Shallow Thoughts

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

Thu, 12 Oct 2017

Letter to the New Mexico Public Education Department on Science Standards

For those who haven't already read about the issue in the national press, New Mexico's Public Education Department (a body appointed by the governor) has a proposal regarding new science standards for all state schools. The proposal starts with the national Next Generation Science Standards but then makes modifications, omitting points like references to evolution and embryological development or the age of the Earth and adding a slew of NM-specific standards that are mostly sociological rather than scientific.

You can read more background in the Mother Jones article, New Mexico Doesn’t Want Your Kids to Know How Old the Earth Is. Or why it’s getting warmer, including links to the proposed standards. Ars Technica also covered it: Proposed New Mexico science standards edit out basic facts.

New Mexico residents have until 5.p.m. next Monday, October 16, to speak out about the proposal. Email comments to rule.feedback@state.nm.us or send snail mail (it must arrive by Monday) to Jamie Gonzales, Policy Division, New Mexico Public Education Department, Room 101, 300 Don Gaspar Avenue, Santa Fe, New Mexico 87501.

A few excellent letters people have already written:

I'm sure they said it better than I can. But every voice counts -- they'll be counting letters! So here's my letter. If you live in New Mexico, please send your own. It doesn't have to be long: the important thing is that you begin by stating your position on the proposed standards.


Members of the PED:

Please reconsider the proposed New Mexico STEM-Ready Science Standards, and instead, adopt the nationwide Next Generation Science Standards (NGSS) for New Mexico.

With New Mexico schools ranking at the bottom in every national education comparison, and with New Mexico hurting for jobs and having trouble attracting technology companies to our state, we need our students learning rigorous, established science.

The NGSS represents the work of people in 26 states, and is being used without change in 18 states already. It's been well vetted, and there are many lesson plans, textbooks, tests and other educational materials available for it.

The New Mexico Legislature supports NGSS: they passed House Bill 211 in 2017 (vetoed by Governor Martinez) requiring adoption of the NGSS. The PED's own Math and Science Advisory Council (MSAC) supports NGSS: they recommended in 2015 that it be adopted. Why has the PED ignored the legislature and its own advisory council?

Using the NGSS without New Mexico changes will save New Mexico money. The NGSS is freely available. Open source textbooks and lesson plans are already available for the NGSS, and more are coming. In contrast, the New Mexico Stem-Ready standards would be unique to New Mexico: not only would we be left out of free nationwide educational materials, but we'd have to pay to develop New Mexico-specific curricula and textbooks that couldn't be used anywhere else, and the resulting textbooks would cost far more than standard texts. Most of this money would go to publishers in other states.

New Mexico consistently ranks at the bottom in educational comparisons. Yet nearly 15% of the PED's proposed stem-ready standards are New Mexico specific standards, taught nowhere else, and will take time away from teaching core science concepts. Where is the evidence that our state standards would be better than what is taught in other states? Who are we to think we can write better standards than a nationwide coalition?

In addition, some of the changes in the proposed NM STEM-Ready Science Standards seem to be motivated by political ideology, not science. Science standards used in our schools should be based on widely accepted scientific principles. Not to mention that the national coverage on this issue is making our state a laughingstock.

Finally, the lack of transparency in the NMSRSS proposal is alarming. Who came up with the proposed NMSRSS standards? Are there any experts in science education that support them? Is there any data to indicate they'd be more effective than the NGSS? Why wasn't the development of the NMSRSS discussed in open PED meetings as required by the Open Meetings Act?

The NGSS are an established, well regarded national standard. Don't shortchange New Mexico students by teaching them watered-down science. Please discard the New Mexico Stem-Ready proposal and adopt the Next Generation Science Standards, without New Mexico-specific changes.

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[ 10:16 Oct 12, 2017    More politics | permalink to this entry | comments ]

Thu, 05 Oct 2017

Tarantula Under Glass, and Micro-Centipedes

Every fall, Dave and I eagerly look for tarantulas. They only show up for a few weeks a year -- that's when the males go out searching for females (the females stay snug in their burrows). In the bay area, there were a few parks where we used to hunt for them: Arastradero, Mt Hamilton, occasionally even Alum Rock. Here in semi-rural New Mexico, our back yard is as good a place to hunt as anywhere else, though we still don't see many: just a couple of them a year.

But this year I didn't even have to go out into the yard. I just looked over from my computer and spotted a tarantula climbing up our glass patio door. I didn't know they could do that!

Unfortunately it got to the top before I had the camera ready, so I didn't get a picture of tarantula belly. Right now he's resting on the sill: [Tarantula, resting after climbing up our glass patio door] I don't think it's very likely he's going to find any females up there. I'm hoping he climbs back down the same way and I can catch a photo then. (Later: nope, he disappeared when I wasn't watching.)

