What's better SCT or Newt?

"Mark D" wrote in message
...
SCT's do have one certain advantage versus Newts, they are more
portable, due to the folded design. Mark

The idea of an SCT being more portable that a Newtonian, must have been with
respect to both being on a GEM.

Given the same apertures, in practice I don't see this being all that
significant, or even true when compared to a Dob. A tripod is often nearly
as long a Dobsonian OTA, and a Dobsonian base is often no larger than an SCT
in a hard carry case.

The Dobsonian base was the great equalizer of Newtonian portability.

Stephen Paul wrote:
The Dobsonian base was the great equalizer of Newtonian portability.

I'd throw in truss tubes and equatorial platforms in there with the
Dobsonian base. A solid-tube 14-inch Newtonian would be a beast, even
on a Dobsonian base. And it still can't track or do long-exposure work
without the equatorial platform.

--
Brian Tung
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Although I have long since sold off all of the SCT that I have owned, I
have gotten the best planetary views ever through an 8" SCT. Jupiter
looked like the Voyager pictures on nights of exceptional viewing.

However, for deep sky, there is nothing like a Newt. Even a 4" scope
like an Astroscan will show the green color of bright nebulae.

All-around scope? Toss-up. When you factor in price, the Newtonians
come up as a better deal.

That is why my primary scope is a 10" Newt on a german equatorial.

"Gil" wrote in
oups.com:

Although I have long since sold off all of the SCT that I have owned, I
have gotten the best planetary views ever through an 8" SCT. Jupiter
looked like the Voyager pictures on nights of exceptional viewing.

While I don't have anything against SCTs, it is hard to take that statement
at face value

http://www.solarviews.com/raw/jup/redspot.jpg


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Brian Tung wrote:
Stephen Paul wrote:

The Dobsonian base was the great equalizer of Newtonian portability.


I'd throw in truss tubes and equatorial platforms in there with the
Dobsonian base. A solid-tube 14-inch Newtonian would be a beast, even
on a Dobsonian base. And it still can't track or do long-exposure work
without the equatorial platform.


I saw this very size solid-tube Dob get shoved into the back of a Scion
xB last Friday. FTM it had some form of alt/az drive, digital setting
circles and an integrating video camera. Sweet setup. Glad I didn't
have to pay for it though.



Shawn

The idea of an SCT being more portable that a Newtonian, must have been
with respect to both being on a GEM.
Given the same apertures, in practice I don't see this being all that
significant, or even true when compared to a Dob. A tripod is often
nearly as long a Dobsonian OTA, and a Dobsonian base is often no larger
than an SCT in a hard carry case.
The Dobsonian base was the great equalizer of Newtonian portability.
======================================

Agreed Steven, I should've perhaps elaborated in this regard as you say.

There's no doubt that a 20" Obsession Dob can be up and running faster
than say a C-14 on a AP1200GTO Mount.

Still, talking about OTA's alone, the SCT does have some size advantages
particularly as far as transporting goes. Mark

While I don't have anything against SCTs, it is hard to take that
statement at face value
======================================
Hi Pierre, I can understand where you are coming from, and perhaps the
poster you responded to, was in fact telling the truth in that this SCT
he viewed through showed the best images he has ever "personally" seen.

If someone were to go through the trouble of building a top quality no
holds barred SCT, lets say with a 1/10 wave or better final wavefront,
Quartz Mirrors, the best coatings, the best Mechanics, specifically
addressing a top quality no shift Focuser, a Rear Cell cooling system
which aided cooldown, I see no reason why an SCT such as this would not
be a desireable scope to some. (If the price was right)

Doubtful though that many would pay the asking price of such a scope,
when folks like Meade, and Celestron are on the scene.

Perhaps most would say "Why should I buy a 8" Schmidt OTA for $5,000,
when I can buy a pretty good one for a 1/5 of that cost"?

Also too, SCT's do have shortcomings versus other compound designs, so
amateurs would probably say why make a high quality SCT, make a good Mak
instead, or an RC?

