The Truth of Celestron C9.25 SCT OTA
Paul K
Hi. Ed Ting wrote about the Celestron C9.25 SCT OTA on his c9.25 review
as follows:
"... Rumor on the street has it that these scopes receive a lot more
attention at the factory than Celestron's other SCTs. The primary is
said to be a paraboloid (as opposed to the aspherical figures on most
SCTs) with a slightly longer f/ ratio than is usual for this type of
scope (this places less optical strain on the secondary, which does not
have to magnify the image as much)..."
By the way, current C9.25 SCT model doesn't have a micrometer. Actually
I think the C9.25 is one of the mass products like other SCTs. Is there
anyone who knows the truth of the C9.25 SCT OTA? Is the C9.25 really
different?
Thanks.
Clear Skies,
Paul
Sent via Deja.com http://www.deja.com/
Before you buy.
Mike McIsaac
I won't address the rumors. I will tell you what I have seen with my
own eyes.
In the past I have owned a C-8 SCT, an Intes-Micro M703 Deluxe MCT, a C-
5 SCT, and a C-9.25 SCT.
The C-9.25 has the finest optics of any of them. Images of Jupiter and
Saturn are refractor-like in their sharpness and contrast. Images of
deep space objects are spectacular. Star images are pinpoints with the
first diffraction ring visible. Defocused star images are perfectly
concentric on both sides of focus (when properly collimated of course).
I don't yet possess the skill to give a SWAG (scientific wild-ass
guess) about what the measured quality of the optics are based on the
star test but from what I see at the eyepiece, they're plenty good
enough for me.
Not bad for an OTA that I purchased for $995 new...
Clear skies!
Mike McIsaac
60*N 150*W
In article <907abe$dh5$1...@nnrp1.deja.com>,
--
*********************************************
186,000 miles per second: its not just a good
idea, its the law!
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Rod Mollise
>
> Hi. Ed Ting wrote about the Celestron C9.25 SCT OTA on his c9.25
review
> as follows:
>
> "... Rumor on the street has it that these scopes receive a lot more
> attention at the factory than Celestron's other SCTs. The primary is
> said to be a paraboloid (as opposed to the aspherical figures on most
> SCTs) with a slightly longer f/ ratio than is usual for this type of
> scope (this places less optical strain on the secondary, which does
not
> have to magnify the image as much)..."
Hi:
The 9.25 is different alright, but not, I believe, to the extent Ed
heard. AFAIK, this scope, like _every_ other Meade and Celestron, has a
spherical primary mirror. It really wouldn't make a lick of sense to do
otherwise. Also, despite other rumors, there's no evidence that it is
lovingly assembled by Black Forest elves under Celestron contract. :-)
But it _is_ a very nice alternative to the C8 for three reasons:
The longer "native" focal length of the 9.25 primary--f/3 vice f/2 for
the C8--evidently makes things easier for Celestron to get right.
The extra ooommmph that over an inch more of aperture provides is very
welcome for deep sky observers.
The scope sells for an amazingly low price.
Drawbacks? Other than the somewhat lackadaisacal approach to QA
embraced by Celestron (and Meade, too), the scope has a longer tube
assembly making it a bid more of a chore to transport and store than a
lovable C8. AND...the CG5 mount Celestron puts this scope on, while not
a joke, and certainly capable of being improved with a little
tinkering, is not anywhere close to being as well matched for the scope
as was the GM8 variant (Losmandy) that Celestron initially placed this
OTA on. While the 9.25 is a very nice instrument, I don't think it
offers _enough_ of a performance increase to call for ditching
C8s/2080s to get one, though.
Peace,
Rod Mollise
Mobile Astronomical Society
http://members.aol.com/RMOLLISE/index7.html
The Home of _From City Lights to Deep Space_:
Rod's Guidebook for the _Urban_ Deep Sky NUT!!
*********************************************************
Clive Gibbons
>
>Hi. Ed Ting wrote about the Celestron C9.25 SCT OTA on his c9.25 review
>as follows:
>
>"... Rumor on the street has it that these scopes receive a lot more
>attention at the factory than Celestron's other SCTs. The primary is
>said to be a paraboloid (as opposed to the aspherical figures on most
>SCTs) with a slightly longer f/ ratio than is usual for this type of
>scope (this places less optical strain on the secondary, which does not
>have to magnify the image as much)..."