In other invertebrate news: we have a sporadic problem with centipedes here in White Rock. Last week, a seven-inch one dropped from the ceiling onto the kitchen floor while I was making cookies, and it took me a few minutes to chase it down so I could toss it outside.

[Tiny baby centipede] But then a few days later, Dave spotted a couple of these little guys on the patio, and I have to admit they're pretty amazing. Just like the adults only in micro-miniature.

Though it doesn't make me like them any better in the house.

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[ 18:47 Oct 05, 2017    More nature | permalink to this entry | comments ]

Thu, 28 Sep 2017

Audio Output from a Raspberry Pi Zero

Someone at our makerspace found a fun Halloween project we could do at Coder Dojo: a motion sensing pumpkin that laughs evilly when anyone comes near. Great! I've worked with both PIR sensors and ping rangefinders, and it sounded like a fun project to mentor. I did suggest, however, that these days a Raspberry Pi Zero W is cheaper than an Arduino, and playing sounds on it ought to be easier since you have frameworks like ALSA and pygame to work with.

The key phrase is "ought to be easier". There's a catch: the Pi Zero and Zero W don't have an audio output jack like their larger cousins. It's possible to get analog audio output from two GPIO pins (use the term "PWM output" for web searches), but there's a lot of noise. Larger Pis have a built-in low-pass filter to screen out the noise, but on a Pi Zero you have to add a low-pass filter. Of course, you can buy HATs for Pi Zeros that add a sound card, but if you're not super picky about audio quality, you can make your own low-pass filter out of two resistors and two capacitors per channel (multiply by two if you want both the left and right channels).

There are lots of tutorials scattered around the web about how to add audio to a Pi Zero, but I found a lot of them confusing; e.g. Adafruit's tutorial on Pi Zero sound has three different ways to edit the system files, and doesn't specify things like the values of the resistors and capacitors in the circuit diagram (hint: it's clearer if you download the Fritzing file, run Fritzing and click on each resistor). There's a clearer diagram in Sudomod Forums: PWM Audio Guide, but I didn't find that until after I'd made my own, so here's mine.

Parts list:

And here's how to wire it: [Adding audio to the Raspberry Pi Zero]
(Fritzing file, pi-zero-audio.fzz.)

This wiring assumes you're using pins 13 and 18 for the left and right channels. You'll need to configure your Pi to use those pins. Add this to /boot/config.txt:

dtoverlay=pwm-2chan,pin=18,func=2,pin2=13,func2=4

Testing

Once you build your circuit up, you need to test it. Plug in your speaker or headphones, then make sure you can play anything at all:

aplay /usr/share/sounds/alsa/Front_Center.wav

If you need to adjust the volume, run alsamixer and use the up and down arrow keys to adjust volume. You'll have to press up or down several times before the bargraph actually shows a change, so don't despair if your first press does nothing.

That should play in both channels. Next you'll probably be curious whether stereo is actually working. Curiously, none of the tutorials address how to test this. If you ls /usr/share/sounds/alsa/ you'll see names like Front_Left.wav, which might lead you to believe that aplay /usr/share/sounds/alsa/Front_Left.wav might play only on the left. Not so: it's a recording of a voice saying "Front left" in both channels. Very confusing!

Of course, you can copy a music file to your Pi, play it (omxplayer is a nice commandline player that's installed by default and handles MP3) and see if it's in stereo. But the best way I found to test audio channels is this:

speaker-test -t wav -c 2

That will play those ALSA voices in the correct channel, alternating between left and right. (MythTV has a good Overview of how to use speaker-test.

Not loud enough?

I found the volume plenty loud via earbuds, but if you're targeting something like a Halloween pumpkin, you might need more volume. The easy way is to use an amplified speaker (if you don't mind putting your nice amplified speaker amidst the yucky pumpkin guts), but you can also build a simple amplifier. Here's one that looks good, but I haven't built one yet: One Transistor Audio for Pi Zero W

Of course, if you want better sound quality, there are various places that sell HATs with a sound chip and line or headphone out.

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[ 15:49 Sep 28, 2017    More hardware | permalink to this entry | comments ]

Sun, 17 Sep 2017

Screen Brightness on an Asus 1015e (and other Intel-based laptops)

When I upgraded my Asus laptop to Stretch, one of the things that stopped working was the screen brightness keys (Fn-F5 and Fn-F6). In Debian Jessie they had always just automagically worked without my needing to do anything, so I'd never actually learned how to set brightness on this laptop. The fix, like so many things, is easy once you know where to look.

It turned out the relevant files are in /sys/class/backlight/intel_backlight. cat /sys/class/backlight/intel_backlight/brightness tells you the current brightness; write a number to /sys/class/backlight/intel_backlight/brightness to change it.

