What's better SCT or Newt?

"Shawn" sdotcurry@bresnananotherdotnet wrote in message
...
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.

I just plugged FTM into Google. Somehow I don't think the response was quite
right...

"Brian Tung" wrote in message
...
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 admit I was thinking of the more typical 8" to 10" solid tubes. A 10"
solid tube Dobsonian is pretty much the limit for passenger sedan
portability (a la my old XT10). My 12.5" requires the station wagon.

Stephen Paul wrote:
"Shawn" sdotcurry@bresnananotherdotnet wrote in message
...

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.


I just plugged FTM into Google. Somehow I don't think the response was quite
right...


LOL! One of my other recent posts refers to such things (trying to help
out a gender confused troll).
Anyhoo...:-)
FTM = For That Matter

Back on topic: The view of the core of M31, the Bubble Nebula, and the
Veil through the 14" were stunning. Detail I never knew was possible
through the eyepiece. I'm as ruined as a porno star. 8" will never be
enough :-)

Shawn

Stephen Paul wrote:


I admit I was thinking of the more typical 8" to 10" solid tubes. A 10"
solid tube Dobsonian is pretty much the limit for passenger sedan
portability (a la my old XT10). My 12.5" requires the station wagon.



This information is somewhat meaningless without the knowing the focal
ratio.

ie:

12.5 inch f/4 - 50 inches - probably a 60 inch tube
10.0 inch f/6 - 60 inches - probably a 66 inch tube
8.0 inch f/8 - 64 inches - probably a 70 inch tube

10.0 inch f/4 - 40 inches - probably a 46 inch tube

The 12.5 inch f/4 would be far easier to fit inside a car than the 8
inch f/8. Weight not withstanding.


Bill
--

William R. Mattil : http://www.celestial-images.com

Mark D wrote:

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

We can only assume that you have done this and therefore speak from
experience ? From what I have seen the time required for assembling a
large Dob can easily equal that of the GEM/C-14 so I would disagree
with your conclusion based upon actual experience.

Bill
--

William R. Mattil : http://www.celestial-images.com

Chris L Peterson wrote:


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.

But with such optical designs necessitating such devices such as the
ubiquitous Ross Corrector the Newtonian compares very favorable with an SCT.


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).

Agreed. Given the same attention to detail


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.

Absolutely true and this fact alone makes such comparisons meaningless.
Each optical design is a compromise and there is no clear cut winner.
Though many people will claim that there is, but then again it's human
nature to like what you have isn't it ? Personally I have no dog in this
hunt as I have at least one of each, including a Calssical Cassegrain
too. They all have their purpose. An SCT is a fine choice for
portability vs aperture, Fork Mount notwithstanding though. That
complicates the SCT just a bit


Bill

--

William R. Mattil : http://www.celestial-images.com

On Wed, 03 Aug 2005 21:24:57 GMT, "William R. Mattil"
wrote:

But with such optical designs necessitating such devices such as the
ubiquitous Ross Corrector the Newtonian compares very favorable with an SCT.

Sure. Just like a standard SCT is improved optically by using a field
flattener. Adding elements to improve correction (regardless of the
original design) generally results in better optics overall.


They all have their purpose. An SCT is a fine choice for
portability vs aperture, Fork Mount notwithstanding though.

Well, there is no need to have your SCT on a fork, although in general
I'll always choose a scope type that can be fork mounted, given the
advantages of that type of mount for my uses.

_________________________________________________

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

"William R. Mattil" wrote in message
...
The 12.5 inch f/4 would be far easier to fit inside a car than the 8 inch
f/8. Weight not withstanding.


Sure, but like I said, I was thinking more along the lines of the more
typical (commerically mass produced) 8" to 10" Dobs. That is the 10" F5 and
the 8" F6.

(By the way, I finally got around to setting up a G-11 with an 8" F4 for
imaging, using a DSLR. Good to hear from you again Bill. It's been a while.)

Stephen Paul wrote:



(By the way, I finally got around to setting up a G-11 with an 8" F4 for
imaging, using a DSLR. Good to hear from you again Bill. It's been a while.)



Stephen,

Congrats ! What DSLR ? Cannon Rebel ? Have some images to show ? I shot
some sample images with the Cannon using MaximDL and the special driver
and was favorably impressed. Great Lunar setup. Are you manually guiding ?


Regards

Bill

--

William R. Mattil : http://www.celestial-images.com

On Wed, 03 Aug 2005 22:07:26 GMT, Chris L Peterson wrote:


although in general
I'll always choose a scope type that can be fork mounted, given the
advantages of that type of mount for my uses.


Hmmm. . .seems to me that a dobsonian is a type of a fork mount and far
less expensive than a machined one with a wedge and skinny arms ;o)


--
Martin R. Howell
"Photographs From the Universe of Amateur Astronomy"
http://members.isp.com/universeofama...nomy%40isp.com