STEAM: Transit of Mercury

With the end of the school year approaching, I was looking for one more STEAM activity I could do. I had relied on mathematics and physics so far because those are the things I am most familiar with. For the same reason, I had avoided chemistry and biology. What other areas of science could I tap into for a good STEAM demonstration for second graders?

Then it hit me: astronomy. It's tricky to do during the day, but if there were some astronomical phenomenon that happened to be timed right, it could be the basis for a good activity.

Searching the Internet, I discovered that a Transit of Mercury was going to happen soon. And it would be during school hours on a weekday! How could you pass that up?

I have been witness to the past two Transits of Venus, and they were pretty spectacular, in an amateur astronomy sort of way. There were just a few issues to overcome.

Astronomy for the Masses

How do you share a planet crossing the face of the sun with a class of second graders and one telescope? The obvious answer is solar projection of some sort. I was dubious about a pinhole projection working. I thought about eyepiece projection onto a large piece of paper, but then there's always that kid who will try to look through the eyepiece when you're not looking, which would be a seriously Bad Thing. Finally I wondered if it was possible to do a rear projection of the sun without blinding all the observers. According to the Internet, you can. And I found a solution that an entire class (or at least a large part of it) could use at the same time. The Sun Gun Telescope.

Okay, technically, it would be the Sun of a Gun, since I can't easily haul a large garden urn to an elementary school (or anywhere else, for that matter), but a five gallon bucket (with handle, no less) is a piece of cake. There were some design issues I didn't like, though.

First, the Sun of a Gun has the rear projection screen directly behind the telescope, so observers are facing the sun (see my comment about untrustworthy children two paragraphs up). Second, the bucket was mounted directly on the telescope, and that seemed a bit awkward. Especially since I was planning to go as cheap as possible in this build. Better to decouple the two. And since I was decoupling the rear projection screen and the projector (telescope), I could stick a right-angle adapter in the path, so observers aren't looking towards the sun.

Thermodynamics

Having never done this before, I wasn't all that comfortable with how much light I would need to gather to project a comfortable image for people to view. Everything I read indicated that as long as the aperture of the scope was 4-inches or less, I wouldn't be in danger of melting anything, but that didn't really answer the question.

So to hedge my bets, I opted to look for the intersection of price and aperture in achormatic refractors. My first inclination was to go with an Orion Short Tube 80mm f/5. I would have to buy a mount for it, but if it survived the solar viewing, I could use it to star hop from the driveway in my light polluted neighborhood.

Then I started reading about chromatic aberration with bright objects in this scope because of it's short focal length and started to second guess myself. That's when I stumbled upon the Meade Polaris 90mm f/10. It's a beginner's kit that comes with tripod, German equatorial mount, a few eyepieces, and a 2x Barlow. All for around US$200. Reviews were favorable. And if I smoked it, I wouldn't be out too much (relative to the cost of things in amateur astronomy). It was a little long in focal length for my liking, but if if survived its solar ordeal, it would be good for planet hopping and moon observing. Being slow, it wouldn't suffer from chromatic aberration as badly as the Orion.

Opening Pandora's Box

One thing I haven't mentioned up till now, but you may have been able to guess based on my writings: I was once an amateur astronomer. But I gave it up. Shortly after the 2004 Transit of Venus, actually. I donated my telescope and eyepieces to a school, managed to shut that demon muse into a cage and suppress the urge to look at the Universe from my backyard. Taking on this project would surely release the demon muse and suck me back in, subjecting me, once again, to aperture fever and gear acquisition syndrome.

I knew this when I started this project. I accepted the consequences. Planetary transits are a rare thing. The next Transit of Mercury isn't until 2019. And after that, you'll have to wait until 2032. A Transit of Venus won't occur in the next 100 years! Having to embrace or do battle with my amateur astronomy demon muse was the price of sowing the sense of wonder that drives scientist, mathematicians, artists, and engineers in the next generation. So be it.

