What's better SCT or Newt?

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
======================================

Hi Bill, While I don't personally own this Obsession Dob I speak of, a
member of my astro club does (But I think it's an 18", not a 20")

He's up, and running at a star party in 10 minutes with his Dob. Same
time for take down.
He's gone in a flash, while I'm still tearing down my gear.

But, I believe he's not assembling this Dob from scratch either. I
believe he leaves his truss tube assembly intact, and that's the
difference.

Evidently, he squeezes it all into a Ford Explorer.

I cannot in no way cart an AP1200GTO to a Star party, or wherever intact
in one piece, or even partially assembled. Not unless I'm willing to
risk damage to the mount in the process.

Even my smaller Byers EQ Mount, and Refractor takes longer that his Dob
in this instance. Mark

On Wed, 3 Aug 2005 16:12:17 -0700, "Martin R. Howell"
wrote:

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)

True enough. But a Dobsonian mount is not a very good choice for
imaging.

_________________________________________________

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

"William R. Mattil" wrote in message
...
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.)

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 ?


Modified Rebel.

I've posted some results on a.b.p.a over the past several months. I really
haven't had a lot of time at it due to sky conditions, so the results are
pretty rudimentary at this point. Thus far I've just done unguided images
for 2 mintues and used stacking.

If you can't find the images on abpa and you'd like me to send you a few,
email me privately at smarshallpaul gmail com.

Some of the images up there are using varying equipment, but the later ones
are the 8" F4 on G-11.

Stephen.

Design is implementation!





Chris L Peterson wrote:

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

On Fri, 05 Aug 2005 02:38:48 -0500, "Laughable!"
wrote:

Design is implementation!

Hardly! They are entirely different things. A spot diagram for a well
corrected SCT design does not reflect the actual optical performance of
every SCT coming off the assembly line. There is always a range of
variation between a design and the individual products. How broad that
variation is depends primarily on things other than the design itself.
This is well demonstrated by the steady improvement in quality of
commercial SCT optics from the mid 1980s to the late 1990s, in spite of
the fact that the designs were not changing.

_________________________________________________

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

execution, design, performance, assembly, use, implimentation, ..........
argue all you want. Its free.



Chris L Peterson wrote:

On Fri, 05 Aug 2005 02:38:48 -0500, "Laughable!"
wrote:

Design is implementation!

Hardly! They are entirely different things. A spot diagram for a well
corrected SCT design does not reflect the actual optical performance of
every SCT coming off the assembly line. There is always a range of
variation between a design and the individual products. How broad that
variation is depends primarily on things other than the design itself.
This is well demonstrated by the steady improvement in quality of
commercial SCT optics from the mid 1980s to the late 1990s, in spite of
the fact that the designs were not changing.

_________________________________________________

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