Newb Q: M31 (Andromeda) In an 8'' Newt

It's not clear to me why it's easier to figure a complex curve on the
corrector plate (lens) rather than parabolizing a spherical mirror.
Please elaborate.

Well

two possible reasons.....

The first I am just making an educated guess backed with a little optics making
experience...

The corrector is pretty much flat...which is the same as saying it is a sphere
with a very large to nearly infinite radius....

The primary mirror is section of a sphere with a very short radius...

As a general rule, when polishing a sphere, the shorter the radius....the more
it "wants" to be/stay a TRUE sphere and the HARDER it is to deform the
polishing tool/strokes to create the non spherical surface (ie the
correction)...so you have two opposing "conditions" that both become more
extreme in their opposition the shorter the radius of the mirror becomes....And
SCT primaries are pretty darn short

To understand this, remember the ONLY surface shape where you can rub surfaces
together is a sphere (with a flat plate just being a sphere of infinite
radius).....the fact that pitch can give a little (and in just the right way)
is what even allows you to polish (barely) non spherical astronomical
mirrors...

Now consider the corrector plate, its basically flat, so you are only
"fighting" one problem, the correction, and not so much the desire to be a good
sphere....of course that also means its easier for the corrector to become all
kinds of other undesirable shapes....ie a LARGE flat surface is a royal pain
for an optican to make...try pricing an 8 inch F6 1/4 wave mirror and an 8 inch
1/4 wave flat and you'll get the idea pretty quick!

Now, up to this point I am just hand waving and havent explicity heard this
before so I dont really know if this is TRUE or just sounds good....


The second reason is the REAL one I am pretty sure....

They take a flat corrector plate.....put in on a mechanical widget....and pull
a vacuum on it.....or perhaps deform it in some other mechanical way....

The flat plate now becomes a shallow bowl shape....but the shape is NOT exactly
spherical....

So, they now grind/polish this warped plate until it becomes a good
sphere....and remember this is the easiest/cheapest/fastest most reliable
surface an optician can make....

When they have a good sphere they remove the plate from the widget....and it
springs back into its old mostly flat self....

Now remember it was NOT spherical (the one surface that is) when it was
deformed, but it was MADE into one in its deformed state....

This means that in some areas of the plate more material HAD to be removed and
in some areas LESS....

So, what you end up with is a plate that is NO LONGER uniformly thick...and
VIOLA, if you done the calcs and the procedure correctly you now have a plate
with the right amount of correction on it!

Now this isnt easy to engineer or get the procedure down pat, but once you do,
you can mass produce decent correctors at an affordable price...

BTW, if you want an amatuer "makeable" version of something like an SCT that
has a FAST (F4 to F6) with EXCELLENT imaging (better than just about anything
else) and only requires making spherical surfaces 2 lenses and 1
mirror....investigage a Laurie? Houghton design...

Very few have been made but I bet an 8 inch F6 one would give even a 7 inch apo
a good run for the money....

take care

Blll

starman wrote:


Being one of the closest galaxies, M-31 would be spectacular if we could
see all of it's size with the naked eye instead of just the central
core.


Reminds me of the time in Tahiti/Morea when I first saw the Magellanic
Clouds. Naked eye, I just thought they were true clouds. Binos
revealed them for what they were -- spectacular. Alas M31 is a tad
farther away (or maybe, considering is size, a good thing)!

Phil

starman wrote in message ...

BllFs6 wrote:

the best part is (three....) it is easier/cheaper to put the optical
correction on the plate because of how they do it rather than putting the
correction on the mirrors...

It's not clear to me why it's easier to figure a complex curve on the
corrector plate (lens) rather than parabolizing a spherical mirror.
Please elaborate.

It's because a little trick is used to manufacture the corrector.
First, a flat piece of optical glass is placed on the mouth of a
vacuum chamber. When the pressure is reduced, the glass will warp
slightly inward in a complex curve (more in the middle than the edges,
of course). The outer surface of the glass is then ground and
polished into a slightly concave spheroid, which is relatively easy.
When the glass is removed from the mouth of the vacuum chamber, it
warps back, and now it's a Schmidt corrector.


- Robert Cook

starman wrote:

Being one of the closest galaxies, M-31 would be spectacular if we could
see all of it's size with the naked eye instead of just the central
core.

We *can* see most of its size with the unaided eye and not just the core when
M31 is observed from a dark sky site. From my rural site, I can see between
two and three degrees of total length with averted vision, and from the site
of the Nebraska Star Party, it looks to be all of 3 degrees and maybe a bit
more. In my 10x50 binoculars, I have measured it out at just about 3 degrees
as well. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely wrote:

starman wrote:

Being one of the closest galaxies, M-31 would be spectacular if we could
see all of it's size with the naked eye instead of just the central
core.

