TELESCOPE

[J. Clarke]

The difficulty with is is that it needs a 2 axis drive to be used for astrophotography.

[J T]

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.

\Pol`i*ti"cian\, n. Latin for career criminal

[fredfighter]

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

[fredfighter]

Where did you get your information about them?

[J. Clarke]

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.

[fredfighter]

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.

[Steve Peterson]

snip

that is why it is a hobby. enjoy

Steve

[CW]

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.

[fredfighter]

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.

[CW]

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.

[fredfighter]

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.

[eclipsme]

yes, but we are talking about telescope optics, a good deal more critical than eyeglasses.

Harvey

[CW]

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.

[fredfighter]

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.

[Oleg Lego]

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

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.