Shallow Thoughts : : astro

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Tue, 10 Dec 2019

Planetarium Show Friday: Hitchhiker's Guide to the Moon

[Schroter's Valley on the Moon] This Friday, Dave and I will be presenting a planetarium show called The Hitchhiker's Guide to the Moon: Visit the Moon Without Leaving Your Home Planet.

I'm jazzed about this show. I think it'll be the most fun planetarium show we've given so far. We'll be showing a variety of lunarfeatures: maria, craters, mountains, rilles, domes, catenae and more. For each one, we'll discuss what the feature actually is and how it was created, where to see good examples on the moon, and -- the important part -- where you can go on Earth, and specifically in the Western US, to see a similar feature up close.

Plus: a short flyover of some of the major features using the full-dome planetarium. Some features, like Tycho, the Straight Wall, Reiner Gamma, plus lots of rilles, look really great in the planetarium.

If you can't get to the moon yourself, this is the next best thing!

The Hitchhiker's Guide to the Moon: 7pm at the PEEC nature center. Admission is free. Come find out how to explore the moon without leaving your home planet!

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[ 18:06 Dec 10, 2019    More science/astro | permalink to this entry | comments ]

Mon, 11 Nov 2019

Mercury Transit: Comparing Between H-alpha and White Light

[Mercury transit 11/11/2019] The Mercury transit is over. But we learned some interesting things.

I'd seen Mercury transits before, but this is the first time we had an H-alpha scope (a little 50mm Coronado PST) in addition to a white light filter (I had my 102mm refractor set up with the Orion white-light filter).

As egress approached, Dave was viewing in the H-alpha while I was on the white light scope. When I saw the black-drop effect at third contact, Mercury was still nowhere near the edge in the H-alpha: the H-alpha shows more of the solar atmosphere so the sun's image is noticably bigger. This was the point when we realized that we should have expected this and been timing and recording. Alas, it was too late.

Mercury was roughly 60% out in the white light filter -- just past the point where the "bite" it made in the limb of the sun -- by the time Dave called out third contact. We guessed it was roughly a minute, but that could be way off.

For fourth contact, Dave counted roughly 45 seconds between when I couldn't see Mercury any more and when he lost track of it. This is pretty rough, because it was windy, seeing was terrible and there was at least a 15-second slop when I wasn't sure if I could any indentation in the limb; I'm sure it was at least as hard in the Coronado, which was running at much lower magnification.

So we had a chance to do interesting science and we flubbed it. And the next chance isn't til 2032; who knows if we'll still be actively observing then.

I wanted to at least correlate those two numbers: 45 seconds and 60% of a Mercury radius.

Mercury is about 10" (arcseconds) right now. That was easy to find. But how fast does it move? I couldn't find anything about that, searching for terms like mercury transit angular speed OR velocity. I tried to calculate it with PyEphem but got a number that was orders of magnitude off. Maybe I'll figure it out for a later article, but I wanted to get this posted quickly.

[Mercury transit 11/11/2019 in H-alpha] I didn't spend much time trying photography. I got a couple afocal snaps with my pocket digital camera through the white-light scope that worked out pretty well. I wasn't sure that would work for the Coronado: the image is fairly dim. The snaps I did get show Mercury, though none of the interesting detail like faculae and the one tiny prominence that was visible. But the interesting thing is the color. To the eye, the H-alpha scope image is a slightly orangy red, but in the digital camera it came out a startling purplish pink. This may be due to the digital camera's filters passing some IR, confusing the algorithms that decide how to shift the color. Of course, I could have adjusted the color in GIMP back to the real color, but I thought it was more interesting to leave it the hue it came out of the camera. (I did boost contrast and run an unsharp mask filter, to make it easier to see Mercury.)

Anyway, fun and unexpectedly edifying! I wish we had another transit happening sooner than 2032.

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[ 12:15 Nov 11, 2019    More science/astro | permalink to this entry | comments ]

Fri, 08 Nov 2019

Mercury Transit Next Monday

[Mercury Transit 2006, photo by Brocken Inaglory]
Mercury Transit 2006, photo by Brocken Inaglory
Next Monday, November 11, is a transit of Mercury across the sun.

Mercury transits aren't super rare -- not once- or twice-in-a-lifetime events like Venus transits -- but they're not that common, either. The last Mercury transit was in 2016; the next one won't happen til 2032.