That at least got me going (ow my eyes, full brightness is migraine-inducing in low light) but of course I wanted it back on the handy function keys.

I wrote a script named "dimmer", with a symlink to "brighter", that goes like this:

#!/bin/zsh

curbright=$(cat /sys/class/backlight/intel_backlight/brightness)
echo dollar zero $0
if [[ $(basename $0) == 'brighter' ]]; then
  newbright=$((curbright + 200))
else
  newbright=$((curbright - 200))
fi
echo from $curbright to $newbright

sudo sh -c "echo $newbright > /sys/class/backlight/intel_backlight/brightness"

That let me type "dimmer" or "brighter" to the shell to change the brightness, with no need to remember that /sys/class/whatsit path. I got the names of the two function keys by running xev and typing Fn and F5, then Fn and F6. Then I edited my Openbox ~/.config/openbox/rc.xml, and added:

<keybind key="XF86MonBrightnessDown">
  <action name="Execute">
    <execute>dimmer<execute>
  </action>
</keybind>
<keybind key="XF86MonBrightnessUp">
  <action name="Execute">
    <execute>brighter<execute>
  </action>
</keybind>

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[ 19:57 Sep 17, 2017    More linux/laptop | permalink to this entry | comments ]

Mon, 04 Sep 2017

Jumpstarting the Raspberry Pi Zero W: Now Available via Humble Bundle!

[Raspberry Pi Zero W] My new book is now shipping! And it's being launched via a terrific Humble Bundle of books on electronics, making, Raspberry Pi and Arduino.

Humble Bundles, if you haven't encountered them before, let you pay what you want for a bundle of books on related subjects. The books are available in ePub, Mobi, and PDF formats, without DRM, so you can read them on your choice of device. If you pay above a certain amount, they add additional books. My book is available if you pay $15 or more.

You can also designate some of the money you pay for charity. In this case the charity is Maker Ed, a crowdfunding initiative that supports Maker programs primarily targeted toward kids in schools. (I don't know any more about them than that; check out their website for more information.)

Jumpstarting the Raspberry Pi Zero W is a short book, with only 103 pages in four chapters:

  1. Getting Started: includes tips on headless setup and the Linux command line;
  2. Blink an LED: includes ways to blink and fade LEDs from the shell and from several different Python libraries;
  3. A Temperature Notifier and Fan Control: code and wiring instructions for three different temperature sensors (plus humidity and barometric pressure), and a way to use them to control your house fan or air conditioner, either according to the temperature in the room or through a Twitter command;
  4. A Wearable News Alert Light Show: wire up NeoPixels or DotStars and make them respond to keywords on Twitter or on any other web page you choose, plus some information on powering a Pi portably with batteries.

All the code and wiring diagrams from the book, plus a few extras, are available on Github, at my Raspberry Pi Zero Book code repository.

To see the book bundle, go to the Electronics & Programming Humble Bundle and check out the selections. My book, Jumpstarting the Raspberry Pi Zero W, is available if you pay $15 or more -- along with tons of other books you'll probably also want. I already have Make: Electronics and it's one of the best introductory electronics books I've seen, so I'm looking forward to seeing the followup volume. Plus there are books on atmospheric and environmental monitoring, a three-volume electronic components encyclopedia, books on wearable electronics and drones and other cool stuff.

I know this sounds like a commercial, but this bundle really does look like a great deal, whether or not you specifically want my Pi book, and it's a limited-time offer, only good for six more days.

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[ 13:21 Sep 04, 2017    More writing | 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.

Totality

[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 ]

Thu, 10 Aug 2017

A Barn-Door Mount for the Eclipse

[Curved rod barn-door mount] I've been meaning forever to try making a "barn door" tracking mount. Used mainly for long-exposure wide-field astrophotography, the barn door mount, invented in 1975, is basically two pieces of wood with a hinge. The bottom board mounts on a tripod and is pointed toward the North Star; "opening" the hinge causes the top board to follow the motion of the sky, like an equatorial telescope mount. A threaded rod and a nut control the angle of the "door", and you turn the nut manually every so often. Of course, you can also drive it with a motor.

We're off to view the eclipse in a couple of weeks. Since it's my first total eclipse, my plan is to de-emphasize photography: especially during totality, I want to experience the eclipse, not miss it because my eyes are glued to cameras and timers and other equipment. But I still want to take photos every so often. Constantly adjusting a tripod to keep the sun in frame is another hassle that might keep my attention away from the eclipse. But real equatorial mounts are heavy and a time consuming to set up; since I don't know how crowded the area will be, I wasn't planning to take one. Maybe a barn door would solve that problem.

