondary mirror.
However, I have begun losing my faith in my zonal readings, because they often seem to give results that are way out of whack compared to other testing methods.
So we decided to do some additional tests: the Double-Pass Auto-Collimation (DPACT) test used by Dick Parker, as well as the Matching Ronchi test (MRT).
The DPACT is very fiddly and exacting in its setup. We used (and modified) the setup lent to us by Jim Crowley and illustrated by him at his Astro Bananas website. Our results seem to show that the mirror is in fact NOT parabolic, rather, overcorrected, which confirms my Foucault measurements. If it were a perfect paraboloid, then the ronchi lines would be perfectly straight, but they definitely are NOT: they curve one way when inside the focal point, and curve the other when the tester is outside the focal point.
We also tested the entire setup on a radio tower that was about half a mile (~1km) distant. We found that the images were somewhat blurry no matter what we did.
We also attempted the MRT on the same mirror. However, requires very accurate photography and cutting-and-pasting skills in some sort of graphics programs. What you are inspecting is the curvature of the Ronchi lines. Here is the result that Alan T and I got last night:
In black is the ideal ronchigram for this mirror, according to Mel Bartels’ website. The colored picture is the one we made with either my cell phone or the device I finished making earlier this week, shown in my previous post. Here are the two images, separated rather than superimposed:
The mirror’s focal length is 47.5″ and the grating has 100 lines per inch, shown somewhat outside of the radius of curvature. The little ‘eyelash’ on the lower left is simply a stray wire that was in the way, and doesn’t affect the image at all. The big hole in the middle is there because the mirror is a cassegrain.
I don’t know about you, but I don’t really see any differences between the real mirror and the theoretical mirror. Do you?
So, what does this all mean?
07 Friday Sep 2018
Posted in astronomy, Optics, science, Telescope Making
Tags
brightness, color balance, exposure, focus, foucault, gain, knife edge, Ronchi, testing, webcam
Here is the latest incarnation of my webcam Ronchi and knife edge (or Foucault) tester. It’s taken quite a few iterations to get here, including removing all the unnecessary parts of the webcam. I attach a still photo and a short video. The setup does quite a nice job of allowing everybody to see what is happening. The only problem is setting the gain, focus, exposure, brightness, color balance, contrast, and so on in such a way that what you see on the screen resembles in any way what your eye can see quite easily.
27 Monday Aug 2018
Posted in astrophysics, education, History, Math, science, teaching, Telescope Making, Uncategorized
Tags
education, engineering, forgetting, France, mathematics, scientists, USA
This was a question on Quora. Here is an answer I wrote:
In the US, judging strictly on what I’ve seen from my time in the classroom as both a student, a teacher, and a visiting mentor of other math teachers, I find that math and science was very often taught as sort of cookbook recipes without any real depth of understanding. The recent National Council of Teachers of Mathematics prescriptions have attempted to correct that, but results have been mixed, and the Common Core has ironically fostered a weird mix of conceptual math marred by teachers being *OBLIGATED* to follow a script, word-for-word, if they want to remain employed. Obviously, if students are really trying to understand WHY a certain mathematical or scientific thing/fact/theorem/theory/law is true, they are going to have questions, and it’s obviously the teacher’s job to figure out how best to answer said questions — which are not likely to have pre-formulated scripts to follow in case they come up — and which are going to take time.
Another thing that is true is that not everything in mathematics has real-world applications in every single person’s life. I taught a good bit of computer programming (aka ‘coding’ today), geometry, arithmetic, probability, algebra, statistics, and conic sections, and in fact I use a LOT of that every week fabricating telescope mirrors to amazing levels of precision, by hand, not for a living, but because I find telescope-making to be a lot of fun and good mental, aesthetic, manual, and physical exercise. But I’m a pretty rare exception!
Most people obviously don’t dabble in math and physics and optics like I do, nor should they!
In fact, I have made it a point to ask professional scientists and engineers that I meet if they actually use, on their jobs, all the calculus that they learned back in HS and college. So far, I think my count is several dozen “Noes” and only one definite “Yes” – and the latter was an actual rocket scientist / engineer and MIT grad and pro-am astronomer (and wonderful, funny, smart person) who deals/dealt with orbital rocket trajectories. (IIRC).