This year's transit isn't ideal for US observers. The transit will already be well underway by the time the sun rises, at least in the western US. Here in New Mexico (Mountain time), the sun rises with Mercury transiting, and the transit lasts until 11:04 MST. Everybody else, check timeanddate's Mercury Transit page for your local times.

Mercury is small, unfortunately, so it's not an easy thing to see without magnification. Of course, you know that you should never look at the sun without an adequate filter. But even if you have safe "eclipse glasses", it may be tough to spot Mercury's small disk against the surface of the sun.

[binocular projection of a solar eclipse] One option is to take some binoculars and use them to project an image. Point the big end of the binoculars at the sun, and the small end at a white surface, preferably leaning so it's perpendicular to the sun. I don't know if binocular projection will give a big enough image to show Mercury, so a very smooth and white background, tilted so it's perpendicular to the sun, will help. (Don't be tempted to stick eclipse glasses in front of a binocular or telescope and look through the eyepiece! Stick to projection unless you have filters specifically intended for telescopes or binoculars.)

Of course, a telescope with a safe solar filter is the best way to see a transit. If you're in the Los Alamos area, I hear the Pajarito Astronomers are planning to set up telescopes at Overlook Park. They don't seem to have announced it in any of the papers yet, but I see it listed on the Pajarito Astronomers website. There's also an event planned at the high school where the students will be trying to time Mercury's passage, but I don't know if that's open to the public. Elsewhere in the world, check with your local astronomy club for Mercury transit parties: I'm sure most clubs have something planned.

I was discussing the transit with a couple of local astronomers earlier this week, and one of them related it to the search for exoplanets. One of the main methods of detecting exoplanets is to measure the dimming of a star's light as a planet crosses its face. For instance, in 55 Cancri e, you can see a dimming as the planet crosses the star's face, and a much more subtle dimming when the planet disappears behind the star. As Mercury crosses the Sun's face, it blocks some of the sun's light in the same way. By how much?

The radius of Mercury is 0.0035068 solar radii, and the dimming is proportional to area so it should be 0.00350682, or 0.0000123, a 0.00123% dimming. Not very much!

But it looks like in the 55 Cancri e case, they're detecting dips of around .001% -- it seems amazing that you could detect a planet as small as Mercury this way (and certainly the planet is much bigger in the case of 55 Cancri e) ... but maybe it's possible.

Anyway, it's fun to think about exoplanets as you watch tiny Mercury make its way across the face of the Sun. Wherever you are, I hope you get a chance to look!

Update: A report from the transit: Mercury Transit: Comparing Between H-alpha and White Light.

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[ 11:36 Nov 08, 2019    More science/astro | permalink to this entry | comments ]

Thu, 13 Jun 2019

Finding Astronomical Alignments in Ancient Monuments (or anywhere else)

Dave and I will be presenting a free program on Stonehenge at the Los Alamos Nature Center tomorrow, June 14.

The nature center has a list of programs people have asked for, and Stonehenge came up as a topic in our quarterly meeting half a year ago. Remembering my seventh grade fascination with Stonehenge and its astronomical alignments -- I discovered Stonehenge Decoded at the local library, and built a desktop model showing the stones and their alignments -- I volunteered. But after some further reading, I realized that not all of those alignments are all they're cracked up to be and that there might not be much of astronomical interest to talk about, and I un-volunteered.

But after thinking about it for a bit, I realized that "not all they're cracked up to be" makes an interesting topic in itself. So in the next round of planning, I re-volunteered; the result is tomorrow night's presentation.

The talk will include a lot of history of Stonehenge and its construction, and a review of some other important or amusing henges around the world. But this article is on the astronomy, or lack thereof.

The Background: Stonehenge Decoded

Stonehenge famously aligns with the summer solstice sunrise, and that's when tens of thousands of people flock to Salisbury, UK to see the event. (I'm told that the rest of the time, the monument is fenced off so you can't get very close to it, though I've never had the opportunity to visit.)

Curiously, archaeological evidence suggests that the summer solstice wasn't the big time for prehistorical gatherings at Stonehenge; the time when it was most heavily used was the winter solstice, when there's a less obvious alignment in the other direction. But never mind that.

