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Guy's Math & Astro Blog

Guy's Math & Astro Blog

Monthly Archives: March 2016

Hitherto Un-noticed properties of Primes

15 Tuesday Mar 2016

Posted by gfbrandenburg in Math

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Tags

Math, mathematics, prime number, probability

Did you know that if a prime number ends in a 9, then the next prime number larger than that is much more likely to end in a 1 than in a 9?

Did you know that if you study coin tosses, it will take you only about four tosses to find a head followed by a tail, but about six tosses to get two heads in a row, even though they are e qually likely?

I didn’t either. Interesting article:

https://www.quantamagazine.org/20160313-mathematicians-discover-prime-conspiracy/

A ‘Maker Faire’ in Reston, VA

15 Tuesday Mar 2016

Posted by gfbrandenburg in astronomy, Telescope Making

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make, maker faire

I went to a “mini maker Faire” in Reston mostly as an exhibitor (on telescope making, hence the home made scopes) but also had a bit of time to visit with other “makers” before the gates opened.

The silly little ears are from a gizmo that was supposed to read and interpret brain waves … either my head was too thick or I broke the gizmo or I am dead: It couldn’t read my brain waves.

It was fun, and I talked to a lot of very interesting people. I regret that I didn’t take Jeff Guerber up on his offer to staff my tables while I went and looked at the other exhibits. Two entire, enormous, modern NoVa public schools (a MS and a HS) were filled with exhibits on all kinds of crafts. Lots of 3D printers, physics stuff, including jugglers in my hall. I have no idea what was in the other ones!

I met a high school girl who had a T-shirt proclaiming her desire to become a mason. She told me she was well aware that there were very few (if any) female masons. I applaud her goal and hope she is successful!

IMG_4321
IMG_4318
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IMG_4319
IMG_4323

Another recent scope at the NCA – ATM workshop at the CCCC

11 Friday Mar 2016

Posted by gfbrandenburg in astronomy, Telescope Making

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ATM, CCCC, NCA

This ultra-short scope, by Todd M, has a mirror of 4.25″ (108 mm) and a pretty short focal length – about 2 feet (60 cm). He made just about everything, right here in the NCA ATM workshop at the Chevy Chase Community Center. He ground, polished, figured, and even helped aluminize the primary mirror; made the primary cell AND the spider and secondary holder; made all of the rest of the mount that you see; and even made the focuser itself from some plumbing parts!

IMG_4104
IMG_4106

It’s a very nice job, meriting a lot of praise. In case you were wondering, the paint was a special, very-high quality and very expensive top-of-the-line alkyd enamel, costing about $200 per gallon – and we have two of them. Explanation: it was an ‘oops’ can that was specially ordered and mixed for someone who changed their mind and couldn’t return it. In exchange for a non-profit donation receipt in the name of NCA, Bill R was able to get the person to donate both gallons to us.

The spider and secondary holder are very similar to the one made by Ramona D that you can see here. The major differences are:

(1) Todd used busted bandsaw blades rather than steel strapping tape for the vanes. (Both were the same price: free.) After looking at both projects, which both turned out quite nicely, my conclusion is that if you want to use bandsaw blades, you have to heat-treat (anneal) them so they will have less of a tendency to break right at the location where you are trying to bend them by 45 degrees. (Heat it up to cherry red and then let it cool slowly in the air, making it softer and less brittle, I am told…)

(2) And of course, it certainly helps to grind down the teeth of the bandsaw blade both for safety and to reduce weird reflections. Strapping tape is about the same thickness as many band saw blades, but the tape is wider and hence more stable and less prone to turn crooked (I think).

(3) Todd used ordinary 1/4″-20 machine screws (aka bolts) to attach the vanes of the spider to and through the walls of the tube. He cut off the heads of the bolts and ground one side flat near the head, and then drilled a little hole in that flat part, tapped (threaded) that, and used a tiny little machine screw to attach the vane to the specially-prepared screw, in a process that I hope is clearly shown in these three drawings.

Begin with a machine screw (bolt)
Begin with a machine screw (bolt)
Cut off the head, use a grinder or saw to make a flat area (or else you can split the screw down the middle)
Cut off the head, use a grinder or saw to make a flat area (or else you can split the screw down the middle)
Drill and tap (i.e., thread) the little hole; attach to vane; feed the far end through a hole in the wall of the tube; attach securely with a washer and nut.
Drill and tap (i.e., thread) the little hole; attach to vane; feed the far end through a hole in the wall of the tube; attach securely with a washer and nut.

(4) Ramona, however, used thumbscrews instead of doing all that cutting, filing and tapping. Actually, our little tiny tapping drills didn’t play well with our bit holders – they kept slipping. So she just drilled holes in the center of each thumbscrew head, and bought three very small nuts and bolts and used them in the place of the little screw that Todd used.

(Thumbscrews like these:)

thumbscrews

Some student-made telescopes

07 Monday Mar 2016

Posted by gfbrandenburg in Uncategorized

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I’ve been helping students at the First Light Saturday science school at the Carnegie Institution for Science for several years now. We’ve done a variety of activities, from making small generators and exploring water power; building and programming robots; measuring the chemical content of foods; growing plants under various conditions (including simulated zero-gravity); and this year, experimenting with light, including building their own small telescopes.

They so far have made three such telescopes: a Galilean, a Keplerian, and a more modern achromatic refractor. Here is what they used to make them. The lenses, all from Surplus Shed, cost a grand total of Five dollars per set. The PVC was a bit less, I think.

galilelan and keplerian telescope

Because of bad weather, our winter term was somewhat shortened. Here is one example of what they will finish – a small refractor on a tripod! (They’ll need to supply their own cat, though…)

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The world’s blackest material

07 Monday Mar 2016

Posted by gfbrandenburg in Uncategorized

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VANTA black is apparently the blackest material in the world – so far. Made out of vertically-oriented arrays of carbon nanotubes grown in situ, reflects only something like 0.035% of the light that hits it. Imagine that coating the inside of a telescope tube! No more stray reflections!

https://en.wikipedia.org/wiki/Vantablack

Possible ‘You’ve been noticed’ card to give to science fair participants?

07 Monday Mar 2016

Posted by gfbrandenburg in Uncategorized

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NCA-business-card

One Way to Make a Telescope Spider

05 Saturday Mar 2016

Posted by gfbrandenburg in astronomy, Telescope Making

≈ 4 Comments

Tags

ATM, diagonal, dobsonian, spider, Telescope

All Newtonian telescopes require a secondary mirror — a flat mirror held at roughly a 45-degree angle to reflect the light from the primary out to the side. Generally this secondary mirror is an ellipsoid, in order to waste as little light as possible.

One major problem is figuring out how to hold this secondary mirror in place securely without interfering with the passage of light from your distant target. The secondary mirror can be held on a stalk, or on crossed arms like a spider’s web.

The images below show how Ramona D made a spider using a piece of extruded aluminum tube with a square cross section, several bolts, a spring, a piece of plastic dowel, some pieces of steel strapping tape, a few thumbscrews, and various small nuts and bolts. She did a very neat job, including threading and tapping several small holes in the aluminum tube.

The idea is not original to me: I got the idea from somebody else on line, but unfortunately, I don’t recall the name of the person to whom I should give credit.

Here are some photos that probably do a better job of explaining how to make it than I could explain in many, many paragraphs.

ramonas spider 1.png

ramonas spider 2

ramonas spider 3

ramonas spider 4

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