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RE: RE: RE: Low-Profile Focusser Problem

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Posted by Pete on July 25, 2000 01:20:29 UTC


I suggest going with a longer telescope tube so that you don`t need an "extender" on your JMI focuser. As suggested earlier, you can purchase sonotube at building supply stores. Another option would be to go back to your old focuser and use the JMI on a new telescope.

At you increase the magnification, image quality gradually gets worse. However, I don`t think a new Barlow lens or an "extender" is the answer to your problem.

If you are certain that the scope is aligned okay, how are the seeing conditions? If the seeing conditions are bad (ie a turbulent atmosphere) then the images will suffer (especially at high power), no matter how good the telescope. A barlow lens will not help.

Are you letting the mirror cool down to the ambient temperature before using it? A warm (or cold) mirror will not produce good images. Again, a barlow will not help here.

Make sure the mirror mount is not pinching or stressing the mirror.

Lastly, you should "star test" the telescope. Point the scope at a bright star (Polaris is good) and move the focuser slightly (~ 1 to 2 mm)inside and outside of focus. The defocused star images inside and outside of focus should look similar. If they are not similar at all, the optics (mirror or secondary) could be bad or the scope is still out of alignment.

Check out this book on star testing:

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