The Build

I acquired the Meade Polaris 90mm from my friendly, neighborhood, telescope store, adding a mirror diagonal to the kit because I was dubious about the prism diagonal's ability to handle the full photon barrage of the sun. Figuring out the necessary eyepiece was a simple exercise in trigonometry, aided by the crib sheet from the Sun Funnel instructions.

After ordering parts for the rear projection bucket, I got to work on assembly. First was to create a light path. Since I wasn't mounting the bucket on the telescope, I opted to widen the hole to about three inches in diameter to give me some wiggle room.

 A paper template guided my cutting. At first, I started drilling holes around the circumference of my template, planning on connecting the dots to punch out a hole. Then I remembered I own a Dremel.

This could take a while...

The more correct tool for the job

 Once the hole was cut, some sand paper was used to smooth the edges and get rid of any hanging chads.

Next, I bolted on an Arca-Swiss-compatible dovetail on the bucket's side near its center of mass so I could mount the bucket on my photography tripod's ball head. To do this, I ground down the head-end threads on some 3/4-inch long 1/4-20 bolts, fed them through the dovetail, and secured them to the bucket with some washers and nuts.

In a pinch, I could probably use this as a smoke ring gun.

Attaching the rear projection screen was a matter of wrapping a large rubber band around the bucket and pulling the screen material taut. Et voila! Sun of a Gun.

To aid in aiming the telescope at the sun, I built a sun finder out of some business cards and plastic corner guard from the home store. The paracord is to secure it to the OTA.

Sun finder.

Hole side goes towards the front of the scope. Rotate the scope until the light dot is on the intersection of cross hairs of the rear card. Tighten things down. Take off the dust cover from the main lens. Fine tune. And start observing.

First light!

First light answered a lot of questions. 90mm is way too much aperture for this projector. I popped the 60mm inner cap of the lens cover for this test, and it was still too dazzling an image. Masking down to 35mm aperture seems to be the sweet spot.

The bucket needs to be baffled. See the orange background? That's light leakage, which means a lower contrast image. Some judicious placement of black card stock on the inside of the bucket solved that problem, and now the sun sits in a sea of black.

Second light!

I wasted a lot of energy worrying about heat. The eyepiece and diagonal never got warm, let alone too hot to touch (at 60mm).

The Meade Polaris 90mm is a cheap piece of crap. The tripod vibrates if you look at it funny. The right ascension motor gear slips inexplicably sometimes when it is being turned. Focus is hard to fine-tune because the gearing is too low. But at least the optics are acceptable.

If you're looking for a first scope (for general observing, not solar projection), I would have a hard time recommending the Meade Polaris family of telescopes. The Astronomers Without Borders OneSky 135mm Dobsonian looks like a much better option, though I've never used one. It's faster to set up (assuming you have a stable table), has a more light collecting ability, and all the reviews I've read say the optics are quite good.

The Transit

On 8 May, the weather forecast for the Transit wasn't looking good, predicting overcast skies. But when I woke up on the morning of the 9th, it was only partly cloudy, so I was hopeful. To prepare the students for what they were about to see, I explained that astronomy requires the use of one's imagination because the scale of things is so large, but the images we see are so small (especially though an amateur telescope).

Mercury is roughly 4900 km (3000 miles) in diameter -- roughly the size of the continental United States -- but would appear as a tiny dot on the image of the sun, one or two millimeters across if my setup worked. By the time I got done explaining to the class what we were going to try to see and got outside to set up the telescope, the sun was behind a cloud, obscured enough that my sun finder was useless.

Fortunately, by the time everything was set up, the cloud had moved sufficiently that I could point the telescope. Eventually, the sun peeked out from behind the clouds, and we got to see the little black dot on the face of the sun that was Mercury for five or ten minutes. After that, the clouds moved back in, and that was it.

Hopefully, some of the students found it interesting.

And now, once again, I really want a TeleVue-85.