Long focal lenght newtonians as planetary scopes

anyone know where You can get the Focuser and diagonal mount set?

I know several of our club members have scopes with just *One* support
holding the secondary and they have very Sweet views through them. No
diffraction spikes of any kind split close doubles beautifully Awesome views
of nebula.
I have been toying with the idea of crafting such a mount for my 6" Newt as
the three heavy wire vanes defract something *Awful* when looking at
something very bright, like Mars or a high mag star. makes splitting doubles
a pain if they are to close. 8*\

--
Clear and Dark Skies for all!

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Tim Longwell

Black River Astronomical Society
http://junior.apk.net/~arstar50/BlackRiver.index.html
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"David" wrote in message
...


Bryan Greer wrote:

...
In this case, the single-arch design represents a 42% reduction in
overall
diffraction compared to the 4-vane, and 22% less diffraction than even
the
3-vane.

The material thickness can be kept the same as the straight-vane spiders
for
small and medium sized reflectors if you're allowed to get away from a
parallel-edged arch. A better idea is to think of the simple cantilever
anchored at one end, and with the load applied to the free end. The
mechanical engineers out there might remember that the material in a
cantilever is uniformly stressed if the anchored end is wider, and it
tapers
down to a point at the free end where the load is applied. This is
another
way of saying the material is utilized in the most efficient manner.
You
can think of the single arch design as two cantilevers (albeit bent
ones),
with each one anchored to the tube wall, and meeting at the hub of the
secondary.

Hello Bryan,


when will Protostar offer such vane ?

Thanks,

David

The
mechanical engineers out there might remember that the material in a
cantilever is uniformly stressed if the anchored end is wider, and it tapers
down to a point at the free end where the load is applied.

Lots of issues with cantilevers like vibration modes, stiffness, adjustability.
I know Del Johnson has a single strut mount but for rigidity and stiffness,
the standard vaned spider is hard to beat. Cantilevers not only depend on
being rigid themselves but the tube also must be stiff.

I still like the music wire spider using guitar tuners and 6 wires.....

jon

Jon Isaacs wrote:
Lots of issues with cantilevers like vibration modes, stiffness,
adjustability.
I know Del Johnson has a single strut mount but for rigidity and
stiffness,
the standard vaned spider is hard to beat. Cantilevers not only depend
on
being rigid themselves but the tube also must be stiff.

I still like the music wire spider using guitar tuners and 6 wires.....

jon


Hi Jon,

Absolutely, and there are two considerations with secondary support designs.
The structural and optical aspects both need to be looked at. As is typical
in life, they pull you in different directions. If you solely focus on
structural requirements, the straight 3-vane design is the most efficient in
terms of use of material. Another way of saying this is that you will get
the most rigidity with the fewest number of vanes in this configuration. As
we all know though, any straight edges will produce concentrated diffraction
effects (i.e., spikes). If you only consider the optical effects, the best
support is no support at all. Unfortunately, rigidity is rather poor.
Curved supports are an attempt to blend the opposing optical and mechanical
needs.

A reasonable approach is to set a threshold for rigidity where it is deemed
sufficient. For visual observing, two criteria need to be met. First, you
don't want the static weight of the secondary mirror to cause excessive
deflection in the support. This will cause the collimation to change as the
telescope is pointed to different altitudes. Secondly, the support should
not excessively resonate. This latter issue could be solved by simply
beefing up the supports, but that results in an increase in overall
diffraction. (That is the mistake many curved support designs make.) A
better way to address vibration is to incorporate dampening elements into
the design.

Finally, none of this will make much difference at all in the
lunar/planetary detail you'll see. Secondary supports need to be
outrageously thick before they impact the MTF in a significant way. This
all falls in the category of aesthetics. Some don't mind diffraction
spikes, while others are bothered by them.

Sincerely,
Bryan Greer

On 10/16/03 00:44 +0900, Bryan Greer wrote:

[ snip lots o' good stuff ]

Finally, none of this will make much difference at all in the
lunar/planetary detail you'll see. Secondary supports need to be
outrageously thick before they impact the MTF in a significant way. This
all falls in the category of aesthetics. Some don't mind diffraction
spikes, while others are bothered by them.

This has turned out to be a most informative thread. Thanks,
everybody, for the great input. I'll stick with the straight
3-vane design of the stock spider. It'll be easier to build and
collimate, and I doubt I'll be bothered by the diffraction
spikes. When I see them in photographs, I find them rather
aesthetically pleasing.

Cheers,

trane
--
//------------------------------------------------------------
// Trane Francks Tokyo, Japan
// Practice random kindness and senseless acts of beauty.
// http://mp3.com/trane_francks/

"Larry Brown" wrote in message
...
My 8-inch f/8 Newtonian blows away every other scope at star parties on
the planets. This includes 4 inch apos, and every configuration of
reflector you can think of, including a 25-inch obsession. SCT's don't
even come close. All of the Newtonians yield sharper and more contrasty
images than any of the SCT''s regardless of size. I've never compared it
to an 8-inch apo, but I have compared it to 8, 11, and 16-inch achromatic
refractors.
Just my experience.
Larry Brown
http://home.fuse.net/astronomy


I have an 8" f8 too which I bought second hand. Long after he had my money
and all the sales pitches were done, the previous owner told me "Don't let
aperture fever rule you. This scope gives better views of the planets, moon,
and the Messier object than I ever got from my StarMaster 11" or my
Obsession 15" scopes." I was surprised by that.

Ed