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A few weeks a go I bought a telescope.

The problem is I am a total beginner. I was able to setup the telescope correctly and I already saw the moon very clearly. Then I moved on to jupiter. When I looked through the telescope a black dot was visible in the middle of Jupiter. I have already turned the focusing wheel but then Jupiter became smaller and smaller and the point became smaller. But even when I turned the eyepiece all the way out the dot was still visible and jupiter was hardly visible.

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    $\begingroup$ That looks like two spider vanes and the shadow of the secodary mirror, possibly decollimated and very out of focus. Collimate it and then bring something in the focus. The manual has details. $\endgroup$ – user34599 Jul 28 '20 at 20:38
  • $\begingroup$ At 800 / 26 = 31x magnification Jupiter is supposed to look small. $\endgroup$ – Mike G Jul 28 '20 at 22:19
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    $\begingroup$ Actually one can coarsely focus with the help of a bright star such that you get the smallest possible image of that star. $\endgroup$ – planetmaker Jul 28 '20 at 22:42
  • $\begingroup$ "There's a little black spot on the sun^H^H^H Jupiter tonight..." $\endgroup$ – Carl Witthoft Jul 30 '20 at 14:37
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What we see in your image is an out of focus object (the white moon shape) with the shadow of the secondary mirror (causing the out of focus object to have a crescent moon shape) and the struts (or "spider vanes") that hold the secondary mirror in place (the vertical and horizontal lines).

Focus on the moon and get that focus sharp. Then try looking at stars (they should appear as pin-points) then try Jupiter and Saturn. Don't touch the focus knob, when looking at the planets, since the focus point for the moon, star and planet is the same (they are all at "infinity" as far as focusing is concerned).

Jupiter will be "small" with that telescope. Perhaps much smaller than you expect. Look a tiny circle of light with four dots (the moon) around it. You may see some banding. Saturn is often better looking, as the rings should be clearly visible.

As a_donda suggests, the telescope may benefit from collimation (getting the mirrors and eyepiece perfectly lined up) The manual should have details (if not, search the internet) It's hard to give specific advice. Look through the manual perhaps there's a faceslapper that you're missing, or get a friend to go through it with you (four eyes are better than two). If you got it from a specialist shop, you could try talking to the shop people. You could try the website of the company that made the telescope, they may have a contact number. You could search for a local astronomy club.

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  • $\begingroup$ Would I see Jupiter better with a different eyepiece? $\endgroup$ – nlschuster Jul 29 '20 at 11:11
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    $\begingroup$ @nlschuster I recommend you start by trying to use your telescope during the day looking at distant object on the ground (NOT in the sky!) this way you can recognize clearly what you are seeing. When you mention you are seeing Jupiter you may be only looking at an out-of-focus "blob" not an image of the planet's disk. If you bring an object that's say 100 meters away into focus then carefully note the position of the focus. To move it to infinity for sky viewing at night you'd have to move inwards (towards the telescope) by about 7 or 8 mm from that point. $\endgroup$ – uhoh Jul 29 '20 at 13:33
  • $\begingroup$ Ok but James K says that Jupiter will be "small" with my telescope. Would jupiter be bigger to seee with an other eyepiece? $\endgroup$ – nlschuster Jul 29 '20 at 14:19
  • $\begingroup$ @a_donda Yesterday i collimated my telescope with a specific laser collimator. I tried to focus on the moon but my Telescope won't focus. It looks like there is not enough focus range. When I pull out my eyepiece a little bit from the eyepiece holder, the image becomes sharp. $\endgroup$ – nlschuster Jul 31 '20 at 7:37
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    $\begingroup$ You don't get into the focus because of the short travel of the focuser, a typical Newton problem. Instead of holding the eyepiece manually, put an extension tube in between. They come in all sizes. And do read the manual. Multiple times if necessary. $\endgroup$ – user34599 Jul 31 '20 at 9:13

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