TELESCOPE

Andrew Barss wrote:
> Jim Weisgram <[email protected]> wrote:
>
> : 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?
>

Experience aquired by using both.

--

FF

eclipsme wrote:
> Andrew Barss wrote:
> > Jim Weisgram <[email protected]> wrote:
> >
> > : 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
> >
>
> 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.
>

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

J. Clarke wrote:
> Jim Weisgram wrote:
>
> >
> > 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.
>
> I'm curious as to whether the availability of high quality molded optics
> alters this.

Are they diffraction-limited?

I doubt it.

--

FF

J. Clarke wrote:
> [email protected] wrote:
>
> >...
> >
> > 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.
>
> The difficulty with is is that it needs a 2 axis drive to be used for
> astrophotography.
>

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

J. Clarke wrote:
> [email protected] wrote:
>
> >
> > J. Clarke wrote:
> >>...
> >> I'm curious as to whether the availability of high quality molded optics
> >> alters this.
> >
> > Are they diffraction-limited?
> >
> > I doubt it.
>
> When you know for sure get back to us.
>

Where did you get your information about them?

--

FF

J. Clarke wrote:
> [email protected] wrote:
>
> >
> > J. Clarke wrote:
> >> [email protected] wrote:
> >>
> >> >
> >> > J. Clarke wrote:
> >> >>...
> >> >> I'm curious as to whether the availability of high quality molded
> >> >> optics alters this.
> >> >
> >> > Are they diffraction-limited?
> >> >
> >> > I doubt it.
> >>
> >> When you know for sure get back to us.
> >>
> >
> > Where did you get your information about them?
>
> 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.
>

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.

--

FF

CW wrote:
> "J. Clarke" <[email protected]> wrote in message
> news:[email protected]...
> > [email protected] wrote:
> >
> > >
> > > J. Clarke wrote:
> > >> [email protected] wrote:
> > >>
> > >> >
> > >> > J. Clarke wrote:
> > >> >>...
> > >> >> I'm curious as to whether the availability of high quality molded
> > >> >> optics alters this.
> > >> >
> > >> > Are they diffraction-limited?
> > >> >
> > >> > I doubt it.
> > >>
> > >> When you know for sure get back to us.
> > >>
> > >
> > > Where did you get your information about them?
> >
> > 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.
> >
> 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.

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.

--

FF

CW wrote:
> 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.

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.

>
> <[email protected]> wrote in message
> news:[email protected]...
> >
> > The upshot is that after a few years the plastic lenses have far
> > fewer scratches than I would normally have in glass lenses.
> >
> > --
> >
> > FF
> >

CW wrote:
> 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.
>

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...)

> depends on how well you grind your mirror and to a lesser extent, how
> well you build the rest of the scope.

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

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.

<[email protected]> wrote in message
news:[email protected]...
>
> The upshot is that after a few years the plastic lenses have far
> fewer scratches than I would normally have in glass lenses.
>
> --
>
> FF
>

On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss
<[email protected]> wrote:

>Jim Weisgram <[email protected]> wrote:
>
>: 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

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.

On Mon, 17 Jul 2006 13:38:14 -0700, Robert Larson <[email protected]>
wrote:

>> depends on how well you grind your mirror and to a lesser extent, how
>> well you build the rest of the scope.
>
>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

=========================
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.

On Mon, 17 Jul 2006 00:35:41 GMT, "CW" <[email protected]> wrote:

>
>"J. Clarke" <[email protected]> wrote in message
>news:[email protected]...
>> Jim Weisgram wrote:
>> > 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.
>>
>> I'm curious as to whether the availability of high quality molded optics
>> alters this.
>>
>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.
>

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

+--------------------------------------------------------------------------------+

[email protected] wrote:
> CW wrote:
>> "J. Clarke" <[email protected]> wrote in message
>> news:[email protected]...
>>> [email protected] wrote:
>>>
>>>> J. Clarke wrote:
>>>>> [email protected] wrote:
>>>>>
>>>>>> J. Clarke wrote:
>>>>>>> ...
>>>>>>> I'm curious as to whether the availability of high quality molded
>>>>>>> optics alters this.
>>>>>> Are they diffraction-limited?
>>>>>>
>>>>>> I doubt it.
>>>>> When you know for sure get back to us.
>>>>>
>>>> Where did you get your information about them?
>>> 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.
>>>
>> 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.
>
> 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.
>
yes, but we are talking about telescope optics, a good deal more
critical than eyeglasses.

