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I have a meade ETX-125 telescope and I have a solar filter, my question is, can I place a small disk in the centre to create an artificial solar eclipse in the hope of seeing corona ejections, if this is possible how big does the disk need to be?

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    $\begingroup$ Would Astronomy be a better home for this question? $\endgroup$
    – Qmechanic
    Jun 22 at 8:03
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    $\begingroup$ @Qmechanic maybe, or even better photography.se . There's a lot of overlap, seeing as the physics of optical systems is something one should understand when mucking with fancy photographic techniques. $\endgroup$ Jun 22 at 14:20
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    $\begingroup$ The term for what you want is "coronagraph". They're a bit tricky. $\endgroup$
    – John Doty
    Jun 22 at 17:36
  • $\begingroup$ Hi @JimiJo. Is it corona ejections you want to see, or the corona? (The corona is what you see during a total solar eclipse.) If you want to see corona ejections, covering 100% of the sun would block a large percentage of them. Only the ones projecting above the limb would be visible. You should research if a hydrogen alpha telescope is what you want so that you can view the entire disk to see flares. $\endgroup$
    – JohnHoltz
    Jun 23 at 15:10

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You can't do this unless your telescope is of a type which has an internal focal plane prior to the last set of lenses. Think of it this way: if you are looking at the sun thru your car's windshield, any obscuration on the windshield simply blocks some of the incoming wavefront, which is (roughly) the Fourier Transform of the object's spatial intensity pattern. To block the sun's disk, you need to block light at the object or image plane, not in phase space.

This can be, and is, done in some of the specialized solar observatories such as one of the 'scopes in Hawaii.

For home use, if you are able to open up your imager (the CCD or similar array inside the camera) and physically block the pixels when the solar disk is imaged, you might get enough contrast to see the corona in the unblocked pixels.

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    $\begingroup$ What you're describing is a type of Coronagraph. The OP doesn't mention an imager, if they try to do this with an eyepiece they will burn a hole through their eyeball and into their brain. It would be best to add a "Never point your telescope at the Sun and try to look through it!" warning along with a procedure to avoid destroying their CCD as soon as the telescope is slightly out of alignment. Perhaps the simplest thing would be to remind them that this does not replace the function of the solar filter, the filter must be in place at all times. $\endgroup$
    – uhoh
    Jun 23 at 20:39
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    $\begingroup$ The once time that "Never point your telescope at the Sun and try to look through it!" warning doesn't apply is when a proper solar filter is securely fastened to the aperture of the telescope. $\endgroup$
    – uhoh
    Jun 23 at 20:44

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