There are some better variations of the Schmidt design, but I've yet to
see really anyone implement them commercially, unless you wish to say
the Meade RCX-400 as being one, which I guess you could say it is. Mark

Chris L Peterson wrote:

On Tue, 02 Aug 2005 01:11:24 -0700, szaki wrote:

Newt has a smaller COB 20-25%, SCT 35% or more, would it show visually?
I own a C-11 and a C-8 right now, but splitting tight doubles is very
ugly with a SCT. So far I haven't got any great views on planets any of
my SCT's, 2 bands in Jpiter.
Finaly I all ways grab my 4" APO, all though it's dimer, but stars looks
like disks/spheres, not seagulls.
JS

You obviously have a problems with the SCTs. They should show perfectly
pinpoint stars centrally. A CO may or may not show its effects visually.
I've never really convinced myself that I could see the difference
between planetary images in obstructed and unobstructed scopes. As far
as tight doubles go, obstructed scopes frequently outperform
unobstructed ones because of the way the Airy disks superimpose. People
who spend a lot of time observing doubles often modify their apertures
with rotatable linear obstructions for that reason.

Theoretically, an SCT is generally better than a Newt because it is more
corrected. In practice, so much depends on the individual quality of the
mirror and other components that it is difficult to generalize.


No its not. Count the optical surfaces. The probability of error at any (or
through
any) surface multiplied by the number of surfaces in each system, gives a
rough
idea of the probability of success with one system vs the other. Not to
mention
the fact they are basically different systems to begin with, unless of course
you
scramble eggs in a hub cap and hit golf balls with the frying pan .... which
many people do in an over populated highly stressed out world!
Jerry




_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

...Getting really good collimation, is the key to the SCT. Many
people stop their collimation at the point where it is only really 'barely
acceptable', rather than really good...

I heartily second that observation. From personal experience, I
remember when I got my 10" LX200 SCT I was disappointed with the
performance out of the box (compared to a little Maskutov I'd been
using prior to that). A browse of the instructions revealed it might be
collimation so I adjusted it. Aha, an acceptable image (better than the
Maskutov as you would expect for a scope of 3-4x the aperture)!
Sometime later, when using higher mag, I noticed the concentric rings
of defocussed stars were not quite concentric, so I adjusted
collimation some more. Wow - even better images (airy disks clearly
visible on all stars now). Finally - after reading Thierry Legaults
info on critical collimation, and getting Suters book on star testing.
I did a TINY adjustment on a night of exceptionally steady seeing (up
to 846x magnification using a Televue zoom EP at 3mm).That noticeably
improved things yet again; but MOST noticeably on planetary detail for
that last tweak.

Now I'm utterly content with the performance of my SCT - to the extent
that I believe, within the variation of mass produced instruments, I've
got one of the "honey's". The point though, even though I KNEW
collimation was important, it was several months before I learned just
HOW important those last miniscule tweaks really are. All part of the
learning experience.

Cheers
Beats

On Wed, 03 Aug 2005 00:25:07 -0500, jerry warner
wrote:


Theoretically, an SCT is generally better than a Newt because it is more
corrected. In practice, so much depends on the individual quality of the
mirror and other components that it is difficult to generalize.

No its not. Count the optical surfaces...

I'm simply discussing the design, not the implementation. If you look at
the spot diagrams of a well corrected SCT, it performs better (on the
whole) optically than a Newt. To get good optical performance from a
Newt any significant distance from the optical axis requires additional
correction.

However, even in practice, the additional surfaces are generally
insignificant. Optical manufacturing techniques are quite good these
days. Plenty of high end refractors have even more surfaces, and yield
very high quality images. The value of additional surfaces in improving
correction exceeds the harm (from light loss or scatter).

But I do agree with you that you can't just do an A/B compare of two
different optical designs. There will always be cases where one or the
other is a better choice, regardless of arbitrary indicators of
performance.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com