In France, when I went to school there 50 years ago and in my experience tutoring some kids at the fully-French Lycee Rochambeau near Washington, DC, is that they go very deeply into various topics in math, and the sequence of topics is very carefully thought out for each year for each kid in the entire nation (with varying levels of depth depending on what sort of track that the students elected to go into (say, languages/literature, pure math, or applied sciences, etc), but the kids were essentially obligated to accept certain ideas as factual givens and then work out more and more difficult problems that dealt with those particular givens. No questions allowed on where the givens came from, except to note the name of the long-dead classical Greek, French, Italian or German savant whose name is associated with it.
As an American kid who was mostly taught in American schools, but who also took 2 full years of the French system (half a year each of neuvieme, septieme, premiere, terminale, and then passed the baccalaureat in what they called at the time mathematiques elementaires, I found the choice of topics [eg ‘casting out nines’ and barycenters and non-orthogonal coordinate systems] in France rather strange. Interesting topics perhaps, but strange. And not necessarily any more related to the real world than what we teach here in the US.
Over in France, however, intellectuals are (mostly) respected, even revered, and of all the various academic strands, pure math has the highest level of respect. So people over there tend to be proud of however far they got in mathematics, and what they remember. Discourse in French tends to be extremely logical and clear in a way that I cannot imagine happening here in the public sphere.
So to sum up:
(a) most people never learned all that much math better than what was required to pass the test;
(b) only a very few geeky students like myself were motivated to ask ‘why’;
(c) most people don’t use all that much math in their real lives in the first place.
27 Tuesday Mar 2018
Posted in astronomy, Math, Optics, Telescope Making, Uncategorized
Tags
By Guy Brandenburg
3/27/2017
I describe here an attempt to quantify progress (or lack thereof) in the removal of the classic, and dreaded, turned-down edge (TDE) present on a 16.5” Newtonian glass mirror blank that I have been trying to “figure” for some years. The figuring process means changing a piece of glass that approximates a small section sliced out of a large hollow sphere, into a highly-accurate paraboloid — with the required level of accuracy being measured in nanometers.
Many amateur and professional telescope makers have maintained that you can only fix figuring errors if you can measure them. Not being able to get good, repeatable measurements of the TDE on my mirror, I had been sort of floundering, failing to get rid of the TDE even after YEARS of work (off and on; mostly off). So a decision was made to try to quantify things.
We recently had some success in matching computer-generated Ronchi images of theoretically-perfect mirrors with photos taken of works in progress, simply by cutting and pasting – which has been recommended by Mel Bartels in particular for quite some time. For the first time, I got the hang of it, and we were able to help a first-timer (Mike L) to figure a 10” plate glass f/5.4 mirror only ¾” thick to just about exactly ¼ lambda, according to our combined, repeated, careful measurements on a mirror that was cooled both by immersion in a room-temperature water bath and by sitting in a closet in the very same testing room for an entire weekend.
Prior to this experiment, I had been taking short videos of the entire mirror, moving the ronchi grating back and forth across the center of curvature. These videos reveal and record a lot of qualitative information about the mirror, including vocal commentary, but I found it impossible to transfer the images to my laptop for closer analysis until I got home, across town, which meant that the turn-around time after testing a mirror was much too long to be of any use. I had tried quite a large number of various strokes suggested by others, by our reading various ATM manuals, and by just thinking; but the very serious TDE on this (for me, relatively ambitious) project never seemed to get any better.
I simply gave up on imaging via video clips, since they were too hard to manipulate or measure on my phone, and which required too much bandwidth to send to my laptop until I got home. This time, I took Ronchi still-images on my cell phone, between 0.2 and 0.5 inches outside of the center of curvature.
(My experience has been generally easier to discern defects in a Ronchigram when the lines curve outwards at the top and bottom, which would mean the test grating is OUTSIDE the COC of a partly-parabolized mirror, as you see on the left in the black-and-white image above. However, when the lines curve inwards at the top and bottom, like the images in the center and to the right, then many serious defects remain hidden. 
Procedure:
A standard 100 LPI grating from Willmann-Bell and a yellow LED were used, on an XYZ stage partly fabricated by me and placed exactly twice the focal length from the primary. Images were taken with an iPhone 6, shooting images zoomed in as much as possible. An attempt was made to have matching ronchigrams, i.e., with the same number of vertical lines showing.
