Celestron G-9¼


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Subject: Re: vote by pjsviolin
By: Anonymous (xxx.xxx.96.200)
In Reply to: pjsviolin (Original Vote)
Date: 09/02/2002 05:26:50 am PDT
I think that the author of this review should take a course in logic, optics, and writing.
First, ALL Schmidt Cassegrain Telescopes have an aspheric corrector plate....NOT a lens in the classic sense. It is NOT an optical window. It corrects for the spherical aberration of the primary mirror, which is spherical.
Second, the primary mirror of all SCTs are spherical, have an F ratio of approximately 2, and use a curved secondary (I can't remember if it's spherical as well), in order to reach the ratio of F10.
The primary CANNOT be a parabola. If it was the scope would be about two times longer than it is, with a secondary obstruction over 50% of the primary diameter.
NO modern cassegrains of ANY type or design uses a flat secondary....The optical tube would be longer than a newtonian with an F10 primary, and would be very short with and F2 primary, which by the way is the actual F ratio of an SCT primary mirror.
Yes, the C9.25 is a great scope...and deserves a 10...but it is not a different scope over any other SCTs. It's primary mirror is a bit longer in focal length than a standard SCT, about F2.4. Which lengthens the OPT tube, and allows a much higher quality primary mirror figure during manufacture.
One last note....occasionally the secondary mirror holder can loosen and shift during shipping. That is the cause of the corrector lens appearing to be "off to one side". This holder is easily repositioned, to achieve optimum optical mechanical alignment.
Otherwise, the first scope was probably a good one. I've owned 3 now, and they were all great performers.



>I just got my second 9.25 the first one had a few bugs. Optics were average and the alignment of mirror in tube in relation to the secondary was poor. never could get dead on collimation and the front glass holding the secondary was over to one side to compensate for poor mirror alignment. Planets looked great but beep sky and faint stars were a problem never could get critical focus so I exchanged for another one and got very lucky this one is much better .Owning two other scopes and haveing extensive knowledge in optics I would have to say the scope is at least 1/6 to 1/7 wave or better the first one was around 1/4 to 1/5 kinda of a lemon in my opinion but for the money they are great scopes. The mount is some what light and the tripod worse. So I completely took it down and got rid of the pancake syurp grease they use and adjusted all bearings and worm gears with very good teflon grease and filled the legs with sand also put rubber washers at the inside and outside of the tray to leg connections. The screw is long and the gap is to much also drill a hole in the middle of the triangle tray and put a bolt and nut through the center and tray holder which already has a nice 3/8 hole at the center WOW tightened things up nicely not perfect but major improvement .Now this Photon Busting 9.25 handles 500+ no problem and the focuser on mine is silky smoth with the teflon grease job can turn with one finger and not wiggle the scope. Hope these tips will help you photon busters out their. Feel free to email me for help and other tricks not listed. Thanks By the way I heard rumor that the 9.25 was a parabloid mirror and not a spherical as all other cass scopes are. I beleave this is true the front corrector plate is just glass not a lens the secondary is a magnifiying curved type not flat. I could be wrong but I don"t think so. UPDATE just spent 3hours collimating this scope and I feel this scope is closer to 1/7 wave instead of 1/6 wave as I posted after adjusting and tweeking at both sides of focus at 600x this thing is absolutly perfect on collimation and the image just out of focus on both sides is identical and the closeing of focus back to critical focus is perfect with no spikes and no oblong star at very slight and I mean slight tweeking of focus knob. I have looked through some fine scopes of all types and I have never seen a scope go back to perfect focus with no spikeing or slight flare at the end just before critical focus. This is absolutly a keeper whish I could find another one this good I would buy it just to have one so well corrected whith no zones and stars that look like diamonds on black velvet. Not bragging just makeing a point that this is one of those rare production units that is a gem on optics and no image shift at 600x. I wish Celestron could be this consistant with all there scopes but cost would be a major factor for quality control this tight.GREAT SCOPE I would like to compare to say a 10 or 11 inch and wouldn"t be suprised to win on a 10 and good run on a 11 if not better.


