ASTRO: Reproduction of Newton's scope

Stellafane often features people who reproduce historical instruments. For
example, in the past there have been copies of William Hershel's smaller
scopes, complete with metal mirrors. Here is a working copy of Isaac
Newton's telescope, the first "Newtonian".

There's a science history professor who shows up every year with his latest
reproduction of mostly medieval era pre-telescope instruments.

George N

"George Normandin" wrote in message
...
Stellafane often features people who reproduce historical instruments. For
example, in the past there have been copies of William Hershel's smaller
scopes, complete with metal mirrors. Here is a working copy of Isaac
Newton's telescope, the first "Newtonian".

I didn't know they had bowling ball mounts back in Newton's day. Heck, I
didn't even know they had bowling. ;-)

"Steve Paul" wrote

...Newton's telescope, the first "Newtonian".

I didn't know they had bowling ball mounts back in Newton's day. Heck, I
didn't even know they had bowling. ;-)

Steve,

They must have had "bowling"....... or why else would the Dutch have
built the "Bowling Green" in New Amsterdam (which is still there in lower
Manhattan) at just about the same time as Newton's scope?

Around the same time the Dutch also outlawed the playing of "Kolf" in the
streets of Beverwyck ("Beavertown", aka Albany, NY) because of the danger of
the balls hitting people or windows! I don't think that the Oneidas had
opened up their "kolf course" at Turningstone Casino back in the 1640's.

....... I don't think they had eyepieces with 100 degree fields of view, or
CCD cameras back in the 1600's either.......

George N

Steve Paul wrote:

"George Normandin" wrote in message
...

Stellafane often features people who reproduce historical instruments.
For example, in the past there have been copies of William Hershel's
smaller scopes, complete with metal mirrors. Here is a working copy of
Isaac Newton's telescope, the first "Newtonian".


I didn't know they had bowling ball mounts back in Newton's day. Heck, I
didn't even know they had bowling. ;-)

Looks more like a cannon ball mount to me.

What's the screw on the mirror back -- the focuser?

Somewhere I remember reading Newton knew he wanted a parabolic mirror
but "never" figured out how to make one. Know if there's any truth to that?

Rick


--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

"Rick Johnson" wrote

I didn't know they had bowling ball mounts back in Newton's day. Heck, I
didn't even know they had bowling. ;-)
.........
What's the screw on the mirror back -- the focuser?

Yep!! It moves the primary end of the scope back and forth. I don't
think it would work on my Obsession 20!


Somewhere I remember reading Newton knew he wanted a parabolic mirror but
"never" figured out how to make one. Know if there's any truth to that?

I just read a book on the history of the telescope, and you are right. The
first parabolic mirrors were not made until the 1700's. Newton did at least
make a decent mirror that would form an image. No one else had done that at
the time. Being Newton (i.e., a general sob), he also made a point of the
fact that he made a working telescope while Cassegrain only had a design,
and Gregory's scope didn't work.

George N

George Normandin wrote:


What's the screw on the mirror back -- the focuser?


Yep!! It moves the primary end of the scope back and forth. I don't
think it would work on my Obsession 20!


George N

Interesting that the much maligned moving mirror of the SCT's focuser
goes back to Sir Isaac. I wonder how much image shift and mirror flop
he had?

Rick

In article , Rick Johnson
wrote:
What's the screw on the mirror back -- the focuser?

I was looking at the tube and it took a couple of seconds for the
"light bulb" moment : it's a tube-in-tube sliding joint. Easily "bodge"
up, or you could wait until the wood had seasoned and "turn" it
(technical sense of "to bodge"), and I'm sure that I've seen that
vintage of telescope with tubes made from multiple small planks, but
also with the tube-in-tube construction.
I recall an "introduction to engineering" programme from the Open
University once which (I think) described these as "one degree of
freedom" joints, with related categories for hinges (also one degree,
but different. I obviously don't remember the programme that well), pin
(one degree), ball+socket (two degrees), and more complex joints. I
think the topic was how to determine if your engineering design had
enough degrees of freedom to achieve the positioning you needed, and the
freedom/ restriction of orientation needed. JCB (American : back-hoe?)
arms were the case in point of enough freedom but with necessary
restrictions. Where was I ? ...
Ob-ASTRO : well, we know that we need 3 degrees of orientation
freedom, but can telescope suspensions be designed to allow one observer
to (for example) point a second observer's scope at "that". (Without
cheating and using electronics ; Greek rules of straight-edge, compass
and well-equipped machine-shop only).

Somewhere I remember reading Newton knew he wanted a parabolic mirror
but "never" figured out how to make one.

Wasn't it Foucault who developed the first practical test for
checking the figure of a mirror while figuring it. Late 1700s? [/self
checks Wikipedia ... nope, mid-18s there, seems a bit late]
But more to the point, in Newton's time the mirror material was
"speculum" metal, whose surface oxidised and dulled on a time scale of
weeks. So mirrors had to be re-figured very frequently. So the
complexities of parabolising a figure may have been more time-consuming
than these constraints made feasible.

--
Aidan Karley,
Aberdeen, Scotland
Written at Wed, 05 Sep 2007 23:49 +0100, but posted later.
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

George,

Thanks for sharing all of your great Stellafane pictures!

Clear Skies,
Scott Smith

"George Normandin" wrote in message
...
Stellafane often features people who reproduce historical instruments. For
example, in the past there have been copies of William Hershel's smaller
scopes, complete with metal mirrors. Here is a working copy of Isaac
Newton's telescope, the first "Newtonian".

There's a science history professor who shows up every year with his
latest reproduction of mostly medieval era pre-telescope instruments.

George N

"Aidan Karley" wrote
....

I was looking at the tube and it took a couple of seconds for the
"light bulb" moment : it's a tube-in-tube sliding joint. Easily "bodge"
up, or you could wait until the wood had seasoned and "turn" it
(technical sense of "to bodge"), and I'm sure that I've seen that
vintage of telescope with tubes made from multiple small planks.....

Aidan,

I have a 6-inch F/5 Newt with a tube I made from 1-inch by .25 inch
strips of oak. It's a beautiful little scope that fits (made on purpose) in
the back of my Chevy Corvette. Unfortunately, it's also pretty heavy. I have
an old 19th Century brass focuser (made in France) that I keep planing on
putting on this scope, but I never seem to get around to it.


Somewhere I remember reading Newton knew he wanted a parabolic mirror
but "never" figured out how to make one.

Wasn't it Foucault who developed the first practical test for
checking the figure of a mirror while figuring it. Late 1700s? [/self
checks Wikipedia ... nope, mid-18s there, seems a bit late]
But more to the point, in Newton's time the mirror material was
"speculum" metal, whose surface oxidised and dulled on a time scale of
weeks. So mirrors had to be re-figured very frequently. So the
complexities of parabolising a figure may have been more time-consuming
than these constraints made feasible.

"Stargazer. The life and times of the Telescope", by Dr. Fred Watson of
the AAO (relatively new book, which I highly recommend) says that in 1721
John Hadley (1682-1744) gave to the Royal Society a 6-inch Newtonian that
performed as well as any refractor of the time. He also states Hadley
"tested" his mirrors using a pin-hole and at least attempted to figure them
into a parabola using sub-diameter tools. However, Foucault's test shows how
good the mirror is, and he used glass, not metal. Hershel just figured his
mirrors until they would show the bricks on a distant wall. John Dobson used
a similar method with the first "big Dob's" - he would 'star test' the
mirror using a streetlight about a mile or so away, and keep figuring until
it showed a good image of the lamp.

George N