>
>By the way, current C9.25 SCT model doesn't have a micrometer. Actually
>I think the C9.25 is one of the mass products like other SCTs. Is there
>anyone who knows the truth of the C9.25 SCT OTA? Is the C9.25 really
>different?
Contrary to the "street talk" <g>, the C9.25 doesn't have a parabolic
primary mirror. It's a typical Schmidt-Cassegrain, which uses a spherical
primary mirror. However, instead of the primary being f/2, it's closer to
f/2.5.
This slightly "slower" mirror makes it easier to produce a good optical
figure for the system.
Cheers,
--
Clive Gibbons
Technician, McMaster University,
School of Geography and Geology.
Frank Bailey
Is the C9.25 really
> different?
>
The ones I've seen have been remarkable. I think the design allows them
to get it dead-on more easily. It seems to be an excellent match for
their production methods, which produce more mediocre 8" scopes than
mediocre 9.25" scopes.
Frank Bailey
Dallas Texas
Brian Tung
> Contrary to the "street talk" <g>, the C9.25 doesn't have a parabolic
> primary mirror. It's a typical Schmidt-Cassegrain, which uses a spherical
> primary mirror. However, instead of the primary being f/2, it's closer to
> f/2.5.
> This slightly "slower" mirror makes it easier to produce a good optical
> figure for the system.
I would venture to guess that the f/2.5 mirror is less susceptible to
TDE than the f/2 mirror.
Is the SCT design in this case also all-spherical?
Brian Tung <br...@isi.edu>
Astronomy Corner at http://astro.isi.edu/
C5+ Home Page at http://astro.isi.edu/c5plus/
atasselli
Rod Mollise schrieb in Nachricht <908c5p$6ek$1...@nnrp1.deja.com>...
>>
>> Hi. Ed Ting wrote about the Celestron C9.25 SCT OTA on his c9.25
>review
>> as follows:
>>
>> "... Rumor on the street has it that these scopes receive a lot more
>> attention at the factory than Celestron's other SCTs. The primary is
>> said to be a paraboloid (as opposed to the aspherical figures on most
>> SCTs) with a slightly longer f/ ratio than is usual for this type of
>> scope (this places less optical strain on the secondary, which does
>not
>> have to magnify the image as much)..."
>
>
>
>The 9.25 is different alright, but not, I believe, to the extent Ed
>heard. AFAIK, this scope, like _every_ other Meade and Celestron, has a
>spherical primary mirror. It really wouldn't make a lick of sense to do
>otherwise. Also, despite other rumors, there's no evidence that it is
>lovingly assembled by Black Forest elves under Celestron contract. :-)
>
>But it _is_ a very nice alternative to the C8 for three reasons:
>
>The longer "native" focal length of the 9.25 primary--f/3 vice f/2 for
>the C8--evidently makes things easier for Celestron to get right.
Rod,
I believe (correct me if I'm wrong) that the 9.25 has an f/2.5 primary while
the C8 has a f/1.95 primary. That should give a secondary magnification of 4
against the usual 5 in the other SCTs and somewhat less fifth order
spherical aberration to correct for.
Clear Skies
Andrea
Clive Gibbons
>
>Is the SCT design in this case also all-spherical?
Hi Brian.
Most likely, though that's a guess on my part.
atasselli
Brian Tung schrieb in Nachricht <908pro$nfc$1...@zot.isi.edu>...
>Clive Gibbons wrote:
>> Contrary to the "street talk" <g>, the C9.25 doesn't have a parabolic
>> primary mirror. It's a typical Schmidt-Cassegrain, which uses a spherical
>> primary mirror. However, instead of the primary being f/2, it's closer to
>> f/2.5.
>> This slightly "slower" mirror makes it easier to produce a good optical
>> figure for the system.
>
>I would venture to guess that the f/2.5 mirror is less susceptible to
>TDE than the f/2 mirror.
>
>
Quite unlikely. According to some sources the secondary is a mild aspheric
with zonal retouching. An all spherical (that is, concentric) design would
have a longer OTA and a huge field curvature but no coma at all. Also the
corrector plate should be made with a stronger aspheric profile.
Clear Skies
Andrea
RAnder3127
but who knows? What I do know is that
of the 30-odd SCT's i've owned, only
two others have been as good as
the one 9.25" I now own.
-Rich
RAnder3127
was a joke. Temp drops "X" degrees,
scope shrinks "X" mm's, micrometer reading not duplicatable. Mirror slop
(minor, but there) also makes the micrometer next to useless for anything
worthwhile.
--Rich