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Subject: Re: vote by pjsviolin
By: Anonymous (xxx.xxx.11.66)
In Reply to: Anonymous (xxx.xxx.96.200) (Original Message)
Date: 11/07/2002 09:23:35 am PDT
>I think that the author of this review should take a course in logic, optics, and writing.
>First, ALL Schmidt Cassegrain Telescopes have an aspheric corrector plate....NOT a lens in the classic sense. It is NOT an optical window. It corrects for the spherical aberration of the primary mirror, which is spherical.
>Second, the primary mirror of all SCTs are spherical, have an F ratio of approximately 2, and use a curved secondary (I can't remember if it's spherical as well), in order to reach the ratio of F10.
>The primary CANNOT be a parabola. If it was the scope would be about two times longer than it is, with a secondary obstruction over 50% of the primary diameter.
>NO modern cassegrains of ANY type or design uses a flat secondary....The optical tube would be longer than a newtonian with an F10 primary, and would be very short with and F2 primary, which by the way is the actual F ratio of an SCT primary mirror.
>Yes, the C9.25 is a great scope...and deserves a 10...but it is not a different scope over any other SCTs. It's primary mirror is a bit longer in focal length than a standard SCT, about F2.4. Which lengthens the OPT tube, and allows a much higher quality primary mirror figure during manufacture.
>One last note....occasionally the secondary mirror holder can loosen and shift during shipping. That is the cause of the corrector lens appearing to be "off to one side". This holder is easily repositioned, to achieve optimum optical mechanical alignment.
>Otherwise, the first scope was probably a good one. I've owned 3 now, and they were all great performers.


I agree to most of what you are saying also. The main difference with the 9.25 is the F2.4 primary ratio as opposed to the F1.9 of all the other SCTs. This allows the secondary to have a shallower 4x figure instead of the steeper 5x curve of the other SCTs. Shallower curves on both surfaces allows more precision in the figuring of the optics. I'm almost positive than any 9.25 OTA can be optimized to a high level with careful alignment of the optics.


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Subject: Re: vote by xxx.xxx.27.147
By: Anonymous (xxx.xxx.96.200)
In Reply to: Anonymous (xxx.xxx.27.147) (Original Vote)
Date: 11/29/2003 11:28:36 pm PDT
>I HAVE OTA ONLY WHICH IS MOUNTED ON VIXEN GP WITH OAK LEGS.
> I HAVE HAD 15 TELESCOPES SINCE 1974 AND THE VIEWS THROUGH THE 9.25 ON
> THE MOON AND PLANETS ARE THE BEST OF ANY SCOPE I HAVE OWNED.
> HOWEVER STAR TEST ON SAME SCOPE HAS BEEN THE WORST OF ANY OF MY SCOPES
> UNABLE TO GET A TIGHT AIRY DISK.I HAVE COLLOMATED SECONDARY MIRROR WHICH
> DOES MAKE BETTER STAR IMAGES BUT STILL NO AIRY DISK, FLARE FLARE EVERY WHERE
> SCOPE HAS BEEN USED SIX TIMES IN PAST TWO WEEKS.I HATE TO BE THE ONE TO
> COMPLAIN BECAUSE I REALLY DO WANT TO LIKE THIS SCOPE IT IS VERY EASY TO USE
> HAS OUTSTANDING CONTRAST,VIEWS OF M42 STRAIGHT THROUGH WITHOUT DIAGONAL
> AND WITH 35mm PANOPTIC IS FANTASTIC ONLY WISH STARS WOULD FOCUS DOWN TO
> PINPOINTS THAT WOULD MAKE THIS SCOPE THE BEST OF THE SCT. I PLAN ON DOING
> MORE OPTICAL TESTS IF OPTICS ARE FOUND TO BE DEFECTIVE I WILL SEND BACK TO
> BE FIXED OR EXCHANGED FOR EXCELLENT OTA AND BE A HAPPY MAN.

If you are getting the planetary views your getting, the mushy star images are due to atmospheric turbulence, and not do to poor optics.


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