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11" SCT



 
 
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  #1  
Old December 15th 04, 10:51 AM
Cie
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Default 11" SCT

Hi,

How is the quality of the latest generation of celestron 11"
SCTs? What is the most lemon one you've received.

Cie

  #2  
Old December 15th 04, 12:19 PM
Roger Hamlett
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"Cie" wrote in message
oups.com...
Hi,

How is the quality of the latest generation of celestron 11"
SCTs? What is the most lemon one you've received.

Cie

There was a page, showing a set of tests carried out on a C11 in Germany,
published here a few weeks ago. I'd say that this was a pretty 'fair'
example (it showed about 1/7th wave PV), with some slight surface
roughness from the machine figuring. Generally, I have not seen a 'bad'
C11, in any recent scope. My own one, is pretty comparable with the German
example, showing fairly smooth optics, with a 'best corrected' point for
SA, about 4.5" behind the rear port (slightly further than is often
quoted), and at this point, nice parallel Ronchi lines.
I'd say the quality was pretty good.

Best Wishes


  #3  
Old December 15th 04, 12:28 PM
Szaki
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C-11 was all ways more expensive high end telescope for more series
observers, even back in the older days, unlike the 8"-er. So, quality was
all ways closely watched the company, they did more hand figuring on them
too.
Very rare to find lemon amongst them, never heard of any.
I own an Ultima 11 OTA, 22lb, sharp scope.
Julius

"Cie" wrote in message
oups.com...
Hi,

How is the quality of the latest generation of celestron 11"
SCTs? What is the most lemon one you've received.

Cie



  #4  
Old December 15th 04, 02:42 PM
Dennis Woos
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quoted), and at this point, nice parallel Ronchi lines.
I'd say the quality was pretty good.


What do you think is the accuracy of the Ronchi test in this configuration?
Thanks.

Dennis


  #5  
Old December 15th 04, 05:28 PM
RichA
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On Wed, 15 Dec 2004 08:42:59 -0500, "Dennis Woos"
wrote:

quoted), and at this point, nice parallel Ronchi lines.
I'd say the quality was pretty good.


What do you think is the accuracy of the Ronchi test in this configuration?
Thanks.

Dennis


Provided spherical aberration is controlled, the thing to look for
in the Ronchi test is rough optics. They should be checked for with
any large SCT.
-Rich
  #6  
Old December 15th 04, 05:31 PM
Roger Hamlett
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"Dennis Woos" wrote in message
...
quoted), and at this point, nice parallel Ronchi lines.
I'd say the quality was pretty good.


What do you think is the accuracy of the Ronchi test in this

configuration?
Thanks.

Dennis

The potential accuracy will depend on the grid spacing, and the number of
lines being shown. My LX200 in the same configuration, shows lines that
bend quite visibly at the ends, (normally a sign of TDE).
I was using a 120LPI grating, and on a normal mirror in the same
configuration, would expect to easily reveal defects around 1/10wave, and
with care, perhaps push to 1/20th wave. However the light source was red,
which downgrades the accuracy compared to the green light normally used.
The Ronchi test is rather 'qualitative', rather than 'quantitative', but
by comparison with simulated results, and mirrors with known defects,
measured by Foucault testing, I feel the scope is in the same 'order' as
the example in the German tests, with defects in the order of 1/7th wave
PV in the green.
At the time, I was using the test for one thing only. To find the point
where the scope was best corrected for SA, and was pleased to find that at
the best point, the lines were much straighter than on the LX200, and were
noticeably smoother...

Best Wishes


  #7  
Old December 15th 04, 07:22 PM
matt
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Roger Hamlett wrote in message ...

"Cie" wrote in message
roups.com...
Hi,

How is the quality of the latest generation of celestron 11"
SCTs? What is the most lemon one you've received.

Cie

There was a page, showing a set of tests carried out on a C11 in Germany,
published here a few weeks ago. I'd say that this was a pretty 'fair'
example (it showed about 1/7th wave PV), with some slight surface
roughness from the machine figuring. Generally, I have not seen a 'bad'
C11, in any recent scope. My own one, is pretty comparable with the German
example, showing fairly smooth optics, with a 'best corrected' point for
SA, about 4.5" behind the rear port (slightly further than is often
quoted), and at this point, nice parallel Ronchi lines.
I'd say the quality was pretty good.

Best Wishes


that was on the Baader-Planetarium page .
Talking with some friends from Germany apparently that interferometric test
was conducted by a fellow named Wolfgang Rohr, who in some people's opinion
is less than credible due to a past test of a Chinese refractor, in which he
arrived at almost the same Strehl of 0.98 .
Later he was contradicted by other measurements and admitted (allegedly)
that he had tested the refractor only over the center 20mm of aperture ,
where it had indeed that high Strehl , then simply claimed the number for
the whole scope.
Please note that I do not have any first hand knowledge if this is the case
or not, and would be interested to learn more . Based on what I understood
from that test report, it just seems incomplete in the sense that the test
setup and method was not described, pictured or in any way specified
accurately .
Any test should be taken with a grain of salt if the test setup is not
published , especially the ones floating over the internet , which has no
peer review and anybody can mae any claims .
I would really be interested in any feedback from the German guys who read
this list which is really the case with that Celestron test. My own
experience with Celestron was along the general lines of that test , the
same surface roughness level and that weird radial pattern but really
nowhere as good a Strehl .

best regards,
matt tudor




  #8  
Old December 15th 04, 08:22 PM
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http://www.pliniocamaiti.it/test_main.htm#C11

How 'bout them radial spokes. They are almost certainly caused by the
radial slots cut into the glass master-block that deforms the corrector
plate during polishing, presumably as the block approaches or exceeds
its usable life (the block wears as it is cleaned. This link is
obviously a severe case, whereas they are just hinted at in the
interferogram from baader. The two C11s I have owned certainly did not
look this bad, more in line with the quite decent .029 rms of the
German result.

  #9  
Old December 15th 04, 08:48 PM
matt
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wrote in message
. com...
http://www.pliniocamaiti.it/test_main.htm#C11

How 'bout them radial spokes. They are almost certainly caused by the
radial slots cut into the glass master-block that deforms the corrector
plate during polishing, presumably as the block approaches or exceeds
its usable life (the block wears as it is cleaned. This link is
obviously a severe case, whereas they are just hinted at in the
interferogram from baader. The two C11s I have owned certainly did not
look this bad, more in line with the quite decent .029 rms of the
German result.


I am not cursive in German or Italian. Attempting to understand what both
websites are saying, it seems that the Baader test image with the 8 spokes
is a phase contrast test image , while the Italian one is just an out of
focus star test . I've seen similar images with spokes from Celestron sct
scopes and the spokes contrast is highly dependent on magnification, how far
out of focus and if the scope has reached thermal balance. Couldn't draw any
conclusion if it's good or bad based on how contrasty the spokes are .

best regards,
matt tudor


 




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