[Gerald Hawkins' Stonehenge alignments from Stonehenge Decoded] In 1963, Gerald Hawkins wrote an article in Nature, which he followed up two years later with a book entitled Stonehenge Decoded. Hawkins had access to an IBM 7090, capable of a then-impressive 100 Kflops (thousand floating point operations per second; compare a Raspberry Pi 3 at about 190 Mflops, or about a hundred Gflops for something like an Intel i5). It cost $2.9 million (nearly $20 million in today's dollars).

Using the 7090, Hawkins mapped the positions of all of Stonehenge's major stones, then looked for interesting alignments with the sun and moon. He found quite a few of them. (Hawkins and Fred Hoyle also had a theory about the fifty-six Aubrey holes being a lunar eclipse predictor, which captured my seventh-grade imagination but which most researchers today think was more likely just a coincidence.)

But I got to thinking ... Hawkins mapped at least 38 stones if you don't count the Aubrey holes. If you take 38 randomly distributed points, what are the chances that you'll find interesting astronomical alignments?

A Modern Re-Creation of Hawkins' Work

Programmers today have it a lot easier than Hawkins did. We have languages like Python, with libraries like PyEphem to handle the astronomical calculations. And it doesn't hurt that our computers are about a million times faster.

Anyway, my script, takes a GPX file containing a list of geographic coordinates and compares those points to sunrise and sunset at the equinoxes and solstices, as well as the full moonrise and moonset nearest the solstice or equinox. It can find alignments among all the points in the GPX file, or from a specified "observer" point to each point in the file. It allows a slop of a few degrees, 2 degrees by default; this is about four times the diameter of the sun or moon, but a half-step from your observing position can make a bigger difference than that. I don't know how much slop Hawkins used; I'd love to see his code.

[Astronomical alignments between pairs of New Mexico peaks] My first thought was, what if you stand on a mountain peak and look around you at other mountain peaks? (It's easy to get GPS coordinates for peaks; if you can't find them online you can click on them on a map.) So I plotted the major peaks in the Jemez and Sangre de Cristo mountains that I figured were all mutually visible. It came to 22 points; about half what Hawkins was working with.

My program found (114 alignments.

[Astronomical alignments between pairs of New Mexico peaks] Yikes! Way too many. What if I cut it down? So I tried eliminating all but the really obvious ones, the ones you really notice from across the valley. The most prominent 11 peaks: 5 in the Jemez, 6 in the Sangres.

That was a little more manageable. Now I was down to only 22 alignments.

Now, I'm pretty sure that the Ancient Ones -- or aliens -- didn't lay out the Jemez and Sangre de Cristo mountains to align with the rising and setting sun and moon. No, what this tells us is that pretty much any distribution of points will give you a bunch of astronomical alignments.

And that's just the sun and moon, all Hawkins was considering. If you look for writing on astronomical alignments in ancient monuments, you'll find all people claiming to have found alignments with all sorts of other rising and setting bodies, like Sirius and Orion's belt. Imagine how many alignments I could have found if I'd included the hundred brightest stars.

So I'm not convinced. Certainly Stonehenge's solstice alignment looks real; I'm not disputing that. And there are lots of other archaeoastronomy sites that are even more convincing, like the Chaco sun dagger. But I've also seen plenty of web pages, and plenty of talks, where someone maps out a collection of points at an ancient site and uses alignments among them as proof that it was an ancient observatory. I suspect most of those alignments are more evidence of random chance and wishful thinking than archeoastronomy.

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[ 14:54 Jun 13, 2019    More science/astro | permalink to this entry | comments ]

Sun, 28 Oct 2018

How to Extend a Moonrise

Last night, as we drove home from the Pumpkin Glow -- one of Los Alamos's best annual events, a night exhibition of dozens of carved pumpkins all together in one place -- I noticed a glow on the horizon right around Truchas Peak and wondered if the moon was going to rise that far north.

Sure enough, I saw the first sliver of the moon poking over the peak as we passed the airport. "We may get an extended moonrise tonight", I said, realizing that as the moon rose, we'd be descending the "Main Hill Road", as that section of NM 502 is locally known, so we'd get lower with respect to the mountains even as the moon got higher. Which would win?

As it turns out, neither. The change of angle during the descent down the Main Hill Road exactly matches the rate of moonrise, so the size of the moon's sliver stayed almost exactly the same during the whole descent, until we got down to the "Y" where a nearby mesa blocked our view entirely. By the time we could see the moon again, it was just freeing itself of the mountains.

