What's better SCT or Newt?

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

"szaki" wrote in message
...
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.

Hi,

Have you looked at the new RCX series to Meade... you know the design used
for the HST ;-)
Here is the pattern,
http://www.astrosurf.org/lombry/Phys...cx400-spot.gif
But for the same diameter or 30% smaller, nothing is worth a true apo.

Thierry
http://www.astrosurf.org/lombry


Finaly I all ways grab my 4" APO, all though it's dimer, but stars looks
like disks/spheres, not seagulls.
JS

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

That seems more indicative of poor seeing even mediocre optics should
show more.
Try collimating your SCTs and see if that helps.
All things being equal (optically) a newt will outperform an SCT on the
planets and doubles. If you have good seeing a 12.5"F6 newt will be
noticeably better than your 11 inch SCT. If you don't have good
seeing your 4" APO is as good as it will get.

Ian Anderson
www.customopticalsystems.com

wrote in message
oups.com...

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

That seems more indicative of poor seeing even mediocre optics should
show more.
Try collimating your SCTs and see if that helps.
All things being equal (optically) a newt will outperform an SCT on the
planets and doubles. If you have good seeing a 12.5"F6 newt will be
noticeably better than your 11 inch SCT.

Hi,
Why do you think that a newtonien is better than an SCT when we know that
the second is corrected for more optical aberrations than the first and its
frontal lens prevents also some turbulence inside the tube ?

Thierry
http://www.astrosurf.org/lombry

If you don't have good
seeing your 4" APO is as good as it will get.

Ian Anderson
www.customopticalsystems.com

Hi,
Why do you think that a newtonien is better than an SCT when we know that
the second is corrected for more optical aberrations than the first and its
frontal lens prevents also some turbulence inside the tube ?


As a rule of thumb two surfaces are better than four. There is no
chromatic aberration introduced by a corrector and there is less
diffraction due to CO. when it comes to hi-res on axis performance as
mentioned in the original post a newt will exceed the performance of an
SCT.

Parallax being a notable exception few Newtonians are made with solid
tubes thus eliminating tube currents. While in theory (and practice)
spherical surfaces are easier to make than aspheres it is much easier
to find a premium Newtonian than a premium SCT.

"szaki" wrote in message
...
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
If stars look like 'seagulls' in any scope, it implies the collimation is
not right. 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. Look at Thierry Legault's pages on
this http://legault.club.fr/index.html, and the star images in particular
on the 'collimation' section. I'd suggest that the 'seagulls', might well
be about the third level of misalignment...

Best Wishes

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.

_________________________________________________

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

Thierry wrote:
"szaki" wrote in message
...

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.


Hi,

Have you looked at the new RCX series to Meade... you know the design used
for the HST ;-)

No, but I've seen it magazines, not a cheap telescope. I just have
problems with 35% COB and heavy rings dancing around stars at high
megnifications. I know Mak-Newts have 19% or less COB.
That's why I can't never can sell my APO, even if it's dimmer at 4", but
stars looks like pearls in it.
JS

Here is the pattern,
http://www.astrosurf.org/lombry/Phys...cx400-spot.gif
But for the same diameter or 30% smaller, nothing is worth a true apo.

Thierry
http://www.astrosurf.org/lombry



Finaly I all ways grab my 4" APO, all though it's dimer, but stars looks
like disks/spheres, not seagulls.
JS

Parallax being a notable exception few Newtonians are made with solid
tubes thus eliminating tube currents. While in theory (and practice)
spherical surfaces are easier to make than aspheres it is much easier
to find a premium Newtonian than a premium SCT.

Huh? There are plenty of Newtonians made with solid tubes. Perhaps
you mean that there are few *premium* Newtonians made with solid tubes.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Agreed, Your SCT sounds like it has collimation issues. Defocus a
bright star considerably, and see if you see what appears to be a
centered shadow from the secondary.

If slightly decentered, it's time to Collimate!

Increase in magnification using higher, and higher mags, and defocus
less, and less until very good collimation is achieved.

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