Celestron C8N-GT


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Subject: Celestron C8N tube assembly
By: Anonymous (xxx.xxx.241.1)
Date: 10/16/2003 03:10:47 am PDT
I already had gotten a mount, a discontinued but still very serviceable Orion Skyview Pro mount for a 6-inch Maksutov, MK-65 tube assembly for a trip this summer. After the trip, I had the chance to aquire the Celestron 8 Newtonian tube assembly which I was able to mount comfortably on this Orion mount. I am actually surprised by how good the balance and stability has been in using this mount with the 8 inch.

I am an experienced amateur who has owned a Celestron 8 Schmidt-Cassegrain, a Star Liner 10 inch f/4 Newtonian, but recently became more of a binocular fan. I now own a couple of giant binoculars, an ITE 100mm Binocular Telescope and a Vixen 127mm Binocular Telescope. A venerable Intes 6 inch MK-65 I purchased ten years ago has been my only telescope lately until the purchase of this tube assembly. It has only been this summer that I became involved in reviving my telescopic interest with the Mars opposition.

I have mainly been a deep sky observer and have gravitated toward the binocular telescopes because of the superior experience of using two eyes under dark skies. Nothing even approches the experience of a good set of powerful 100mm and larger binoculars for this task. No telescope, whether Maksutov, Newtonian or Refractor can equal this experience. However, having said this, none of my giant binoculars were satisfactory for viewing the planets! The image scale was far too small to make out any details on Mars even in opposition.

Here my six inch came in handy. The MK-65 has been superb. Textbook diffraction rings in and out of focus, and very detailed contrasty images have been absolutely wonderful for viewing Mars. However, when I tried to use this scope for deep-sky objects the results have been, as was the case when I bought it, a disappointment. The images of even seeming brighter Messier objects do not stand out. Therefore, when the opportunity to aquire this tube assembly came I took it.

First, I had to get the instrument in collimation. This was not easy! The tube assembly does not come with a traditional mirror cell, but rather the main mirror is mounted on a cell that is directly attached to the tube. In most Newtonians you will have a mirror cell that is an inch to half and inch smaller than the diameter of the tube. This makes it easy to collimate the mirror just by adjusting the screws (that have springs attached to the screws that by tension hold the mirror in place). This scope does not have this at all. There are no springs. The lack of springs makes the collimation a bit awkward because the mirror is directly mounted on a rudimentary cell that is really a rim attached to the tube!

Second, the secondary cell is glued to the spider holder so that no adjustment of screws can take place here. The screws of the spider can be adjusted but only for adjusting the location of the secondary up and down or sideways to allign the secondary in the tube in relation to the main mirror. Same thing with the middle screw which holds the holder to the spider. It can only be adjusted so that the secondary mirror can be rotated.

The only way I could collimate this was to actually bend the spider (slightly) so the tilt of the mirror was perpendicular to the main mirror. For whatever reason, when I got this the tilt was slightly off so that the secondary reflection on the main mirror showed one side larger than the other. I used a cheshire collimation eyepeice to get this just right, adjusting the spider so the secondary was centered, then adjusting the secondary itself so that the reflection the main mirror and secondary were as centered as much as possible. This collimation process took about a week due to time constraints, and the need for outdoor testing under the stars.

Even when it was slightly out of collimation I could focus down on stars pretty well, but the focus point was easily lost. On Mars the resulting loss of sharpnes was very apparent, so I had to continue to tweak it until I felt I had acheived a perfect alignment as is possible. When I could get a sharp image of Mars I knew I had acheived good collimation. Good collimation for Newtonians is critical.

Once collimated I have been pleased by the much brighter images of the stars I get vis-vis the Maksutov. What has been surprising is that under higher magnification (approaching 250x to 300x) the image has been comparable in quality to the Mak when looking at Mars. The image has actually been sharper than in the Mak at 200x , even though the contrast is slightly better in the Mak. The brightness of the image though almost compensates for the slight loss of contrast, as Syrtis Major and other dark markings on Mars can be easily discerned.

I have yet to really test this under dark skies at deep sky objects, but already the test on Mars shows that the eight inch holds its own against the MK 65. I am sure that the 8 inch should easily rival the Mak for deep sky though it will not be able to approach the aesthetic experience of the binocular telescopes. I am rediscovering that for larger image scale nothing can beat a telescope.


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Subject: Re: Celestron C8N tube assembly
By: Anonymous (xxx.xxx.139.49)
In Reply to: Anonymous (xxx.xxx.241.1) (Original Message)
Date: 01/06/2004 07:40:07 pm PDT
Hi,

I own the C8-NGT and the secondary is indeed collimatable. Once you loosen the three allen head screws, the philips can be used to move the secondary closer/farther to/from the primary, and to rotate it. And the three allen head screws can be used to adjust the tilt. True, the focuser itself can't be collimated, so shimming is required if it points significantly far away from the center of the tube. See the manual for the SkyView Pro 8 EQ Reflector, since it has the identical spider assembly to the one in the C8-N, and that manual is superior to the one Celestron includes with the C8-N:

http://www.telescope.com/text/content/pdf/IN_175_SkyViewPro_8EQ.pdf

Hope this helps.

I love my C8-NGT in case anyone's still reading. ;-) The optics and the mount are excellent.



> [snip]
>Second, the secondary cell is glued to the spider holder so that no adjustment of screws can take place here. The screws of the spider can be adjusted but only for adjusting the location of the secondary up and down or sideways to allign the secondary in the tube in relation to the main mirror. Same thing with the middle screw which holds the holder to the spider. It can only be adjusted so that the secondary mirror can be rotated.
>
>The only way I could collimate this was to actually bend the spider (slightly) so the tilt of the mirror was perpendicular to the main mirror. For whatever reason, when I got this the tilt was slightly off so that the secondary reflection on the main mirror showed one side larger than the other. I used a cheshire collimation eyepeice to get this just right, adjusting the spider so the secondary was centered, then adjusting the secondary itself so that the reflection the main mirror and secondary were as centered as much as possible. This collimation process took about a week due to time constraints, and the need for outdoor testing under the stars.
>
> [snip]


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