Neat! Made me think of The Little Prince: his home asteroid B6-12 (no, that's not a real asteroid desgination) was small enough that by moving his chair, he could watch sunset over and over again. I'm a sucker for moonrises -- and now I know how I can make them last longer!

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[ 19:32 Oct 28, 2018    More science/astro | permalink to this entry | comments ]

Fri, 23 Feb 2018

PEEC Planetarium Show: "The Analemma Dilemma"

[Analemma by Giuseppe Donatiello via Wikimedia Commons] Dave and I are giving a planetarium show at PEEC tonight on the analemma.

I've been interested in the analemma for years and have written about it before, here on the blog and in the SJAA Ephemeris. But there were a lot of things I still didn't understand as well as I liked. When we signed up three months ago to give this talk, I had plenty of lead time to do more investigating, uncovering lots of interesting details regarding the analemmas of other planets, the contributions of the two factors that go into the Equation of Time, why some analemmas are figure-8s while some aren't, and the supposed "moon analemmas" that have appeared on the Astronomy Picture of the Day. I added some new features to the analemma script I'd written years ago as well as corresponding with an expert who'd written some great Equation of Time code for all the planets. It's been fun.

I'll write about some of what I learned when I get a chance, but meanwhile, people in the Los Alamos area can hear all about it tonight, at our PEEC show: The Analemma Dilemma, 7 pm tonight, Friday Feb 23, at the Nature Center, admission $6/adult, $4/child.

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[ 10:23 Feb 23, 2018    More science/astro | permalink to this entry | comments ]

Sun, 27 Aug 2017

Total Eclipse

[2017 Solar eclipse with corona] My first total eclipse! The suspense had been building for years.

Dave and I were in Wyoming. We'd made a hotel reservation nine months ago, by which time we were already too late to book a room in the zone of totality and settled for Laramie, a few hours' drive from the centerline.

For visual observing, I had my little portable 80mm refractor. But photography was more complicated. I'd promised myself that for my first (and possibly only) total eclipse, I wasn't going to miss the experience because I was spending too much time fiddling with cameras. But I couldn't talk myself into not trying any photography at all.

Initially, my plan was to use my 90mm Mak as a 500mm camera lens. It had worked okay for the the 2012 Venus transit.

[Homemade solar finder for telescope] I spent several weeks before the eclipse in a flurry of creation, making a couple of solar finders, a barn-door mount, and then wrestling with motorizing the barn-door (which was a failure because I couldn't find a place to buy decent gears for the motor. I'm still working on that and will eventually write it up). I wrote up a plan: what equipment I would use when, a series of progressive exposures for totality, and so forth.

And then, a couple of days before we were due to leave, I figured I should test my rig -- and discovered that it was basically impossible to focus on the sun. For the Venus transit, the sun wasn't that high in the sky, so I focused through the viewfinder. But for the total eclipse, the sun would be almost overhead, and the viewfinder nearly impossible to see. So I had planned to point the Mak at a distant hillside, focus it, then slip the filter on and point it up to the sun. It turned out the focal point was completely different through the filter.

[Solar finder for DSLR, made from popsicle sticks] With only a couple of days left to go, I revised my plan. The Mak is difficult to focus under any circumstances. I decided not to use it, and to stick to my Canon 55-250mm zoom telephoto, with the camera on a normal tripod. I'd skip the partial eclipse (I've photographed those before anyway) and concentrate on getting a few shots of the diamond ring and the corona, running through a range of exposures without needing to look at the camera screen or do any refocusing. And since I wasn't going to be usinga telescope, my nifty solar finders wouldn't work; I designed a new one out of popsicle sticks to fit in the camera's hot shoe.

Getting there

We stayed with relatives in Colorado Saturday night, then drove to Laramie Sunday. I'd heard horror stories of hotels canceling people's longstanding eclipse reservations, but fortunately our hotel honored our reservation. WHEW! Monday morning, we left the hotel at 6am in case we hit terrible traffic. There was already plenty of traffic on the highway north to Casper, but we turned east hoping for fewer crowds. A roadsign sign said "NO PARKING ON HIGHWAY." They'd better not try to enforce that in the totality zone!

[Our eclipse viewing pullout on Wyoming 270] When we got to I-25 it was moving and, oddly enough, not particularly crowded. Glendo Reservoir had looked on the map like a nice spot on the centerline ... but it was also a state park, so there was a risk that everyone else would want to go there. Sure enough: although traffic was moving on I-25 at Wheatland, a few miles north the freeway came to a screeching halt. We backtracked and headed east toward Guernsey, where several highways went north toward the centerline.

