$3000 and which scope???

On Thu, 16 Oct 2003 00:54:11 -0400, "Stephen Paul" ...reflected:


"Alan W. Craft" wrote in message
.. .
On Wed, 15 Oct 2003 16:41:14 -0400, "Stephen Paul"
...reflected:

"Alan W. Craft" wrote in message
.. .
On 14 Oct 2003 06:50:58 -0700, (Rod Mollise)
...reflected:

Frankly, images in SCTs are _better_ with the
r/c in place than _without_...

So, one has to add an extra element to IMPROVE an image at an
f/10 ratio?

More's the pity.

No more a pity than needing a Paracorr to IMPROVE an image in an F5
parabola... you seem to be overlooking the bigger pitcure here.

At that point, I wasn't comparing specific telescopes. I was
referring
only to an f/10 ratio, hence an 8" f/10 Newtonian would not require any
sort of correcting element at all.

Of course, the tube would be monstrous in length.

I can't win for losing.


Well, you do tend to wander around the idea pool a bit. Not that that's a
bad thing, but it does make it harder to argue in favor of, or against your
position.

In general your question seemed to be whether the 8" F5 was a better general
purpose instrument than an 8" F10 SCT. But you clearly have an underlying
bias favoring the Newt (or more to the point, dissing the SCT), which is not
quite sound.

Quite, as I don't believe I've "dissed" it. Rather, I only want it acknowledged
that, with it being a member of the Cassegrain family, the Schmidt-Cassegrain is
primarily a higher-magnification instrument; consequently it is not as versatile
(wide-field to planetary) as a Newtonian of half its focal length, particularly when
given one maker's(Meade, I believe it was) failed attempts to satisfactorily produce
an f/6.3 instrument; an attempt obviously born of public demand for a Schmidt-
Cassegrain to exhibit said versatility.

Bottom line: It's the Dark Ages...Anacortes and Herb York are yet to be. Wells
has dropped you off in the middle and on top of a equally-dark plateau; but not
without a thing or two to occupy your time:

(1) 8" f/10 Schmidt-Cassegrain, straight from the factory and unaccessorised...

(1) 8" f/5 Newtonian, identically configured...

(1) Ubiquitous 32mm Erfle(or of some other moderate wide-field design, which
of course excludes a Nagler), and last but not least...

(1) 4mm orthoscopic

Now, which will be the more versatile of the two?

Further, have you ever seen Parks "H.I.T." series? At first glance, it's a
fast Newtonian, but then, with a timely exchange of secondaries, 'tis a slow
Cassegrain...

looks up and whistles I wonder from where THAT idea originated?

I think it's also clear that you went with the 8" F5 for good reasons, and
that you went with a well regarded brand, and there's absolutely no fault in
your logic concerning OTA size and maximum true field of view, and the rest
of your justifications. You did good.

I'm just not sure why you feel you need to diss...

That's the second time you've emotively-employed the term "diss..."

...the SCT to further justify
your choice. It's really not necessary. The 8" F5 Newt isn't better or
worse. It's simply the option you chose to excercise. I chose the 8" F10 SCT
as a general purpose instrument for both visual and photographic pursuits. I
then added a Dobsonian to the fray for quick looks from the driveway, quick
setup in the backyard, and easy transport to dark skies. I'd have been just
as happy with an altazimuth fork mount in the form of the NX11GPS as a quick
setup scope for visual use only, because it isn't really about field of
view. It's about easy setup, and easy use.

If you aren't going to be imaging, then an equatorial mount isn't a
necessity.

I covered that contigency as well, if and when the mood strikes, not to
mention the fact that we've yet to haggle as to which of the two designs is
best astrophotographically...

In fact, most of us find that the GEM, or EQ Fork is a
detrimental factor in setup that causes us to throw in the towel on nights
where a Dobsonian or some other quickly setup scope would have us going for
it, even under marginal conditions.

I think a Dobsonian-style Newtonian would've been the best for
me to begin with, but it's too late now, yet not enough of a regret to
cause me to reverse the decision regardless of loss.

At this point there's only two scopes _I_ would like to put on a GEM, a 10"
F6 Newtonian, or a C14. Problem is, the mount for either would need to be
robust, and would likely cost much more than the 10" F6 Newtonian OTA, and
close to the price of the C14.

