Slashdot Mirror

For those who are interested, it's possible to get the feeds from the orbital solar observatories and make your own movies of the Sun in action. A nice piece of software to automate this is jhelioviewer: http://jhelioviewer.org/ You can even purchase a small solar telescope that will allow you to view the sun safely at hydrogen alpha wavelengths (at which a lot of features are visible). A popular beginner scope is the Meade PST: http://www.meade.com/product_pages/coronado/scopes/pst.php (Lunt is another good manufacturer). With that you can see solar flares, prominences, sun spots, etc. Prominences are particularly fun because they change visibly over the time-course of minutes; so you can literally see the Sun watch the sun change before your eyes. Here's a link on what's possible to see visually: http://www.prairieastronomyclub.org/resources/solar-observing/observing-the-sun-in-h-alpha/

Only yesterday I was looking longingly at the Meade site and wondering if I could possibly justify a whole eight inch LX200 rather than one of those little ETX series things - I can't help thinking they're the equivalent of desktop routers vs a Cisco 6500. In theory the recent drop in the dollar should make them effectively half price (as I'm in the UK and £1 == $2.03 or so today). Sadly it doesn't seem to work that way :(

It is a good father-son hobby. I built my own 8" newtonian about 27 years ago, dad and I spent a lot of time grinding the mirror, heading down to Meade to buy parts, eyepieces, an equatorial mount, etc. I learned more about my father during that nine month project than I had in my previous sixteen years of existence on this ball of dirt we call the earth.

We had many years of eyepiece time enjoying and documenting our observations

I still have that telescope, and I think of my recently-departed father whenver I use it.

Oh, yeah, we both learned early on not to drink and grind optics. :-)

-Scott

It might be a bad idea to get them into astronomy, especially since a *good* beginer telescope should cost around $400 and will not get used nearly as much as a $400 computer. Don't get a telescope for anything under $200 without *thoroughly* researching it first. It will most likely be a mistake (unless it is a bargain, I got a great 4 inch refractor with AutoStar for $200 at Sam's Club. It is an older model but is is *great*. I think it would have costed around $500 when it was in production). Some good beginner scopes: Orion StarBlast Astro Telescope, Orion SkyQuest XT6 IntelliScope, and Meade ETX series. An alternative is digital photography. You can start them out with an inexpensive camera, and in a few years get a batter one with more advanced features (like a Canon Powershot A95). Or Model rocketry (mute sound!) which is very fun and not too expensive. It can be hands-on (building rockets) or you can get a pre-built kit.

Atmospheric (or lens) diffraction, I'd say. If you were out in space, you could probably block it out with the tip of a pin.

More info on this here.

Here is a cool telescope. You set it on the ground, GPS signals tell it where and when it is, and you type whatever you want to see into the handset. Then the telescope finds it for you.

Celestron (and others) make similar models, but I like Meade.
-aiabx

I'm going to assume you want this for astronomy, not peeping tom activities.

I agree with the folks who say "small". I bought a Meade 4.5" equatorial reflector (Mead 4500, no longer in production) a few years ago and I'm very happy with it. My only regret is that there are no computer controls available for it. At the time, it was important to me to learn to use a telescope. I'm ready to move up to a 6" or 8", but I'm not in a big hurry.

When I did the research, I looked at a number of 'net articles by astronomers - the consensus recommendation to the amateur novice was 4.5" minimum with an equatorial mount (easiest for manual tracking). My telescope is fairly portable - I can easily drag it into the back yard, but does take a little work and care to take it anywhere else. Reflectors are very long, but mine fits across the back seat of my bronco.

I recommend:

• Min 4.5"
• Automated tracking. If dad's got a computer, a tracking system that can be also tied into software like Starry Night to do the aiming/tracking is even better (imho)
• Potential for "stocking stuffers" - what accessories are available? Eyepieces, CCD arrays, filters, etc. I have a solar filter (among other stuff) which is the only way to watch a solar eclipse.