East of Glendo, there were crowds at every highway pullout and rest stop. As we turned onto 270 and started north, I kept an eye on OsmAnd on my phone, where I'd loaded a GPX file of the eclipse path. When we were within a mile of the centerline, we stopped at a likely looking pullout. It was maybe 9 am. A cool wind was blowing -- very pleasant since we were expecting a hot day -- and we got acquainted with our fellow eclipse watchers as we waited for first contact.

Our pullout was also the beginning of a driveway to a farmhouse we could see in the distance. Periodically people pulled up, looking lost, checked maps or GPS, then headed down the road to the farm. Apparently the owners had advertised it as an eclipse spot -- pay $35, and you can see the eclipse and have access to a restroom too! But apparently the old farmhouse's plumbing failed early on, and some of the people who'd paid came out to the road to watch with us since we had better equipment set up.

[Terrible afocal view of partial eclipse] There's not much to say about the partial eclipse. We all traded views -- there were five or six scopes at our pullout, including a nice little H-alpha scope. I snapped an occasional photo through the 80mm with my pocket camera held to the eyepiece, or with the DSLR through an eyepiece projection adapter. Oddly, the DSLR photos came out worse than the pocket cam ones. I guess I should try and debug that at some point.

Shortly before totality, I set up the DSLR on the tripod, focused on a distant hillside and taped the focus with duct tape, plugged in the shutter remote, checked the settings in Manual mode, then set the camera to Program mode and AEB (auto exposure bracketing). I put the lens cap back on and pointed the camera toward the sun using the popsicle-stick solar finder. I also set a countdown timer, so I could press START when totality began and it would beep to warn me when it was time to the sun to come back out. It was getting chilly by then, with the sun down to a sliver, and we put on sweaters.

The pair of eclipse veterans at our pullout had told everybody to watch for the moon's shadow racing toward us across the hills from the west. But I didn't see the racing shadow, nor any shadow bands.

And then Venus and Mercury appeared and the sun went away.


[Solar eclipse diamond ring] One thing the photos don't prepare you for is the color of the sky. I expected it would look like twilight, maybe a little darker; but it was an eerie, beautiful medium slate blue. With that unworldly solar corona in the middle of it, and Venus gleaming as bright as you've ever seen it, and Mercury shining bright on the other side. There weren't many stars.

We didn't see birds doing anything unusual; as far as I can tell, there are no birds in this part of Wyoming. But the cows did all get in a line and start walking somewhere. Or so Dave tells me. I wasn't looking at the cows.

Amazingly, I remembered to start my timer and to pull off the DSLR's lens cap as I pushed the shutter button for the diamond-ring shots without taking my eyes off the spectacle high above. I turned the camera off and back on (to cancel AEB), switched to M mode, and snapped a photo while I scuttled over to the telescope, pulled the filter off and took a look at the corona in the wide-field eyepiece. So beautiful! Binoculars, telescope, naked eye -- I don't know which view was best.

I went through my exposure sequence on the camera, turning the dial a couple of clicks each time without looking at the settings, keeping my eyes on the sky or the telescope eyepiece. But at some point I happened to glance at the viewfinder -- and discovered that the sun was drifting out of the frame. Adjusting the tripod to get it back in the frame took longer than I wanted, but I got it there and got my eyes back on the sun as I snapped another photo ...

and my timer beeped.

I must have set it wrong! It couldn't possibly have been two and a half minutes. It had been 30, 45 seconds tops.

But I nudged the telescope away from the sun, and looked back up -- to another diamond ring. Totality really was ending and it was time to stop looking.

Getting Out

The trip back to Golden, where we were staying with a relative, was hellish. We packed up immediately after totality -- we figured we'd seen partials before, and maybe everybody else would stay. No such luck. By the time we got all the equipment packed there was already a steady stream of cars heading south on 270.

A few miles north of Guernsey the traffic came to a stop. This was to be the theme of the afternoon. Every small town in Wyoming has a stop sign or signal, and that caused backups for miles in both directions. We headed east, away from Denver, to take rural roads down through eastern Wyoming and Colorado rather than I-25, but even so, we hit small-town stop sign backups every five or ten miles.