We all succumb to some level of economic restraints. For me, the inexpensive
solution was a used Ultima 8-PEC, a used MX5-C, a Phillips ToUcam Pro, and a
10" F5 Orion XT10. Problem is, once I got the XT10, I found myself
correcting the images in ways I didn't have to with the SCT. That meant
expensive solutions, and the "cheap" factor went out the window. So, in that
regard I favor the SCT as the better general purpose telescope. It works
well with relatively inexpensive accessories.

I understand precisely.

A C8 on a CG-5, with a set of Ultima eyepieces from 35mm to 7.5 mm, and an
external battery to run the clock drive and heater strip on the corrector,
isn't going to run into a whole lot of money. More than an 8" F6 Dob, sure,
but not so much once you add a platform drive.

The true downside to the SCT on a GEM or EQ fork, is counted in setup and
breakdown efforts. But that same downside exists for an 8" F5 Newt on a GEM.

Oh, it exists right now with the FS-102 and the GP-DX, but once they're up
and tracking, it's done, then to enjoy several hours's observing...

....but afterwards, I'll have to take it all apart again, and lug it all back inside...

I'm tired already.

Peace,
Stephen

Peace to you, Stephen.

Alan

(Jon Isaacs) wrote in message ...
I assume that a field flattener for the Parks would increase the
focal ratio by a small amount, so the practical difference
between a cat running at f/6.3 and your Parks would be negligible
in terms of FOV. At that point, it becomes a matter of optics and
mounts, I suppose.

The difference between an 8 inch CAT running a F6.3 reducer and an 8 inch F5
Newtonian running a Paracorr is that the Newtonian can take full advantage of 2
inch eyepieces whereas the CAT is limited in this regard. Also, one can remove
the Paracorr and achieve a larger FOV.

Alan's question was regarding astrophotography. I wouldn't think that
you'd want to remove image correctors in such an instance because coma
will be most unattractive in a photograph. In such a case, then, I
think a cat with an f/6.3 flattener/reducer will compete very nicely
with a fast Newt with its own image correction.

Understand, I'm guessing here, so I'm open to correction in case I'm
mistaken.

--
trane

"Stephen Paul" wrote in message ...
"Trane Francks" wrote in message
...

If you have an exit
pupil that is larger than the pupil dilation of your eye, you'll
see the central obstruction in the scope.

Well, that's not exactly correct. You can exceed the eye's pupil up to the
point where the obstruction becomes visible. I believe there is some data on
how large a percentage the CO must be in comparison to the diameter of the
light beam which can pass through your eye, but I don't have that
information.

Fair enough.

I am happy to say that without knowing my maximum pupil dilation, and
although I'm in my mid-40's, I can still use a 7mm exit pupil in an F5

I don't know how large my exit pupil might be. I do know that the 35mm
Ultima in my 4 GT has too much exit pupil for solar observation. I
find myself using the 25mm SMA and 17mm Ploessl most of the time. It's
just more comfortable viewing.

in spite of my age. Point being that this too is a rule of thumb. Alan may
be able to get away with larger exit pupils as well, but maybe not. He is
going to have to experiment, which is a fun part of any hobby :-).

Arguably an expensive part of the hobby, too. ;^)

correction problems. After roughly 3.5mm exit pupil I need correction for
astigmatism, and that means a lens between my eye and the eyepiece. This is
really only a problem for brighter telescopic objects, say stars of NELM 5

Thankfully, all my other vision problems aside, astigmatism isn't one
about which I need to worry. When observing, I only use my glasses
when I use the finder. The rest of the time, it's au natural.

viewing M44 and M45 for example. Both of which require a very large field of
view, hence a larger exit pupil.

I'm looking forward to seeing both in the wee f/8 Dob once it's
finished. It will be very, very interesting to see how the 35mm Ultima
performs with that mirror. I'm not expecting great results, but I'll
be pleased if the image remains reasonably well focused out to the
edge.

--
trane

On Thu, 16 Oct 2003 17:04:39 +0900, Trane Francks ...reflected:
snip
Both parabolic and spherical mirrors are convex.

I thought parabolic mirrors were concave...

Alan

Ritesh posted:
In
fact, if using a widefield 2" eyepiece sans Paracorr and moving say
Vega to the edge of the field doesn't show obvious coma, the newbie
should immediately know that something is wrong. It's inherent in the
design and must show up if the scope is operating properly.


