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Invisible Astronomy December 29, 2010

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If you’ve ever wondered why we need space telescopes, this is a lovely example.


It’s an entire galaxy, fairly large and bright and nearby. But don’t look for it in your backyard telescope, it’s hidden behind the dust of our own galaxy. That dust is no problem for Spitzer’s infrared eyes. However, the wavelengths of IR light that can penetrate the dust can’t make it through our atmosphere, so we have to put the telescope in space to get this image.

Jupiter, the Bringer of Jollity | Bad Astronomy | Discover Magazine December 25, 2010

Posted by aquillam in Astronomy, MichiganAstro.
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This was just too lovely not to share.

Jupiter, the Bringer of Jollity | Bad Astronomy | Discover Magazine.

Also, I rather like the debate about the difference between professional and amateur astronomers going on in the comments.

Galileo’s Telescope July 30, 2009

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Galileo's Telescope

Galileo's Telescope

Galileo’s lone surviving in-tact telescope paid a visit to the Franklin institute this year.  The telescope is there until September 7, when it will return to Italy. It is part of a larger (huge even) exhibit on Galileo and the age of the Medici. I have another blog post about the exhibit (with a little about the Franklin too).

Coming up on the ‘scope, the most striking thing (to me) was just how long it was relative to its diameter. I mean, I’ve see pictures, and I know how a Galilean telescope is constructed, but jeez! It looks more like a medieval weapon than a telescope! If you look carefully, you can see that the tube is actually sagging.

A Galilean style telescope is constructed so that the focal point of the objective is at the same position as the focal point of the eyepiece, so the tube length is equal to the sum of the two focal lengths. This way, you get an upright image. Also, it is easy to determine the length of the tube, and there is less travel for trying to focus it when you throw in that third lens (your eye). However, it generally has a tiny exit pupil and a dim image, so it’s a hard style of telescope to use. Astronomers and instrument makers of 400 years ago also had a problem caused by the low quality and strictly spherical shape of the lenses:  chromatic aberration.

Chromatic aberration is caused because the angle the light bends actually depends on the wavelength: blue light refracts a little less that red light, so the focal point for blue light is actually slightly closer to the lens than the focal point for red light. You can really see this looking through something like a crystal ball or round fishbowl. You can usually see straight through the middle, but as you look toward the edges you get a rainbow effect. Galileo was stuck with spherical lenses, so he had to find other ways around the problem.

One way is to make the lens as flat as possible, so the light isn’t being bent as sharply.  This means a really long focal length, and that is why the tube is so long.  There was a sketch in one of the books in the exhibit that appeared to be a telescope of about 3 inches in diameter, but a good 15 feet long!

Aperature mask helps cut down on chromatic aberation

An aperture mask on the objective reduces chromatic aberration

An aperture mask also helps. The steeper the angle of incidence, the greater the angle of refraction. Light hitting the center of the lens should be coming in at nearly a 0º angle, but the angle increases as you get away from the center.  An aperture mask like this one will block the light that would others wise hit the lens at too steep an angle.

Looking at this end you can also see that the ‘scope is in fact made of wood, and you might be able to see the copper tension ring that holds the lens in place. (more on the assebly in my next blog post)

Supposedly, there is a note in Galileo’s handwriting on the eyepiece aperture mask telling the magnification of the telescope. However the lighting was such that I couldn’t see anything (you pretty much had to crawl on the floor to see the eyepiece end.)

oh, and in case you’re wondering, I don’t know how the focus is, but yes, light does still get through it (you can see the tiny dot to the lower right in the image below)


A tiny point of light in the lower right shows that, at the very lease, light does still get through.

Galileo and the Medici July 29, 2009

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The Franklin InstituteGalileo’s lone surviving in-tact telescope paid a visit to the Franklin institute this year.  The telescope is there until September 7, when it will return to Italy. Until then it is part of an exhibit on Galileo and the Age of the Medici.

Unfortunately, it is also summer camp time at the Franklin, so, outside of the Galileo exhibit, chaos rules. The museum was overfilled, understaffed, and rather disorganized. I believe it is a minor miracle there was only one accident in the cafeteria while we were there.

However, we went there to see the Galileo exhibit, so anything else was kind of a bonus. So, on to the exhibit.

First off, if you go, be aware that there is no seating in the exhibit.  Well, ok, there is actually one bench. I suppose they don’t really expect people to read everything like I do, so that’s probably enough for all the people that just pass through.

There are a huge number of artifacts.  I’m not actually sure how many, but I thought there were a lot. And of all different kinds. There were drawing compasses, compasses for measuring height by triangulation, levels, cross-staffs, sundials of every sort, nocturnals, astrolabes, sextants, quadrants, books, diagrams, glassware, lab instruments, drawing tools, and probably a dozen other things I’ve forgotten by now. I was a little disappointed that there wasn’t more explanation about how to use some of the items.  For example, the information on the plaques told us that nocturnals were used to tell time like a sundial, but at night, and that it was important to align them with north (and this was on pretty much every nocturnal plaque.) However, it didn’t actually have any instructions on how to read one, and it really isn’t that hard: align the pointer with the pointer stars in the big dipper and read the sidereal time. If there is a dial for the date, set that first and the time you read will be the mean solar time. Still, seeing all the different types of objects, or all the different versions of some objects (there were sundials I would never have recognized as sundials) was really interesting.

They had some audio tours available for download on the website.  These were very helpful and added a lot to the exhibit.  As long as you knew about them and had them fully working (oops) before you went. Also, there are 3 audio tours, and they all cover the exhibit from start to finish, but not the same artifacts, so if you want to hear all three you have to go back and start over (the student tour covers many of the same things as the main tour, so you may not need to go through again, but it is worth hearing). It may be better to listen to the audio before you go, then decide which one you really want to listen to. Then you would also know if your file isn’t working…

Outside the exhibit was a hands-on area.  It was full of demos about optics, telescopes, and where astronomy stood 400 years ago.  If my feet hadn’t hurt so much, that area would’ve been great!

The big thing of course was Galileo’s telescope.  I’ll leave that for another post though.