Telescope Mirrors?
Wednesday, November 18th, 2009 at
9:41 pm
Hi,
I want to build my own telescope and now I need to know how I can build the mirrors, any ideas?
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It is possible to make your own telescope mirrors at home, but it’s not easy or fast. You’ll need an area that can be easily cleaned, and a sturdy workbench. Then you need the glass blank (the mirrors are made of glass), a glass “tool” (like the blank, only smaller), optical pitch (like gooey tar) and optical abrasives in various grades from coarse to fine. Shaping the glass takes many days or weeks of work from the coarse grinding to the final figuring and polishing, and there is a lot of skill involved, which you can perhaps learn from books on the subject, but some of the more difficult parts might require the advice of someone with experience. When finished, the mirrors need to be aluminized in a vaccum chamber—this can possibly be done at home, but it’s certainly cheaper to send it to an optical shop with all the right equipment, because it requires a fairly clean vaccum chamber and an aluminum evaporator. This is a lot of work, a big committment in time, and does require some money for tools and supplies. It is probably cheaper, and certainly much easier, to buy a mirror ready-made from an optical company like Edmund Scientific. Making your own mirrors can be an interesting and rewarding experience. Good luck.
There are several companies that offer “kit” mirrors. I suggest you buy a mirror it will save you time and expense in the long run as the mirror is the telescope.
There are several books. Here are a couple. I used the first one.
You can go about this a few ways. You can order the mirror already made, or you can order a glass blank and grind the mirror yourself, then send it out to have it silvered.
Grinding unless you have a grinder to “hog out” the general shape of the mirror, is very labor intensive. Workshops on how to grind your own mirror are offered around the country through different astronomical societies. Whether you grind you own mirror or not you should consult with the people on Amature Telescope Making websites such as:
http://www.atmsite.org
http://www.atm-workshop.com
http://www.stellafane.com
There are also many books available on the subject.
If you wish to order the mirror already made, here are some places to get it from.
http://www.rfroyce.com/ < -Highly acclaimed!
http://www.scopecity.com < --Pricey in my opinion
If you want to grind it yourself you can buy the blank from:
http://www.newportglass.com
If you grind it you will also need to buy a grit kit and many other supplies and you will need a clean workshop to work in. You will also need to have testing devices handy such as a focault tester to properly figure the mirror.
I hope this helps.
Its always better to get a real Telescope. Nowadays there are great cheap Telescopes for beginners.
Its easier to make your own Telescope when you are familiar with a real one.
Right after that you can purchase a book on how to make your own Teles or just Google
you need to start with a kit ( pre-ground not polished )www.e-scopes.cc YOU can grind your own mirror seach the web lots of kits out there -TELESCOPE MIRROR KITS -
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The key factor is aperture – that is how big the primary mirror or objective lens is – since this governs how much light the telescope can collect. More light means that you can see fainter objects and higher magnifications are practical. For a first telescope where the budget is a consideration you are usually best looking at a Newtonian reflector telescope since these have the most bang for buck. Other scopes may have better optical quality or be better for certain tasks, but at the bottom end of the price scale a Newt usually wins hands down.
Less simple is the choice of mount. Here there are two options to consider: you have the Dobsonian (Dob) and the German equatorial mount (GEM). The Dob resembles a cannon mounted on a gun carriage, whereas the GEM is mounted on top of a tripod as you would expect.
Dobs are the cheaper of the two options, meaning you can get a larger or better quality scope and also slightly easier to set up since there is no polar alignment to worry about. There are people who get obsessed by aperture and will always say buy a Dob for this very reason, but it is important to retain a sense of perspective. The law of diminishing returns applies and in any case Dobs do have some limitations in use. They give you aperture cheap but that is it.
GEMs are slightly more flexible in use, and can follow the night sky very easily (the higher the magnification, the faster the stars appear to move). However, _some_ GEMs are unusably shaky in use. It isn't really that many – most shaky mounts are a different kind called altazimuth – but try to find some favourable reviews before you buy. (I've ignored the conventional altazimuth mount here since it combines the disadvantages of both the Dob and the GEM with none of the benefits.) GEMs do need careful setting up, but the process isn't really any slower, since for either option the limiting factor is the time needed for the scope to cool down when you take it outside.
The choice between the two is ultimately personal. There is no doubt that a GEM is more flexible and capable but it is also less convenient.
In the $200-300 range you are looking at a 6" Dob or 4.5-5.1" GEM – if you go for a GEM try hard for a 5.1" scope as there are some dodgy 4.5" scopes out there. I'm not going to recommend specific scopes but Celestron, Orion and Sky-Watcher all have scopes to suit you in their ranges and are well respected. I would steer clear of short tube scopes if you have the option, though. These are shorter and more compact but optical quality suffers because of the reduced focal ratio.
Yes, you can build your own scope. If you want to go down this route get a good book on amateur telescope construction since it is too complex to go into here. Be warned that you will not save much money without expending a _lot_ of time. If you buy in optical components and build them into a standard sized scope you are going to save little if anything.
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Assuming that the telescope is of high quality construction, and that the atmospheric conditions are perfect for observations, then the resolution is limited by diffraction.
Light passing through the lens interferes with itself creating a ring-shaped diffraction pattern, known as the Airy pattern, if the wavefront of the transmitted light is taken to be spherical or plane over the exit aperture. The result is a blurring of the image. An empirical diffraction limit is given by the Rayleigh criterion invented by Lord Rayleigh:
The images of two different points are regarded as just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other. If the distance is greater, the two points are well resolved and if it is smaller, they are not resolved. Mathematically, this translates into:
D=1.220 x w/sin r
Where D is the diameter of the main mirror or lens
w is the wavelength of the light
r is the resolution required in radians
In this case we have:
w = 5000 Angstroms
r = 0.4 arc seconds = 0.000111111 degrees = 1.93925 x 10^-06 radians
sin r = 1.93925 x 10^-06 radians
Therefore
D= 1.220 x 5000 / 1.93925 x 10^-06 Angstroms
D = 3,145,538,295 Angstroms
D = 31.46 centimeters
or about 12 inches.
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T-Pain’s Daniel Marino “Dolphin Killer” hearse – Click above to watch video
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[Source: YouTube]
Continue reading VIDEO: T-Pain introduces the “Dolphin Killer” Hearse
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