Many eyepiece introduce so much astigmatism or other aberrations near the edge
of the field that they completely swamp the coma of Newtonians of f/5 and
above (in fact, many people confuse this for coma, as with certain focus
positions, the horrid astigmatic image near the field edges can look a bit
like it). I can see a *little* coma at the edge of the 49.2 arc minute true
field produced using my 10 inch f/5.6 and my Meade 14mm Ultrawide eyepiece
(83.1 degree Apparent field of view), but it is very slight and *much* less of
a problem than the significant astigmatism I see in my 30mm Widescan III
eyepiece (85 degree AFOV) or even my 30mm Ultrascopic (52.3 degree AFOV).
Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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On 10/17/03 14:10 +0900, Alan W. Craft wrote:

On Thu, 16 Oct 2003 17:04:39 +0900, Trane Francks ...reflected:
snip
Both parabolic and spherical mirrors are convex.

I thought parabolic mirrors were concave...

Argh. That should have read: Both parabolic and spherical mirrors
are concave, as used in typical reflectors. *SIGH* There are such
things as convex parabolic mirrors, but I can't imagine offhand
how such a beast might be used in astronomy.

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

In article , Trane Francks
wrote:

On 10/17/03 14:10 +0900, Alan W. Craft wrote:

On Thu, 16 Oct 2003 17:04:39 +0900, Trane Francks
...reflected:
snip
Both parabolic and spherical mirrors are convex.

I thought parabolic mirrors were concave...

Argh. That should have read: Both parabolic and spherical mirrors
are concave, as used in typical reflectors. *SIGH* There are such
things as convex parabolic mirrors, but I can't imagine offhand
how such a beast might be used in astronomy.

Some extremely-curved parabolic and spherical mirrors have been used to
obtain 360-degree views of the sky in a single shot. Sort of like using
a fisheye lens without having to buy a fisheye lens.

They typically only show the sky up to about 30 degrees, unless the
camera is mounted above the mirror, in which case you get most of the
sky, except camera-shaped spot where the camera lies above the mirror.

IIRC, they've been used for things like surveying lightning, or aurorae.

On Fri, 17 Oct 2003 16:55:29 +0900, Trane Francks ...reflected:

On 10/17/03 14:10 +0900, Alan W. Craft wrote:

On Thu, 16 Oct 2003 17:04:39 +0900, Trane Francks ...reflected:
snip
Both parabolic and spherical mirrors are convex.

I thought parabolic mirrors were concave...

Argh. That should have read: Both parabolic and spherical mirrors
are concave, as used in typical reflectors. *SIGH* There are such
things as convex parabolic mirrors, but I can't imagine offhand
how such a beast might be used in astronomy.

trane

I believe hyperbolic secondaries used in classical
Cassegrains are convex; the only instance in astronomy.

I've never heard of a convex parabolic mirror, however.

I thought that small, cheap reflectors had convex
spherical mirrors, but I can't remember what the
primary was like in the only reflector I've owned:
a Sears 3-inch plastic-tubed "Newtonian" that
I got at the Crosstown Sears Surplus in Memphis
back in the dim '80's for $30.

I don't even remember seeing anything worthwhile
with it, but I do recall that the tube was of a darker
blue.

Alan

On 10/18/03 11:22 +0900, Alan W. Craft wrote:

I thought that small, cheap reflectors had convex
spherical mirrors, but I can't remember what the

No, those are also concave. At high focal ratios, a spherical
concave mirror and a parabolic concave mirror are very, very
close in shape. At f/8 or slower, a spherical mirror can provide
adequate performance. Not as crisp an image in the outer FOV as a
parabolic, mind, but acceptable.

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

Spherical aberration is not limited to the field's edge as it will soften
the entire image. Also, even at f/8 a spherical mirror will perform
poorly. At best one ends up with a clunky f/12 optical tube. I would steer
clear from spherical mirrored Newtonians altogether.

Del Johnson



"Trane Francks" wrote in message
...

No, those are also concave. At high focal ratios, a spherical
concave mirror and a parabolic concave mirror are very, very
close in shape. At f/8 or slower, a spherical mirror can provide
adequate performance. Not as crisp an image in the outer FOV as a
parabolic, mind, but acceptable.

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