Loose Visual back / Baffle tube - Celestron SCT 9.25
When I attach any component ie; adapter, diagonal, extender etc to the visual back, the whole tube rotates with the equipment. This causes the eyepiece and all to flop down while observing at an angle. It is evident that the baffle tube / visual back base has worked it's way loose. How can I re-tighten this? If dismantling the scope is required is there a locking nut or grub screw responsible? Any recommendations would be helpful.
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Do you have a "diagonal" with this small telescope? It looks like this: http://www.amazon.com/Orion-1-25-Prism-Telescope-Diagonal/dp/B0000XMWYY/ref=sr_1_4?ie=UTF8&qid=1313867352&sr=8-4
Put this into the focuser and then put the eyepiece into the diagonal. That all there is to it. If you are just putting the eyepiece into the telescope without the diagonal --- then you will need to OVER extend the focuser in order to come to focus as you described.
The old Celestron C90's used 0.935" diameter accessories. You can check this yourself by measuring the opening at the back of the C90 with a ruler. Preferably, you would want to get a 1-1/4" visual back for the C90 so that you can use the far more common 1-1/4" diameter telescope eyepieces and accessories.
If you are planning on doing terrestrial observations, you can use a Porro prism to get a correctly oriented image, or you can use a 90 degree angled Amici roof prism to get a correctly oriented image. If you plan on doing celestial viewing, a simple 90 degree diagonal will suffice since it really doesn't matter if the image is reversed left to right.
Various focal length eyepieces merely give you different magnifications with the telescope. To find the magnification, simply divide the telescope's focal length (1200mm) by the focal length of the eyepiece. A 25mm eyepiece, for example, would give you a magnification of 1200/25 = 48 power.
Modern eyepieces are available which provide a variety of magnifications, yet which also have a consistent eye relief (distance from the top of the eyepiece to your eyeball) of around 20mm. This is very handy, especially when using higher magnifications, since these eyepieces are much easier to look through and since they can be used if you must wear eyeglasses to correct for astigmatism within your eyes.
Orion telescope center has knowledgeable staff who can help you choose a good set of eyepieces and accessories to use with your C90.