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misty eyed


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I have at last penetrated the gloom to spot the moon with my Celestron130EQ this morning;there is a dull spot in the middle of the view-is this a collimation issue ( I have collimated with a Cheshire eyepiece and all seems fine) or just the poor quality of the supplied e/p?

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I have at last penetrated the gloom to spot the moon with my Celestron130EQ this morning;there is a dull spot in the middle of the view-is this a collimation issue ( I have collimated with a Cheshire eyepiece and all seems fine) or just the poor quality of the supplied e/p?

The moon is bright and constricts your pupil and you're possibly also not placing your eye at the correct distance or on the correct spot. The trouble is a low magnification means the light bundle for a point in the field is fairly wide at the exit of the scope+eyepiece system (your eye's lens then focuses that bundle on the retina), and the shadow of the secondary can then fill a good portion of your eye pupil if that 's not fully dilated (and the moon is bright to make the pupil constrict).

What helps:

-Try to move your eye closer and further away from the eyepiece and move it sideways to see if the effect disappears, because incorrect eye placement can also cause field illumination issues.

-Apply more magnification, so that the exit pupil of the system becomes smaller (and easier to catch with the eye pupil) and the smaller surface brightness lets your eye pupil dilate a bit again.

-At low power, consider using a variable polarising filter so that you eye pupils don't have to constrict as much. It's also going to help avoid being essentially night blind with one eye and stumbling upon things when you stop looking at the moon. If your eye piece has long eye relief (i.e. correct eye placement is some distance away from the eyepiece), a cheap alternative is sunglasses.

It's not a collimation issue.

Edited by sixela
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doesnt sound like collimation. cd be if you are using a very low power e/p that you are seeing the shadow of the secondary mirror? Or it could be that your eye is not in quite the right place. If you're using a celestron "silvertop" plossl you need to get your eye in just the right place as the eye relief is very large (at least that's what I found).

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Dunno what a silvertop is-it's the 10mm that came with the scope.I:) have ordered a 32mm Omni so hopefully a miraculous improvement will be effected.

er, it's juts a plossl with a silver top. :) If you're getting the dull spot with a 10mm e/p it won't be the shadow of the secondary.

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