Earlier someone recommended the Meade ETX series. I think if I were buying right now, that would be at the top of my list. Right now they're even offering their AutoStar tracking system ($149) and tripod for free with a purchase. I would think the 105 or 125 would do very nicely. With a little legwork, you might be able to get a better price than the standard retail. We ended up driving down to Phoenix and spending all day going through the camera and astronomy shops (keep in mind this was 1996 or so) - fun and saved about $100.

I'm going to assume you want this for astronomy, not peeping tom activities.

I agree with the folks who say "small". I bought a Meade 4.5" equatorial reflector (Mead 4500, no longer in production) a few years ago and I'm very happy with it. My only regret is that there are no computer controls available for it. At the time, it was important to me to learn to use a telescope. I'm ready to move up to a 6" or 8", but I'm not in a big hurry.

When I did the research, I looked at a number of 'net articles by astronomers - the consensus recommendation to the amateur novice was 4.5" minimum with an equatorial mount (easiest for manual tracking). My telescope is fairly portable - I can easily drag it into the back yard, but does take a little work and care to take it anywhere else. Reflectors are very long, but mine fits across the back seat of my bronco.

I recommend:

• Min 4.5"
• Automated tracking. If dad's got a computer, a tracking system that can be also tied into software like Starry Night to do the aiming/tracking is even better (imho)
• Potential for "stocking stuffers" - what accessories are available? Eyepieces, CCD arrays, filters, etc. I have a solar filter (among other stuff) which is the only way to watch a solar eclipse.

Earlier someone recommended the Meade ETX series. I think if I were buying right now, that would be at the top of my list. Right now they're even offering their AutoStar tracking system ($149) and tripod for free with a purchase. I would think the 105 or 125 would do very nicely. With a little legwork, you might be able to get a better price than the standard retail. We ended up driving down to Phoenix and spending all day going through the camera and astronomy shops (keep in mind this was 1996 or so) - fun and saved about $100.

Get the largest aperature you can afford. Both Meade & Celestron make models with an 8" apperature with auto goto in the price range you suggest. You can easily hook either of them up to a laptop by using a webcam. Sample photos from a webcam of Jupiter can be found here.

Use a Philips webcam (Forget the name of it, but it is mentioned on the web page listed above) because it has the most sensitive CCD of the webcams, and takes the best photos. You can also get an adapter for around $20 to hook up the webcam to your computer, or you can easily make one.

Also, if you don't absolutely need the auto goto, you can get a good Dobsonian mounted telescope pretty cheap. Check out Orion Telescopes for some good Dobsonian mounted scopes, and some good Newtonian reflectors in the price range you wanted.

And oh yeah... $1,500 is by no means an ultra expensive telescope! A high quality mirror alone can cost several thousand dollars.

The Meade Autostar telescopes are great for someone who is wanting to look at the cosmos but isn't sure where to look. You set it up, do some alignment, then select from thousands of different objects to look at. It will automatically locate to that object.

The reason they are called the Leonids is that the main orbital path the meteroids are on before they strike the earth is such that it points back in the general direction of the constellation Leo at the point where the earth crosses the comet's orbit each year (meteor showers come from debris broken off a comet).

If you make a black-on-white copy of a starchart, and draw a line on it for each meteor you see when it happens, with an arrowhead in the direction of travel, at the end of the night you will see the most of the paths generally radiating away from Leo, like spokes radiating from the hub of a bicycle wheel. This is like what you'd see if you stood in the middle of a multilane highway as cars sped past you, facing where they come from - you'd see the cars angling to the right and left, but "radiating" from one spot in the distance.

If a meteor's path is very short, it is headed in your general direction. If it just a bright spot, then it is headed straight for you, so you know when to duck. If it is very long, it is headed away from you.

I don't know if it is still practiced, but there used to be organized efforts among amateur astronomers to map meteor paths during showers so their orbits could be calculated. Now I guess it would be more practical and accurate to do it with radar. To do make such a calculation, the observers also need to write down the time they saw each meteor.

