The OP's comment under another answer says:
I would expect the flare to move depending on how I move with my telescope. What I saw, however, was an object that was always moving with the planet at the same relative place to it.
So it may in fact not be lens flare, but instead some kind of misalignment between two parallel surfaces. Like an optical wedge type of thing. A wedge prism for example, with proper antireflection coatings will not tend to show a double image, but here the image is very intense and the smudge is very weak, so it could be a double reflection between two almost-parallel surfaces.
A test would have been to see if the gap between Jupiter and the smudge scaled with magnification by changing eyepieces; if Jupiter is twice as big is the gap twice as big also?
I see a lot of chromatic aberration,
and this is a 3 inch refractor. I wonder if someone took the objective lens apart, separating the achromat pair (assuming it's an air gap, which some are) and then put them back together incorrectly? It's possible that could result in a small wedge error, depending on how the lens was keyed and spacers implemented.
No, the Astromaster 76EQ is a 76 mm f/9.21 Newtonian Reflector! One of the eyepieces has a built-in image corrector(?!) and the other doesn't?
The secondary mirror has a 46% obstruction by diameter (21% by area) which sounds absolutely huge for an f/9.21 Newtonian, so something may be going on inside.
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