(This was a weak point of the experiment. For one, it’s hard to hold cell phone steady enough, and an observer will notice that the images do NOT have exactly the same number of lines. That’s because I did not have a printout of the previous image right in front of me to make comparisons to. All that needs to be fixed in subsequent iterations. Also, other imaging devices need to be tried, as well.)
I was in fact able to email individual photograph frames to my laptop at the lab. After downloading the clearest images to my laptop, I used plain old MS Windows Paint to shrink and crop the useful portion of the picture, and then pasted the result into a Geometry software (Geometer.s Sketchpad, or GSP) that I was already familiar with. GSP was then used to draw a circle around the circumference of the image of the nearly-perfectly-circular glass disk, adjusting this as well as possible. This process automatically generated the center of the disk. Using that center, a second, and smaller, circle was drawn whose circumference was placed at the location along the ronchi lines where they appeared to begin to turn outwards. GSP was then to measure directly the radii of the two circles and to compute their ratio.
A final ratio of 0.7, just to pick a number that is easy to compute, means that just about half of the area of the mirror is covered by a wide rolled-down edge, since the ratio of areas is equal to the square of the ratio of the respective radii, and 0.7 squared is 0.49, or 49%.
In the diagram above, the images go in chronological order but COUNTER-clockwise, from upper left (labeled #1), which was made in mid- or early March, through the next three images, all taken on March 22. In between each image, various strokes were employed in figuring sessions for anywhere between 15-20 minutes to attempt to fix the TDE. All the figuring sessions involved sub-diameter laps anywhere from 8 to 12 inches in diameter that had been warm-pressed upon the mirror. The strokes were both forward and back and incorporated enough of a ‘W’ stroke to cover the entire mirror, using cerium oxide on either tempered burgundy or Acculap pitch, depending. The edge of the tool was allowed to go up to the edge of the mirror, +/- maybe 5 mm. The goal was simply to wear down the glass in the center until it caught up with the amount that the edge had been worn down. None of the laps seemed to have full contact with the mirror out to the very edge; thus the end of the stroke was NOT at the edge of the mirror.
You will notice that these ratios, circled in green, seem to increase monotonically from 69% to 80%, which is gratifying: if this real, then the fraction of the mirror that is NOT covered by TDE has gone from about 47% to about 67%, as you can see here. (Note: in figure #1, the large circle was denoted circle AB, and the smaller circle was denoted circle CD. I know that points A and C are not identical, but they are rather close; that error will be fixed in subsequent iterations.)
However: the key question is: IS THIS REAL? Or am I merely fooling myself?
I don’t know yet.
I certainly hope it is real.
But it needs to be checked with subsequent investigation.
My attempt at limiting my own subjectivity or wishful thinking was to try to draw the circles at the place where the more-or-less vertical lines began turning outwards. Hopefully that location really corresponded to the place where the turned/rolled edge began. However, it is entirely possible that the precise apparent location of the beginning of the TDE very much depends on exactly how many lines appear in the Ronchigram, thus, precisely how far from the COC the grating is located.
Unfortunately, often times I have to dismantle the entire apparatus, because we have to close up shop for the night, or somebody else needs to use the tester on another mirror. Thus, it is nearly impossible to ensure that the measurement apparatus remains undisturbed.
My next steps, I think, are these:
I would like to thank Isaac and Elias Applebaum for their diligent and noted explorations in solving a similar question relating to fixing or preventing TDE. That STEM project won them a number of well-deserved awards.
05 Monday Mar 2018
Just got back from an exciting astro expedition to Hopewell Observatory with one of the other members. Great fun!
Anybody living on the East Coast in March 2018 has just lived through a very strong, multi-day gale. The same weather system brought snow and flooding to the northeast, and here in the DC-Mar-Va area, it was cut off power to many (including my mother-in law) and caused almost all local school districts to close — even the Federal Government! Two of my immediate neighbors in DC had serious roof damage.
Today, Sunday, Paul M and I decided the wind had calmed enough, and the sky was clear enough, for an expedition to go up and observe. We both figured there was a good chance the road up to the observatory would be blocked by trees, and it turns out that we were right. My chainsaw was getting repaired – long story, something I couldn’t fix on my own – so I brought along work gloves, a nice sharp axe, loppers, and a 3-foot bowsaw. We used all of them. There were two fairly large dead trees that had fallen across the road, and we were able to cut them up and push them out of the way.