Even so, the meteors won't all be radiating from a single point. There will be a lot of randomness. Part of this will be because the meteoroids are spread out in space, to either side of the comets orbit, each on its own slightly different orbit.

Also, as it approaches the earth, the earth's gravity will disturb the orbit of the meteoroid. If the meteoroid is heading straight to the center of the earth just before it hits, then it will just go faster. If it's heading a ways to one side of the earth, then its path will be deflected in towards the earth, and when it hits it will be at a highly deflected path. If it's even farther to the side, it won't hit the earth but it's orbit will be disturbed, and many orbits of a planet through a comet's path will introduce a lot of scatter in future showers.

Now let me shill for amateur astronomy. I'm grinding my own telescope mirror. You can join the Amateur Telescope Maker's mailing list and they'll tell you how - read the FAQ. Dan Cassaro can sell you a mirror grinding kit. You can get books with instructions (you need a whole book, it's pretty involved) from Willman-Bell. You can find lots of tips on the Telescope Making WebRing.

Or you can buy telescopes from Meade and Celestron or shop at the shop at the astronomy mall. Finally, there's a new ATM portal at www.telescopemaking.com.

I made several telescopes when I was a teenager, and have recently taken up grinding glass again after a long hiatus. I was also pleased to find the Central Maine Astronomical Society is in my area and joined last night while visiting their new observatory.

Telescope mirrors can be made by hand with suprisingly simple equipment. An eight-inch diameter telescope will run you about $250, maybe less if you're creative, for the mirror kit, eyepiece, aluminizing, and mounting.

There may be a telescope making or astronomy club in your area. A good way to find out is to subscribe to the ATM mailing list. Another way is to follow some of these links:

• Chabot Telescope Maker's Workshop (Oakland, California)
• Sidewalk Astronomers (Los Angeles and San Francisco)
• Amateur Telescope Makers of Boston
• Stellafane - Springfield Vermont, where the hobby was started in the USA

If you don't want to build a telescope, you can buy one. The telescopes made by Meade and Celestron are well known. You can find ads for dealers in the pages of Sky and Telescope Magazine, which you'll find in many bookstores.

A large number of astronomy products may be found through the Astronomy Mall.

Although the price differential for small telescopes like 6 or 8 inches is not that great between making it oneself and purchasing, the cost of purchasing really large instruments is really prohibitive, while large ones are actually affordable to make, comparable to purchasing a computer. If you start off making an 8 inch mirror, your next mirror can be much larger, say 16 inches, and amateurs commonly make mirrors from 20 to 30 inches, and I think there is a 72 inch mirror nearly complete made by some amateurs. My goal is to have a 40 inch observatory in my backyard.

Although I've listed U.S. organizations and companies, telescope making is practiced world-wide. A while back someone from Iraq subscribed to the ATM list and asked for help obtaining a kit. There are lots of subscribers from Europe and a number from Asia and Africa. Follow the links, and maybe you'll find a club in your home town, or at least within a reasonable distance!

I cannot describe the awe that comes from beholding the wonders of the heavens through a telescope made with one's own hands.

• Amateur Telescope Making by Albert Ingalls
• How to Make a Telescope by Jean Texerau

The stubby Celestron and Meade telescopes that are popular with amateur astronomers who prefer to purchase their instruments are of a type called a "schmidt-cassegrain". This has a nearly flat corrector plate in the front, that actually has a shallow fourth-order curve ground into it to correct spherical aberration, a deep prolate spheroidal primary mirror, and a convex secondary mirror mounted on the back of the center of the corrector plate.

It's the convex secondary that makes the telescope a cassegrain. The 200 inch on Palomar is a cassegrain. I don't have a schmidt-cassegrain to show you but here's how an ordinary cassegrain is laid out.

The use of the schmidt corrector plate allows one to make the telescope very short, with a small ratio of focal length of the primary to its diameter, without making images away from the center of the field blurry.