We'd brought the Rav4 partly for this reason. I kept my eyes glued on OsmAnd and we took dirt roads when we could, skirting the paved highways -- but mostly there weren't any dirt roads going where we needed to go. It took about 7 hours to get back to Golden, about twice as long as it should have taken. And we should probably count ourselves lucky -- I've heard from other people who took 11 hours to get to Denver via other routes.

Lessons Learned

Dave is fond of the quote, "No battle plan survives contact with the enemy" (which turns out to be from Prussian military strategist Helmuth von Moltke the Elder).

The enemy, in this case, isn't the eclipse; it's time. Two and a half minutes sounds like a lot, but it goes by like nothing.

Even in my drastically scaled-down plan, I had intended exposures from 1/2000 to 2 seconds (at f/5.6 and ISO 400). In practice, I only made it to 1/320 because of fiddling with the tripod.

And that's okay. I'm thrilled with the photos I got, and definitely wouldn't have traded any eyeball time for more photos. I'm more annoyed that the tripod fiddling time made me miss a little bit of extra looking. My script actually worked out better than I expected, and I was very glad I'd done the preparation I had. The script was reasonable, the solar finders worked really well, and the lens was even in focus for the totality shots.

Then there's the eclipse itself.

I've read so many articles about solar eclipses as a mystical, religious experience. It wasn't, for me. It was just an eerily beautiful, other-worldly spectacle: that ring of cold fire staring down from the slate blue sky, bright planets but no stars, everything strange, like nothing I'd ever seen. Photos don't get across what it's like to be standing there under that weird thing in the sky.

I'm not going to drop everything to become a globe-trotting eclipse chaser ... but I sure hope I get to see another one some day.

Photos: 2017 August 21 Total Solar Eclipse in Wyoming.

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[ 20:41 Aug 27, 2017    More science/astro | permalink to this entry | comments ]

Mon, 14 Aug 2017

A Homemade Solar Finder, for the Eclipse

While I was testing various attempts at motorizing my barn-door mount, trying to get it to track the sun, I had to repeatedly find the sun in my telescope.

In the past, I've generally used the shadow of the telescope combined with the shadow of the finderscope. That works, more or less, but it's not ideal: it doesn't work as well with just a telescope with no finder, which includes both of the scopes I'm planning to take to the eclipse; and it requires fairly level ground under the telescope: it doesn't work if there are bushes or benches in the way of the shadow.

For the eclipse, I don't want to waste any time finding the sun: I want everything as efficient as possible. I decided to make a little solar finderscope. One complication, though: since I don't do solar observing very often, I didn't want to use tape, glue or, worse, drill holes to mount it.

So I wanted something that could be pressed against the telescope and held there with straps or rubber bands, coming off again without leaving a mark. A length of an angled metal from my scrap pile seemed like a good size to be able to align itself against a small telescope tube.

[Constructing a solar sight] Then I needed front and rear sights. For the front sight, I wanted a little circle that could project a bulls-eye shadow onto a paper card attached to the rear sight. I looked at the hardware store for small eye-bolts, but no dice. Apparently they don't come that small.I settled for the second-smallest size of screw eye.

The screw eye, alas, is meant to screw into wood, not metal. So I cut a short strip of wood a reasonable size to nestle into the inside of the angle-iron. (That ripsaw Dave bought last year sure does come in handy sometimes.) I drilled some appropriately sized holes and fastened screw eyes on both ends, adding a couple of rubber grommets as spacers because the screw eyes were a little too long and I didn't want the pointy ends of the screws getting near my telescope tube.

I added some masking tape on the sides of the angle iron so it wouldn't rub off the paint on the telescope tube, then bolted a piece of cardboard cut from an old business card to the rear screw eye.

[Homemade solar sight] Voila! A rubber-band-attached solar sight that took about an hour to make. Notice how the shadow of the front sight exactly fits around the rear sight: you line up the shadow with the rear sight to point the scope. It seems to work pretty well, and it should be adaptable to any telescope I use.

I used a wing nut to attach the rear cardboard: that makes it easy to replace it or remove it. With the cardboard removed, the sight might even work for night-time astronomy viewing. That is, it does work, as long as there's enough ambient light to see the rings. Hmm... maybe I should paint the rings with glow-in-the-dark paint.

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[ 15:25 Aug 14, 2017    More science/astro | permalink to this entry | comments ]