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8" SCT v. 11" SCT



 
 
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  #21  
Old September 28th 05, 06:13 AM
Stephen Paul
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"John Deer" wrote in message
...
I found a huge improvement in quality between the C8 and C11.
Perhaps there is less variation in the C11.

If you want to see more deep sky objects a 10" or 12" dob makes more
sense.


To you maybe.


  #22  
Old September 28th 05, 07:57 AM
decaf
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Actually, I don't think it makes any difference at all to the brightness of
the moon. This is true in a telescope of any size. Surface brightness
cannot be increased by a passive optical system as it would be a violation
of thermodynamics.


The Moon, or ANY luminous object will look brighter in a larger
telescope than
a smaller one for a given magnification, or the only gain would be
resolvimg
power. Actual surface brightness does NOT need to be increased for an
object
to appear brighter; the distribution of availible light energy
increases the brilliance
of an object for a given magnification. The larger the aperture, the
more light energy
is gathered and availible to spread over a given expanse of image
plane.
This is not the same as claiming that any telescope will show the Moon
brighter
per unit area than it actually is, or looks to the naked eye ( which it
can't). Nor
did I imply that.

DC

  #23  
Old September 28th 05, 08:00 AM
decaf
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BTW, I gave the Newt
a tune-up today so I can get back to using it for DSO's. It is
currently my biggest aperture scope. I need to get a new focuser for
it.


Bill,
I guess I missesd it, but hat ever happened to that 18" er?

Dan

  #24  
Old September 28th 05, 10:48 AM
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If you want to see more deep sky objects a 10" or 12" dob makes more
sense.


To you maybe


Actually a 10 or 12 inch DOB will have a much shorter focal length than
the C11 and use 2 inch eyepieces effectively at that focal length thus
allowing for a significantly wider possible field of view. This will
allow one to see some larger DSOs that would otherwise not fit in the
FOV of a C11. In the other sense, the Newtonian could fit more DSOs in
the eyepiece at any given moment due to that larger FOV.

jon

  #26  
Old September 28th 05, 05:08 PM
Brian Tung
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matt wrote:
don't forget that commercial SCT's have a limited field of view due to
baffle tube vignetting . None exceeds 1.25 degrees , so there's no such
thing as a wide field SCT.


Haven't done the math, but the vignetting at about 2 degrees in a C5+
with a reducer, although visible, is not visually offputting. I wouldn't
image through it, though.

--
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
  #27  
Old September 28th 05, 05:34 PM
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Haven't done the math, but the vignetting at about 2 degrees in a C5+
with a reducer, although visible, is not visually offputting. I
wouldn't
image through it, though.
---------------

I have a C5. With a F6.3 reducer/corrector, the focal length ends up
at about 800mm and a field of view with a 1.25 inch eyepiece of around
2 degrees, though the hole in the back of the scope is quite a bit
smaller than 27mm so vignetting is going to be a factor.

jon

  #28  
Old September 28th 05, 05:52 PM
Brian Tung
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Jon Isaacs wrote:
I have a C5. With a F6.3 reducer/corrector, the focal length ends up
at about 800mm and a field of view with a 1.25 inch eyepiece of around
2 degrees, though the hole in the back of the scope is quite a bit
smaller than 27mm so vignetting is going to be a factor.


Yes, I know that vignetting is a factor. When I said I hadn't done the
math, I meant that I hadn't computed the extent of the vignetting at the
edge of the (maximum) field.

The question raised earlier was whether the baffle tube introduces extra
vignetting above and beyond what the rear port does. Of course, the C5+
rear port introduces vignetting that it doesn't in the larger SCTs.

--
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
  #30  
Old September 28th 05, 08:55 PM
Stephen Paul
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"Brian Tung" wrote in message
...

The question raised earlier was whether the baffle tube introduces extra
vignetting above and beyond what the rear port does. Of course, the C5+
rear port introduces vignetting that it doesn't in the larger SCTs.


Here are the measurements for the inside of all of the [Celestron Primary]
baffle tubes:

5" 25mm
8" 38mm
9.25" 48mm
11" 54mm
14" 54mm

(Reposted from June 2003)


 




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