However, there was a large and very dangerous ‘widow-maker’ tree (two images above) that had fallen across the road, but it was NOT on the ground. Instead, was solidly hung up on the thick telecommunications line at about a thirty-degree angle to the ground. The power lines above it didn’t seem to be touched. You could easily walk under the trunk, if you dared (and we did), and you probably could drive under it, but of course the motion of the car just might be enough to make it crack in half and crush some unlucky car and its driver. Or maybe it might make the phone line shake a bit …
No thanks.
So, we didn’t drive under.
I called the emergency phone for the cell phone tower (whose access road we share) to alert them that the road was blocked and could only be cleared by a professional. I also attempted to call a phone company via 611, without much success — after a long wait, the person at the other end eventually asked me for the code to my account before they would forward me to somebody who could take care of it. Very weird and confusing. What account? What code? My bank account? No way. We will both call tomorrow. Paul says he knows some lawyers at Verizon, whose line he thinks it is.
But then: how were we going to turn the cars around? It’s a very narrow road, with rocks and trees on one side. The other side has sort of a ravine and yet more trees. Paul realized before I did that we had to help each other and give directions in the darkness to the other person, or else we would have to back up all the way to the gate! Turning around took about four maneuvers, per car, in the dark, with the other person (armed with astronomer’s headlamp, of course) yelling directions on when to turn, how much to go forward, when to stop backing up, and so on. Success – no injuries! We both got our cars turned around, closed them up, got our cutting tools, gloves and hats, and then hiked the rest of the way up, south and along the ridge and past the big cell phone tower, to the Observatory buildings themselves, moving and cutting trees as we went.
As we were clearing the roadway and walking up the ridge, we peered to the west to try to find Venus and Mercury, which had heard were now evening planets again. It wasn’t easy, because we were looking through LOTS of trees, in the direction of a beautiful multi-color, clear-sky sunset featuring a bright orange line above the ridge to our west. Winter trees might not have any leaves, but they still make the search for sunset planets rather tough. Even if you hold perfectly still, one instant you see a flash that’s maybe a planet, or maybe an airplane, and then the branches (which are moving in the breeze, naturally) hide it again. So what was it? Paul’s planetarium smartphone app confirmed he saw Venus. If the trees weren’t there, I think we also would have seen Mercury, judging by Geoff Chester’s photo put out on the NOVAC email list. I think I saw one planet.
In any case, everything at the observatory was just fine – no tree damage on anything, thanks to our prior pruning efforts. The Ealing mount and its three main telescopes all worked well, and the sky and stars were gorgeous both to the naked eye and through the scopes. Orion the Hunter, along with the Big Dog and the Rabbit were right in front of us (to the south) and Auriga the Charioteer was right above us. Pleiades (or the Subaru) was off high in the west. Definitely the clearest night I’ve had since my visit to Wyoming for the solar eclipse last August, or to Spruce Knob WV for the Almost Heaven Star Party the month after that.
Paul said that he and his daughter had been learning the proper names of all the stars in the constellation Orion, such as Mintaka, Alnilam, and Alnitak. As with many other star names, all those names are Arabic, a language that I’ve been studying for a while now [but am not good at. So complicated!] Mintaka and Alnitak are essentially the same Arabic word.
After we got the scopes working, Paul suggested checking out Rigel, the bright ‘leg’ of Orion, because it supposedly had a companion star. {Rajul means “leg”} We looked, and after changing the various eyepieces and magnifications, we both agreed that Rigel definitely does have a little buddy.
I had just read in Sky & Telescope that Aristotle (from ancient Greece) may have given the first written account of what we now call an “open cluster” in the constellation Canis Major (Big Dog – that’s Latin, which I studied in grades 7 – 12) called Messier-41, only a couple of degrees south of Sirius, the brightest star in the sky. A passage in a book allegedly written by Aristotle (roughly 230 BC) seems to indicate that he could see this object with averted vision. (He was trying to establish that it was a fuzzy patch in the sky that was most definitely NOT a comet, just like Charles Messier was doing almost exactly two thousand years later!)
M41 was quite attractive. But no, we didn’t then look at M42. Been there, done that many times before. And no, what you see with a telescope does not have all those pretty colors that you see in a photograph.