Harvey

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.

<[email protected]> wrote in message
news:[email protected]...
>
> CW wrote:
> > 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.
>
> 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.
>
> >
> > <[email protected]> wrote in message
> > news:[email protected]...
> > >
> > > The upshot is that after a few years the plastic lenses have far
> > > fewer scratches than I would normally have in glass lenses.
> > >
> > > --
> > >
> > > FF
> > >
>

snip
> But I had fun.

that is why it is a hobby. enjoy


Steve

On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss
<[email protected]> wrote:

>Jim Weisgram <[email protected]> wrote:
>
>: 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


Because *better* is subjective.

On Fri, 14 Jul 2006 17:28:42 -0400, [email protected] (J T)
wrote:

>http://members.aol.com/sfsidewalk/dobplans.htm
>
>
>
>JOAT
>Politician \Pol`i*ti"cian\, n. Latin for career criminal

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.

Jim Weisgram <[email protected]> wrote:

: 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

Andrew Barss wrote:

> Jim Weisgram <[email protected]> wrote:
>
> : 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?

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

> -- Andy Barss

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

Jim Weisgram wrote:

> On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss
> <[email protected]> wrote:
>
>>Jim Weisgram <[email protected]> wrote:
>>
>>: 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
>
> 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.

I'm curious as to whether the availability of high quality molded optics
alters this.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

[email protected] wrote:

>
> eclipsme wrote:
>> Andrew Barss wrote:
>> > Jim Weisgram <[email protected]> wrote:
>> >
>> > : 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
>> >
>>
>> 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.
>>
>
> 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.

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

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

[email protected] wrote:

>
> J. Clarke wrote:
>> Jim Weisgram wrote:
>>
>> >
>> > 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.
>>
>> I'm curious as to whether the availability of high quality molded optics
>> alters this.
>
> Are they diffraction-limited?
>
> I doubt it.

When you know for sure get back to us.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

[email protected] wrote:

>
> J. Clarke wrote:
>> [email protected] wrote:
>>
>> >
>> > J. Clarke wrote:
>> >>...
>> >> I'm curious as to whether the availability of high quality molded
>> >> optics alters this.
>> >
>> > Are they diffraction-limited?
>> >
>> > I doubt it.
>>
>> When you know for sure get back to us.
>>
>
> Where did you get your information about them?

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.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss
<[email protected]> wrote:

>But how do
>you know it would be better than anything someone could produce
>in a factory for ten times the money?

Zero-rate your time. Optics is quite cheap, but it's awfully labour
intensive.

The Andy Dingley entity posted thusly:

>On Sun, 16 Jul 2006 02:49:58 +0000 (UTC), Andrew Barss
><[email protected]> wrote:
>
>>But how do
>>you know it would be better than anything someone could produce
>>in a factory for ten times the money?
>
>Zero-rate your time. Optics is quite cheap, but it's awfully labour
>intensive.

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.

Andrew Barss wrote:
> Jim Weisgram <[email protected]> wrote:
>
> : 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
>

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

[...snip...]
>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 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.

"J. Clarke" <[email protected]> wrote in message
news:[email protected]...
> [email protected] wrote:
>
> >
> > J. Clarke wrote:
> >> [email protected] wrote:
> >>
> >> >
> >> > J. Clarke wrote:
> >> >>...
> >> >> I'm curious as to whether the availability of high quality molded
> >> >> optics alters this.
> >> >
> >> > Are they diffraction-limited?
> >> >
> >> > I doubt it.
> >>
> >> When you know for sure get back to us.
> >>
> >
> > Where did you get your information about them?
>
> 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.
>
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.

On Tue, 18 Jul 2006 09:50:56 -0400, Bob G.
<[email protected]> wrote:

>On Mon, 17 Jul 2006 13:38:14 -0700, Robert Larson <[email protected]>
>wrote:
>
>>> depends on how well you grind your mirror and to a lesser extent, how
>>> well you build the rest of the scope.
>>
>>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
>
>=========================
>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.
>
>
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.

"J. Clarke" <[email protected]> wrote in message
news:[email protected]...
> Jim Weisgram wrote:
> > 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.
>
> I'm curious as to whether the availability of high quality molded optics
> alters this.
>
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.