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Subject: on your C9.25? By: Anonymous (xxx.xxx.14.4) Date: 10/01/2004 02:27:14 pm PST When you're in focus dont you see the shadow of the central obstruction plainly?? Once you use an eyepiece greater then 32mm the central obstruction is actually seen in the center of the focused image. How is it that you're not seeing that in the center of your 50mm eyepiece? Subject: Re: on your C9.25? By: Anonymous (xxx.xxx.11.240) In Reply to: Anonymous (xxx.xxx.14.4) (Original Message) Date: 11/01/2004 01:34:04 pm PST >When you're in focus dont you see the shadow of the central obstruction plainly?? Once you use an eyepiece greater then 32mm the central obstruction is actually seen in the center of the focused image. > >How is it that you're not seeing that in the center of your 50mm eyepiece? ANSWER- 32 is not a magic number or limit. Multiply the focal ratio of the telescope by 7 and that gives you the lowest power (highest mm) focal length eyepiece that is useful. For example 7 x 4.5 = 31.75 (32)....7 x 5 = 35.... By this you can see that the eyepiece is most useful on telescopes of f/6 or slower (i.e. 7 x 6 = 42). The first reviewer had a f/4.9 scope..... 7 x 4.9 = 34.3....so any focal length eyepiece greater than 34.3 would show the central obstruction in daylight. The 42 would create an exit pupil of 8.6 mm, which is greater than anyone's eye can open. This is the wrong eyepiece for that scope. On an f/10 instrument such as the 9.25 SCT, the longest useful focal length eyepiece would be 70mm......( i.e. 7x10) so the 42 is well within bounds of usefulness with that scope, and you wouldn't see the obstruction. Subject: Re: on your C9.25? By: Mozhoven In Reply to: Anonymous (xxx.xxx.11.240) (Original Message) Date: 11/10/2004 10:43:50 pm PST >>When you're in focus dont you see the shadow of the central obstruction plainly?? Once you use an eyepiece greater then 32mm the central obstruction is actually seen in the center of the focused image. >> >>How is it that you're not seeing that in the center of your 50mm eyepiece? > >ANSWER- 32 is not a magic number or limit. >Multiply the focal ratio of the telescope by 7 and that gives you the lowest power (highest mm) focal length eyepiece that is useful. For example 7 x 4.5 = 31.75 (32)....7 x 5 = 35.... By this you can see that the eyepiece is most useful on telescopes of f/6 or slower (i.e. 7 x 6 = 42). > >The first reviewer had a f/4.9 scope..... >7 x 4.9 = 34.3....so any focal length eyepiece greater than 34.3 would show the central obstruction in daylight. The 42 would create an exit pupil of 8.6 mm, which is greater than anyone's eye can open. This is the wrong eyepiece for that scope. > >On an f/10 instrument such as the 9.25 SCT, the longest useful focal length eyepiece would be 70mm......( i.e. 7x10) so the 42 is well within bounds of usefulness with that scope, and you wouldn't see the obstruction. ~~~~~~~~~~~~~~~~~~~~~~ Hello, this is the first reviewer here. True, it is too much for my scope by itself. The secondary is plainly visbile, however for general "finding" it is great, especially for the price. I have found after using it for some time that I prefer it Barlowed at 22mm, it outperforms other eps of the same focal length in terms of confort and field curvature/flatness. It can get bulky and heavy barlowed, but hey - you can't beat the 2" feel. Thanks for the replys - I'm keeping the 42mm for the time being, it's too good of a deal. ~~~~~~~~~~~~~~~~~~~~~~ Page 1 of 1 | ||||||||