Instead, we looked on a multi-sheet star atlas (that stays in the observatory) near M41 and found three other open clusters, all really beautiful. We first found M38 and thought that in the C-14 and 6″ Jaegers, it looked very anthropoid or like an angry insect, if you allowed your mind to connect the beautiful dots of light on the black background. In the shorter 5″ refractor made by Jerry Short, it looked like a sprinkling of diamond dust. This cluster must have been formed rather recently. We then found M36, which was much less rich, but still quite pretty. Lastly, we found M37, another open cluster, which has a very bright yellow star near the center, against background of much fainter stars. It seemed to me that those other stars might be partly obscured by a large and somewhat translucent cloud of dust. We saw a web of very opaque dust lanes, which we confirmed by readings on the Web. Really, really beautiful. But I’m glad we don’t live there: too dangerous. Some of the stars are in fact red giants, we read.
Then we looked straight overhead, in the constellation Auriga. We decided to bypass the electronics and have Paul aim the telescope, using the Telrad 1-power finderscope, at one of the fuzzy patches that he saw there. He did, and my notes indicate that we eventually figured out that he found Messier-46 (yet another open cluster) with his naked eye! Very rich cluster, I think, and we even found the fan-shaped planetary nebula inside!
At this point we were getting seriously cold so we moved over just a little, using the instruments, to find M47, again, a very pretty open cluster.
Realizing that the cold and fatigue makes you do really stupid things, and that we were out in the woods with no way to drive up here in case of a problem, we were very careful about making sure we were doing the closing up procedures properly and read the checklist at the door to each other, to make sure we didn’t forget anything.
On the walk back, we saw the Moon coming up all yellowish-orange, with the top of its ‘head’ seemingly cut off. When it got a bit higher, it became more silver-colored and less distorted, but still beautiful.
I really thought all of those open clusters were gorgeous in their own right, and I think it would be an excellent idea to make photographs of them, but perhaps black dots on white paper, and give them to young folks, and ask them to connect the dots, in whatever way they feel like doing. What sorts of interesting drawings would twenty-five students come up with?
I am not sure which of our various telescopes would do the best job at making astro images. I have a CCD camera (SBIG ST-2000XM), with a filter wheel. What about just making it a one-shot monochromatic black and white image? I also have a Canon EOS Revel XSI (aka 450D, I think). Compare and contrast… The CCD is really heavy, the Canon quite light. I also have a telephoto lens for the Canon, which means that I have essentially four telescopes to choose from (but not a big budget!). One problem with the C-14 and my cameras is that the field of view is tiny: you can only take images of very small bits of what you can see in the eyepiece with your naked eye. This means you would need to make a mosaic of numerous pictures.
In any case, no imaging last night! Not only did I not feel like hauling all that equipment for a quarter of a mile, after all that chopping, sawing, and shoving trees, it turns out I had left my laptop home in the first place. D’oh!
I had previously found every single one of these open clusters when I made my way through the entire Messier list of over 100 objects, with my various home-made telescopes, which had apertures up to 12.5 inches. However, I don’t think I had ever seen them look so beautiful before! Was it the amazing clarity of the night, or the adventure, or the company? I don’t know!
But this was a very fun adventure, and this photography project – attempting to make decent images of these six open clusters – promises to be quite interesting!
31 Wednesday Jan 2018
Posted in astronomy, Math, Optics, Telescope Making, Uncategorized
I just finished a 6″ f/8 Dobsonian telescope as a gift for my great-nephews, one of whom I discovered is VERY interested in astronomy and happens to live in a place with pretty dark skies – about the middle of Maryland’s Eastern Shore. The mirror is excellent, and the mechanical parts all work very well — in my opinion. Let’s see what the recipients think.
I finally succeeded in putting in the mirror yesterday afternoon in a very stiff and cold wind right outside my house, estimating where the mirror should go by aiming at a distant chimney. This takes patience because it’s trial-and-error, no matter how much you calculate beforehand!
Later that evening, after the nearly-full Blue Moon came over the trees at DC’s Chevy Chase Community Center where we have our telescope-making classes, fellow ATMer and all-around interesting person Jim Kaiser helped me collimate it by pointing the scope at the illuminated curtains in the windows of the CCCC. We then verified that the Moon actually did come to a focus with the eyepiece nearly all the way screwed in. If you want to focus on closer things than the Moon or galaxies, you need to screw the eyepiece out towards you, the observer in the cheap but effective helical focuser that was lying around the shop.
