TELESCOPE

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J. Clarke wrote:

Where did you get your information about them?
--

FF


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snipped-for-privacy@spamcop.net wrote:

Follow events in the photographic equipment industry and you will be aware that that industry made a transition from ground to molded optics about ten years back. All of the major manufacturers use molded optics for their highest quality lenses--the major driver for the change was that it is possible to make aspheric elements inexpensively using that technology.
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--John
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J. Clarke wrote:

But do you have a source for information about them?
A quick web search doesn't reveal anything useful about how accurately the surfaces are figured.
Telescopes (exclusing refractors) have been using aspherical surfaces for a couple of hundred years.
Few telescope objectives are anywhere near as fast as a typical camera lens, but few camera lenses resolve as well as a telescope objective.
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FF


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wrote:

ten
Cheap is just that, cheap. I pracically have to beat it into their skulls when getting glases. GLASS LENSES ONLY. Plastic ones are still crap and likely will be for my lifetime.
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CW wrote:

I beg to differ. I cannot see any difference optically between pastic and glass eyeglass lenses. But because the plastic ones are lighter they don't slipe down my now as much. That means I don't push them back up as often so they don't get smudged as much. That means I don't have to clean as much.
The upshot is that after a few years the plastic lenses have far fewer scratches than I would normally have in glass lenses.
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You sit behind a desk for a living don't you? I work in a machine shop. A pair of plastic lenses will last for one or two days. Glass, a couple of years.

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CW wrote:

Yes, though I do work on my car, do gardening and woodworking. Not much exposure to metal chips though. I understand where you are coming from. Glass is harder than almost any metal, not so any plastic.

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The one downside to glass. Grinding. The dust is no problem if brushed off, it's the sparks. If a grinding spark hits glass lenses, the surface tension of the glass will be broken at that spot and there will remain a tiny bump. Plastic is better for this situation as the hot particles just roll off.

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CW wrote:

The hot particles don't just roll out of my beard so I have a preference for using a full face shield when grinding. Though I never managed to actually set fire to my face (it might improve my looks) I did notice one pant leg smouldering while grinding.
--

FF

(Yeah, I know, "Liar liar...)


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snipped-for-privacy@spamcop.net wrote:

yes, but we are talking about telescope optics, a good deal more critical than eyeglasses.
Harvey
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On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss

Zero-rate your time. Optics is quite cheap, but it's awfully labour intensive.
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Mon, Jul 17, 2006, 9:11pm (EDT+5) snipped-for-privacy@codesmiths.com (AndyDingley) doth proclaim: Zero-rate your time. Optics is quite cheap, but it's awfully labour intensive.
It's kinda sad that you have to remind people not to rate their time if they're making something for themselves, or as a gift. But it's very true, you can't realistically do it. No matter how much you might wish, your time is only worth money if you're going to get paid for it
On some of my projects for my shop even if I rated my labor at $1 (one dollar U.S.) per hour the labor bill would be so high I couldn't afford to pay myself, and would have to pass on making whatever it was. Just now I'm designing a sled for my bandsaw. I've probably been on it 10-12 hours already. Nothing on paper, it's all in my head, but keep coming up with improvments. I'll meke it from scrap plywood. Now who'd pay $10-12 for a scrapwood bandsaw sled? Not me. So all my work for myself is strictly pro bono. LOL
JOAT Politician \Pol`i*ti"cian\, n. Latin for career criminal
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If you want a great book with five plans for various telescopes, I highly recommend Build Your Own Telescope, by Richard Berry. Covers Building a 4" f/10 Reflector, Building a 6" f/8 Dobsonian Reflector, Building a 6" f/8 Equatorial Reflector, Building a 10" Dobsonial Reflector, and Building a 6" f/15 Refractor. The first telescope plans I've actually been able to understand. Got my copy in a used bookstore, for maybe around $5. Very good read, lots of other 'scope info too. http://www.allbookstores.com/search?type=title&q=BUILD+YOUR+OWN+TELESCOPE
JOAT Politician \Pol`i*ti"cian\, n. Latin for career criminal
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The Andy Dingley entity posted thusly:

For anyone interested in grinding a mirror, but wanting to do woodwork, there's the "Mirror-o-Matic", a mirror grinding machine.
There is a Yahoo group dedicated to it at http://groups.yahoo.com/group/Mirror-O-Matic
The group is described as:
"This list is for discussion about the "Mirror-o-Matic" telescope mirror grinding machine, as developed by Dennis Rech. Likely topics might be about construction and modification of the machine, and about grinding and polishing technique."
The basic framework is baltic birch plywood. There are complete plans, many photos, and discussions about construction and grinding.
Enjoy.
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Andrew Barss wrote:

Experience aquired by using both.
--

FF


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