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How some I sometimes see the "hole" in the light in Reflectors

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We have a little Heritage 100p and some different eyepieces. During the day we see a "hole in the image". With the entirely inappropriate 40mm projection eye piece you can see it at night too

I think I know what is going on (eyepiece exit pupil eye ball entry pupil stuff) but cant get a proper scientific explanation my kids are happy with . . .

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I think you probably have it spot on, what part of your explanation are your children not happy with?

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With a 40mm eyepiece you will be getting an exit pupil (i.e. the width of the image presented at the eyepiece) of 10mm. This is too much as the human iris can't open that wide.

Also, with Newtonians, if the exit pupil is too large then the secondary mirror becomes visible, which is the black hole that you are seeing.


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