We *can* see most of its size with the unaided eye and not just the core when
M31 is observed from a dark sky site. From my rural site, I can see between
two and three degrees of total length with averted vision, and from the site
of the Nebraska Star Party, it looks to be all of 3 degrees and maybe a bit
more. In my 10x50 binoculars, I have measured it out at just about 3 degrees
as well. Clear skies to you.
--
David W. Knisely

I've seen about two degrees with averted vision too but imagine seeing
all of it with the naked eye looking directly at it. It would look like
a saucer floating in the sky.


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Robert Cook wrote:

starman wrote in message ...

BllFs6 wrote:

the best part is (three....) it is easier/cheaper to put the optical
correction on the plate because of how they do it rather than putting the
correction on the mirrors...

It's not clear to me why it's easier to figure a complex curve on the
corrector plate (lens) rather than parabolizing a spherical mirror.
Please elaborate.

It's because a little trick is used to manufacture the corrector.
First, a flat piece of optical glass is placed on the mouth of a
vacuum chamber. When the pressure is reduced, the glass will warp
slightly inward in a complex curve (more in the middle than the edges,
of course). The outer surface of the glass is then ground and
polished into a slightly concave spheroid, which is relatively easy.
When the glass is removed from the mouth of the vacuum chamber, it
warps back, and now it's a Schmidt corrector.

- Robert Cook

I forgot about that 'trick'. I've read that a mirror can be parabolized
by heating it to a certain temperature, then polishing it to a sphere.
When the mirror returns to normal temperature it has a parabolic
surface.

Here's more on that subject:

http://www.digilife.be/club/johan.va.../thermally.htm


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starman wrote in message ...

Being one of the closest galaxies, M-31 would be spectacular if we could
see all of it's size with the naked eye instead of just the central
core.

Well, just think, we have something almost as good: the Milky Way!
The problem with the Milky Way is that we can't see its spiral
structure from our position. But if we could, then we would also see
its core, and that would be a significant source of light pollution.

You can see what a difference this makes (on a smaller scale) by
comparing a near edge-on galaxy such as M104 (where the core is
somewhat visible) to a truly edge-on galaxy such as NGC 891 or NGC
3628.

I would imagine that living in the Virgo Cluster would be a mixed
blessing. I once estimated that about 35 giant galaxies would be
visible to the naked eye there. I didn't consider the effects of 15
or so naked cores on the overall visibility, although i did mention
something about denizens of M87 having to wait until Virgo A sets
before they could see anything. (M87 has no dust lane to hide its
massively bright nucleus, and even at 55 million light years away, M87
is *easily* visible through 50mm binoculars from my suburban back
yard.)

The Magellanic Clouds--particularly the LMC--are a good compromise.
Now, all we need to do is pack up and move to Tahiti...


Clear skies!

--
------------------- Richard Callwood III --------------------
~ U.S. Virgin Islands ~ USDA zone 11 ~ 18.3N, 64.9W ~
~ eastern Massachusetts ~ USDA zone 6 (1992-95) ~
--------------- http://cac.uvi.edu/staff/rc3/ ---------------

starman wrote in message ...

Robert Cook wrote:

starman wrote in message ...

It's not clear to me why it's easier to figure a complex curve on the
corrector plate (lens) rather than parabolizing a spherical mirror.
Please elaborate.

It's because a little trick is used to manufacture the corrector.
[snip]

I forgot about that 'trick'. I've read that a mirror can be parabolized
by heating it to a certain temperature, then polishing it to a sphere.
When the mirror returns to normal temperature it has a parabolic
surface.

The reason for using a corrector lens is not quite as simple as it's
been made out to be--note that SCTs (and even Schmidt-Newtonians) are
typically more expensive than Newtonians of equivalent aperture. The
driving force behind the consumer SCT, more than anything else, is its
compact size, along with the location of its focal plane. It's not
just a matter of convenience, but of efficiency and cost-effectiveness
in performing certain tasks, such as astrophotography.

In order to make an exceptionally short reflector, you need an
extremely fast primary mirror--as fast as f/2 to really make it
worthwhile. However, such a fast parabolic mirror, as you would use
in a Classical Cassegrain (along with a hyperboloidal secondary),
would have an overwhelming amount of coma. One solution to this
problem is the Ritchey-Chrétien design (most large professional
telescopes are of this design), which has a hyperboloidal primary and
an even more hyperboloidal secondary. The problem now is that no
inexpensive way to create this type of figure has yet been found, so
the price for a telescope of this design that is available to amateurs
currently runs about $1500-$2500 per inch of aperture.

Commercial SCTs (and MCTs) are kind of like a poor man's
Ritchey-Chrétien. They can't match the optical performance of a good
RC telescope, but they can be just as compact, while providing
reasonable correction for the various aberrations. Regarding cost,
this is a more legitimate comparison than SCTs versus Newtonians,
which was sort of implied earlier.


- Robert Cook