TELESCOPE

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http://members.aol.com/sfsidewalk/dobplans.htm
JOAT Politician \Pol`i*ti"cian\, n. Latin for career criminal
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On Fri, 14 Jul 2006 17:28:42 -0400, snipped-for-privacy@webtv.net (J T) wrote:

It might interest some to know that John Dobson is still around (I know this because he was taking his morning walk by my house the other day) and he still travels around the country teaching folks how to grind their own mirrors, make telescopes, etc, using mounts that look just like your link. The tubes are from cardboard concrete forms. Dobson recommends you take apart a pair of pawn shop binoculars to get the eyepiece.
You end up spending a few hundred dollars to construct a scope that is better than any you could purchase for thousands.
The plans on the web site appear to be scans taken from his instructions that he passes out when you make a scope in his class.
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: You end up spending a few hundred dollars to construct a scope that is : better than any you could purchase for thousands.
I'm willing to think that a few hundred dollars could make for a nice scope, with some serious work involved. But how do you know it would be better than anything someone could produce in a factory for ten times the money?
    -- Andy Barss
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On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss

Because *better* is subjective.
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Andrew Barss wrote:

If you grind your own mirror, which in mho is the hardest part - the woodworking is pretty basic - there is the possibility of getting a mirror with much greater accuracy than can be purchased for a similar amount.
Also, with attention to detail and design, other parts of the scope can be improved upon over the typical import scope.
Yes, it is possible to buy a mirror blank and turn it into a mirror worth thousands, and I think this is what the op is referring to. For a few hundred dollars in total parts, you can actually make a scope worth perhaps a couple of thousand.
Perhaps if the word 'can' were inserted... "You 'can' end up spending a few hundred dollars to construct a scope that is better than any you could purchase for thousands.'
depends on how well you grind your mirror and to a lesser extent, how well you build the rest of the scope.
Harvey
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I took John Dobsons class and have a great telescope that I take up to the intersection by my house a few times a year. Not only was it relatively inexpensive, but it is indeed a great telescope.
Fortunately we have a number of used record stores in and around the Haight Ashbury - I was given an old scratched LP by one for the bearing surface on the base that allows the device to rotate smoothly through a complete circle. I used UHMW jig/fixture material for the other bear surface, although in John's class they do supply small teflon pieces. I had to try the UHMW as we sell it - it worked as well as teflon.
As I recall the entire device is generally called a Dobsonian telescope because John Dobson devised a fantastic design that could be built cheaply by almost anyone. The optics have been around for a long long time (Newton).
By the way, the telescope is a great people magnet. Even in my residential block - perhaps six to eight blocks from the real heart of the Haight-Ashbury - I will have ten to twenty people stop by to ask what I am doing. When they find out, they invariably ask if they can view. My eight inch telescope fills the eyepiece with the moon, and when it is full the light is almost blinding. The rings of saturn are incredbile as well. My wife also agrees that more than half of those that stop are single women, so if one were single ... well I'm not.
The mount took perhaps a day's work, the mirror much longer. At the very end of the grinding and polishing John did help with some slight problems I had, using a trick he had only figured out in the relatively recent past.
I didn't see the start of this tread, but I would highly recommend making one of these - fun, educational, and a gift to the community when you share the night sky with them.
Robert Larson
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wrote:

========================Astronomy is another of my Hobbies...
"my" Observatory is finally under construction as the Dome itself was finally delivered last week...thus my SCT will finally have a home and I will not have to contend with cooling down the scope before putting my eye to the eyepiece.. and dew will become a "little" less of a problem
I have to admit however that I never attempted to grind my own mirrors or build my own scope... BUT I have had the pleasure to let some of the neighborhood folks look at Saturn and hear their reactions (WOW) And a couple of local teens stop by almost every time I set up
Clear Skyies to y'all
Bob G.
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On Tue, 18 Jul 2006 09:50:56 -0400, Bob G.

Years ago (don't ask) I ground, posished and figured a 10" Newtonion reflector, f7.2. Whenever we had guests over when I had the telescope set for viewing Saturn they would stand on a picnic bench and look down the tube to see if I had set up a fake picture or something in the tube to fool them. They could't believe the image was so impressive.
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eclipsme wrote:

Yes, 'can' is quite apropos. However, you _can_ also buy the optics and make a scope that resolves fine detail and has a much lower limiting magnitude than a store bought or mail-order scope costing several times as much.
The simple mount developed by John Dobson is smoother than what you will get in any scope for less than $1000 and can be made for the price of a cople of sheets of plywood, a handful of screws, a bit of glue and paint and some formica and plastic scraps.
--

FF


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snipped-for-privacy@spamcop.net wrote:

The difficulty with is is that it needs a 2 axis drive to be used for astrophotography.
--
--John
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J. Clarke wrote:

To be accurate, his altazimuth mount was never intended to be mechanically driven in the first place, nor were his scopes intended for photography.
Howver, you can plop the whole scope down on a Ponce' platform, which traditionally is built as a constant speed one-axis drive. Your exposure times wil be limited by considerations of balance and travel in the drive mechanism
--

FF


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Andrew Barss wrote:

You compare the measured optical performance and find that yours is better than theirs?

--
--John
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On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss

Hmm, I was paraphrasing Dobson there, and I trust that he knows.
Better optically, per inspection and correction of the primary mirror using a technique of using a bright point of light and out of focus eyepiece to obtain an interference pattern image at the eye; from that hand corrections are made to the primary mirror until the interference pattern is "correct". Commerically made mirrors don't approach the precision of this method until you get into the big bucks.
Again, paraphrasing Dobson.
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Jim Weisgram wrote:

I'm curious as to whether the availability of high quality molded optics alters this.
--
--John
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A lens or mirror cannot be molded to any better standards than it can be ground (in any process that I'm aware of). Chances are that the molded product would be inferior as it can be no more accurate than it's mold and likely less so.
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Significant advances in molded optics have produced optics of very high precision, but nothing yet at the precision of ground optics.
Note that the answer to original question, that for several hundred dollars in parts, one can produce and optical system worth many thousands of dollars. This is the equivalent to the fact that with several hundreds of dollars worth of wood, one can produce furniture that would cost many thousands of dollars. The only thing standing between the wood and the furniture is many hundreds of hours of labor and some quality woodworking tools. Same thing holds for the telescope. If one's time is free, yes, a few hundred dollars of components results in something that they would have to pay thousands for. But most likely, one will have put several thousand dollars worth of hours into the project.
+--------------------------------------------------------------------------------+ If you're gonna be dumb, you better be tough +--------------------------------------------------------------------------------+
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[...snip...]

If I recall correctly, with Dobson teaching the class (he can be a very crabby guy, by the way, but he very much believes in what he is doing in helping people learn astronomy and how to do their own scopes) the investment in grinding and polishing your own mirror and building the scope and the mount is something like 20-50 hours. I did this about 10 years back with an 8" mirror, so I don't recall the total hours that well. Most of the class did 10" mirrors and their grinding time would be a bit more.
If I was a consultant, I guess I would be earning about $100/hr in my chosen field (not astronomy) and so I guess I lost money on the deal.
But I had fun.
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snip

that is why it is a hobby. enjoy
Steve
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J. Clarke wrote:

Are they diffraction-limited?
I doubt it.
--

FF


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snipped-for-privacy@spamcop.net wrote:

When you know for sure get back to us.
--
--John
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