This is an advanced kind of design for an amateur to make oneself, although many amateurs have. Here's how one guy made a schmidt corrector plate.

The typical amateur starter scope is the "newtonian reflector". This has a concave parabolic mirror at the back end of the tube, and a few inches inside its focus is an optically flat mirror at 45 degrees. The optical path shown in the diagram is for the light from a single star, an image is formed from light sources spread across a small angle, and a small image is formed at the focal plane where it's examined by the eyepiece (a high-power magnifier) or photographed with film, a CCD or I guess even a webcam.

If you make a parabolic mirror with too short a ratio of focal length to diameter (the f-number, like the f ratio on a camera lens), then the images away from the center are blurred. This is called "coma". A parabola only focuses light perfectly if it's parallel to its axis and tilting the beam introduces coma. A ratio of 1 to 4 is about the shortest you can make it - f/4. My 6 inch is f/8, my 10 inch is f/3.5, and my 8 inch is f/6.

Having a longer focal length gives you greater magnification. Having a shorter one gives you a wider field of view, within the limits of the coma. Having a shorter focal ratio also makes it easier to fit in a car, an important consideration for making the scope enjoyable. Those Celestrons are nice because they'll easy fit in the trunk of a car or even in airline luggage (with a hard case) but it comes at the expense of a fancier design.

For the first homebuilt scope one usually grinds the primary and buys the flat diagonal mirror from a vendor. More advanced amateurs make their flats too but again optically flat surfaces are hard to make.

Making a primary that doesn't have too short a focal ratio is not too bad because the grinding process naturally makes a sphere. You grind a sphere of the right radius of curvature, fine grind through successively finer grits, then polish. You then use an optical test to get the mirror perfectly spherical, then deepen the center to move from a sphere to a parabola of revolution, testing carefully as you go.

The way I ground my mirrors was with pyrex mirror blanks on plate glass tools. Initially each is flat. They are both pretty thick, my 8 inch is about 1.25 inches thick, to stiffen them so they don't lose their figure. You have to have a figure that is perfect to about 1/8 of a wavelength of visible light in variation across the whole face of the glass, so any bending is disastrous. The 1/8 wave limit is the same for mirrors of all sizes so it's much harder to figure larger ones - best to start small. I would recommend an 8 inch for a first mirror. I have heard of people doing much larger first telescopes though.

What you do is sprinkle some granulated silicon carbide and water on the tool, place the mirror blank face down on it and push it back and forth until the silicon carbide ("carborundum") breaks down. (This is the same abrasive as you find on black wet-or-dry sandpaper, only in free-flowing powdered form). Then you add more abrasive and water and repeat. When too much mud builds up you wash it off and add more abrasive again.

To grind a concave curve into the mirror blank you place it on top, face down, grind with long strokes and have it hanging mostly off the side. Also you put pressure on it, either pushing hard or putting weights on it. This concentrates the grinding action in the middle and a shallow sphere develops.

Every few strokes you rotate the mirror a little, and once a minute or so you rotate the tool a little, with the idea that every part of the mirror gets ground over every part of the tool in every direction.

These days it has become popular to "hog out" a mirror with a metal ring tool, like a pipe cap as I'm about to try, then after rough grinding you make a fine-grinding tool out of small bathroom tiles mounted in dental stone or portland cement. This is in part because it's getting harder to get telescope making kits, unfortunately because it's so easy to buy a Celestron people don't make their own as much anymore. So people conserve the glass just for the mirrors and make the tile tools instead.

Be aware, before you say "well it's easier to buy a Celestron", that the price of a telescope goes up astronomically with increasing diameter - my 8 inch kit was $78 including shipping, I'll probably spend a few hundred to make a nice clock-driven mount, but the 10 meter telescope on Mauna Kea cost $90 million! If you know how to make your own, it is within your reach to grind your own 20 inch, which will have astounding views, but few of us could hope to afford to purchase a twenty inch commercial scope.

I know people who have ground 30 inch scopes and I know of some amateurs who are now figuring a 67 inch mirror!