Perhaps more useful, it would mean that my sun photos would all be rotated approximately the right amount, in case I wanted to make an animation. I've taken photos of lunar and partial solar eclipses, but stringing them together into an animation turned out to be too much hassle because of the need to rotate and position each image.

I've known about barn-door mounts since I was a kid, and I knew the basic theory, but I'd never paid much attention to the details. When I searched the web, it sounded complicated -- it turned out there are many types that require completely different construction techniques.

The best place to start (I found out after wasting a lot of time on other sites) is the Wikipedia article on "Barn door tracker", which gives a wonderfully clear overview, with photos, of the various types. I had originally been planning a simple tangent or isosceles type; but when I read construction articles, it seemed that those seemingly simple types might not be so simple to build: the angle between the threaded rod and the boards is always changing, so you need some kind of a pivot. Designing the pivot looked tricky. Meanwhile, the pages I found on curved-rod mounts all insisted that bending the rod was easy, no trouble at all. I decided to try a curved-rod mount first.

The canonical reference is a 2015 article by Gary Seronik: A Tracking Platform for Astrophotography. But I found three other good construction guides: Optical Ed's "Making a Curve Bolt Barn Door", a Cloudy Nights discussion thread "Motorized Barn Door Mount Kit", and Massapoag Pond Photography's "Barn Door Tracker". I'm not going to reprise all their construction details, so refer to those sites if you try making your own mount.

[Barn-door mount, showing piano hinge] The crucial parts are a "piano hinge", a long hinge that eliminates the need to line up two or more hinges, and the threaded rod. Buying a piano hinge in the right size proved impossible locally, but the folks at Metzger's assured me that piano hinges can be cut, so I bought one longer than I needed and cut it to size. I used a 1/4-20 rod, which meant (per the discussions in the Cloudy Nights discussion linked above) that a 11.43-inch radius from the hinge to the holes the rod passes through would call for the nut to turn at a nice round number of 1 RPM.

I was suspicious of the whole "it's easy to bend the threaded rod ina 11.43-inch circle" theory, but it turned out to be true. Draw the circle you want on a sheet of newspaper, put on some heavy gloves and start bending, frequently comparing your rod to the circle you drew. You can fine-tune the curvature later.

I cut my boards, attached the hinge, measured about 11.4" and drilled a hole for the threaded rod. The hole needed to be a bit bigger than 5/8" to let the curved rod pass through without rubbing. Attach the curved rod to the top wood piece with a couple of nuts and some washers, and then you can fine-tune the rod's curvature, opening and closing the hinge and re-bending the rod a little in any place it rubs.

A 5/8" captive nut on the top piece lets you attach a tripod head which will hold your camera or telescope. A 1/4" captive nut on the bottom piece serves to attach the mount to a tripod -- you need a 1/4", not 3/8": the rig needs to mount on a tripod head, not just the legs, so you can align the hinge to the North Star. (Of course, you could build a wedge or your own set of legs, if you prefer.) The 3/4" plywood I was using turned out to be thicker than the captive nuts, so I had to sand the wood thinner in both places. Maybe using half-inch plywood would have been better.

[Wing nut on barn-door mount] The final piece is the knob/nut you'll turn to make the mount track. I couldn't find a good 1/4" knob for under $15. A lot of people make a wood circle and mount the nut in the center, or use a gear so a motor can drive the mount. I looked around at things like jam-jar lids and the pile of metal gears and sprinkler handles in my welding junkpile, but I didn't see anything that looked quite right, so I decided to try a wing nut just for testing, and worry about the knob later. Turns out a wing nut works wonderfully; there's no particular need for anything else if you're driving your barn-door manually.

Testing time! I can't see Polaris from my deck, and I was too lazy to set up anywhere else, so I used a protractor to set the hinge angle to roughly 36° (my latitude), then pointed it approximately north. I screwed my Pro-Optic 90mm Maksutov (the scope I plan to use for my eclipse photos) onto the ball head and pointed it at the moon as soon as it rose. With a low power eyepiece (20x), turning the wing nut kept the moon more or less centered in the field for the next half-hour, until clouds covered the moon and rain began threatening. I didn't keep track of how many turns I was making, since I knew the weather wasn't going to allow a long session, and right now I'm not targeting long-exposure photography, just an easy way of keeping an object in view.

A good initial test! My web searches, and the discovery of all those different types of barn-door mounts and pivots and flex couplings and other scary terms, had seemed initially daunting. But in the end, building a barn-door mount was just as easy as people say it is, and I finished it in a day.

And what about a motor? I added one a few days later, with a stepper and an Arduino. But that's a separate article.

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[ 19:25 Aug 10, 2017    More science/astro | permalink to this entry | comments ]