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I have a Dobson Omegon with a 640mm focal length and a 102mm aperture.
My finderscope is broken but I can do without.
I have located the planets with the naked eye but I do I do not see them through my eyepiece.

I can focus perfectly on the moon, using a 20mm eyepiece. (I have a 6mm eyepiece as well.)

How should I proceed?

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Always start with the low magnification (20 mm) eyepiece. If the target is not in the eyepiece field at first, methodically sweep the surrounding area. Any sweep pattern that makes sense to you is fine. When the telescope is pointed near a bright planet, stray light may brighten the background slightly. After you find and center the target, switch to the high magnification (6 mm) eyepiece if needed.

Of course it's much easier to aim a telescope with a working finder. If your finder is not repairable, most modern consumer telescopes have a dovetail fitting which makes it easy to install a replacement.

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    $\begingroup$ Well now that both Saturn and Jupiter are out of my field of view , I will wait for tomorrow. $\endgroup$ – Marc Oct 19 '18 at 21:31
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    $\begingroup$ @Marc The "Tyranny" of Earth's rotation for astronomers ;-) It takes some patience, I used to practice using a far away light at night, or top leaf on a far-away tree during the day. This is a good answer. Stick to lowest magnification first (don' worry about seeing detail on the planet) where the planet will appear star-like. Only after you get good at finding it, then start working your way up in magnification. $\endgroup$ – uhoh Oct 20 '18 at 0:35
  • $\begingroup$ 5mm eyepeices on the cheapest celestron hurt my soul... I coughed while aligned and had to switch back to 20mm to find mars again. $\endgroup$ – Magic Octopus Urn Oct 21 '18 at 4:07

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