Anyway it takes several hours of work to rough grind your mirror, more if you're doing a short f/number, less if you have a higher one, also less for smaller mirrors and more for larger ones. My 6" f/8 was about as deep as the thickness of an american dime, I don't know a little more than a millimeter.

Then you fine grind, grinding for a few hours with successively finer grades of abrasive. Usually you rough grind with 80 mesh silicon carbide - it is graded by sieving it through a mesh with 80 wires in it (same as the sandpaper sizes). Then you grind with #120, #220, #320, #400 and then several very fine grades of aluminum oxide whose sizes are given in microns.

The idea is that each finer grade erases the pits left by the previous grade. Between each grade you must scrupulously clean yourself, the mirror and tool and your work environment lest a coarse particle get into a finer stage and cause a scratch.

With each grade the mirror and tool surfaces will become more and more accurately spheres, within the limits of the sizes of the grits. This is because a sphere is the only shape that allows two surfaces to be placed anywhere against each other in any position or rotation (a flat surface is the limit of this as the radius goes to infinity). If there are any high spots, they will get more pressure and grind off quickly; any low spots will miss out on grinding and the surrounding surface will come down to match.

Then you polish. You make a "pitch lap", using either another dental stone base or the glass grinding tool, covered with refined, thickened pine pitch. You cut channels in the pitch with a knife or mold them in with a silicone mold. Then you cover the pitch lap with a suspension of cerium oxide in water, or else ferrous oxide (same as rust but finely powdered - "jeweler's rouge"). Then again you stroke the mirror on the pitch lap.

During fine grinding and polishing you use shorter strokes, and alternate which is on top, the mirror or the tool, to keep the depth constant. You also stroke a little side-to-side, in a W pattern. This evens everything out.

To test the mirror you use the Foucault test or the Ronchi Test. The foucault test appatatus I link to is much fancier than you need, although nicer to use - you can do it all with your naked eye and the tester, you don't need a camera.

In each test you use a light emanating from a pinhole or narror slit just to the side of the center of curvature of the mirror. The image of the pinhole or slit will form an equal distance to the other side, where you can place a knife edge (Foucault) or screen (Ronchi) across it and hold your eye there and look at the mirror.

It's kind of hard to explain but each of these has the effect of dramatically magnifying deviations from spherical surfaces in the mirror. A dramatic demonstration is to have someone hold their hand in the beam - you can see the distortion in the beam caused by the warm air rising from their hand.

You can easily make out a bump or hollow that's a fraction of a wavelength high on the glass.

Then you make your mirror perfectly spherical by preferentially polishing off the high spots. If you did the fine grinding and polishing well you won't have to work hard to do this.

Unfortunately what we want is a parabola, not a sphere. This must have a precisely controlled error in each test. This is a little more than I want to get into, but basically your preferentially polish out the center of the mirror so it's deeper in the middle than appropriate for a sphere by a little bit. Get it just right and you have a parabola, and your mirror will focus perfectly.

Then you package it securely and send it off to one of the people who does vacuum aluminization. They clean the mirror extremely well, place it in a high vacuum, and evaporate aluminum off of tungsten wires. The aluminum vapor sticks to your glass and you have a telescope mirror.

Mike

• Mead LX200 telescope ~$2500US
• WM-918 weather station
• both run with linux using wx200d and xephem
  then you just need a CCD camera and an internet connection for remote viewing :)

They've got some nice beginner scopes now that have a computerized motor attached that has an astronomical database attached - you can just tell it what object to point it. Can't get any simpler than that. Also, the optical quality of the scopes are quite nice from what I've read. They're also small enough to carry around, so you can go out into the country, away from urban light pollution and air pollution, and get a real clear view.

The models are in the ETX-60, 70, 90, and 125 lines, with the 'Autostar Computer Controller'.

Pretty sweet. Check them out at www.meade.com.

a Meade LX200 16" is a great gift to give, and receive!
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