This scope incorporates a couple of innovations by me, and a bit of artistic whimsy.
First small innovation: I made the secondary diagonal mirror holder so that no tools are needed at all: you just rotate the part holding the elliptical mirror and turn a little thumbscrew to collimate it quickly and easily, while you watch. Here is a sketch of how I made it.
Second innovation can be seen near my right hand (to your left) atop the cradle: two 1/4″-20 machine screws with simple homemade knobs on top, going through threaded inserts (T-nuts would work too), which push against a piece of lumber in the shape of prism with an isosceles right triangle at each end. I call this the tube brake, which can be applied or released quite easily, whenever needed. Small springs (almost impossible to see in this photo) pull this brake up against the corner of the tube, while the machine screws press it down. If you want to change the position of the eyepiece because a taller or shorter person has arrived, no problem. A few CCW turns of the wooden knobs releases the brake, you rotate the tube to the desired position, and then you lock it down again with a few clockwise turns. If you add or remove a heavy eyepiece or a finder or whatever, same procedure, except this time you can slide the scope up and down inside the cradle.
The artistic whimsy is partly seen in a photo Jim took of me after we got it collimated but before we rushed back inside: lots of colors, thanks to several tons of paint cans salvaged by fellow ATMer Bill Rohrer from being thrown away by a third party who lost his warehouse lease, and also because smurf blue is the favorite color of one of the boys. The altitude bearing is made out of the Corian countertop that my wife and I got rid of a few months ago when we had our kitchen remodeled. (30 years ago we did it ourselves, mostly. This time we hired professionals. They are SOO much faster and better at this than us!)
So that my young relatives can keep this thing looking good, they also get four or five quart or pint cans of paint – the ones I used on the scope. Free, of course. The more we get rid of put to use, the better. They can repaint anything that gets scratched, you see?
You can also see some wood-cutting fun above and below. This retired geometry teacher had a lot of fun figuring out how to lay out and cut out stars with 5, 6, and 7 points, as well as a crescent moon and a representation of Saturn seen with its rings edge-on. I guess you could show Saturn’s rings a 30 to 45 degrees to the viewer, if you instead carved it out of solid wood or did wood burning, but I just had a hand-held jigsaw and a Dremel knockoff. And plus, this is supposed to be a scope that is USED rather than just admired for its artsy parts.
I designed what I wanted onto two sheets of paper and then taped them to the plywood. This worked, but it wasn’t the most wonderful plywood, so on many of the pull strokes, the wood splintered a bit. So that side got to face inside.. Painting all those little nooks and crannies was tough!
(The purpose of the cut-outs was simply to make the telescope lighter. It’s got a very heavy and sturdy base. Each square inch of plywood removed saves about 7 grams. Also, more holes means more hand-holds!)
18 Thursday Jan 2018
Posted in astronomy, education, Optics, Safety, Telescope Making, Uncategorized
This weekend, I’m hosting a small workshop at the Community Museum in Gaithersburg, MD, where interested persons from 8 to 88 years of age can make their own telescope in an hour or two. We will be using surplus but high-quality achromatic primary doublet lenses as well as inexpensive eyepieces, along with PVC tubing and some really cool tripods to hold it steady. We will some basic optics experiments to help explain how these gizmos work, and will have spray paint and colored tape to decorate the tubes.
If you are interested, here is the necessary information:
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Sunday at 1 PM – 4 PM
3 days from now · 36–50°Partly Cloudy
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Gaithersburg Community Museum
9 S Summit Ave, Gaithersburg, Maryland 20877
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You will also see how various types of telescopes such as reflectors, refractors, and catadioptrics are put together and operate, using actual examples, including the type made and used by Galileo around 1609.
$30 City of Gaithersburg residents/$35 non-residents. Space is limited to 15 so pre-registration is required. To register go to RecXpress at https://online.activenetwork.com/gaithersburg/Start/Start.asp. It’s activity #49690.
For more information or if you have trouble registering call the museum at 301-258-6160 or museum@gaithersburgmd.gov
27 Monday Nov 2017
Posted in astronomy, astrophysics, nature, Safety, science, Telescope Making
Some very nice folks from the Australian Broadcasting Corporation came and interviewed me on film for a bit on folks who make their own telescopes to see the great August 2017 eclipse. Here is the link:
( https://www.facebook.com/abcnews.au/videos/10157157152414988/ )
