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I was looking at the Jupiter for the first time today and I have noticed a smudge near it. Could someone explain what it is, please? I was looking online but did not find anything. So I thought that some kind of defect in my setup but I couldn't get rid of it. I mean the thing below its moons.

Edit1: My telescope is a Celestron AstroMaster 76 ESQ. The smudge was there no matter what eyepiece I have used. I was able to see it even without a camera.

Edit2: I was watching it for about 10 minutes, and the smudge was moving along with the rest of the objects.

I took just this one image for the StackExchange purposes. I have put the camera in front of an eyepiece to take the picture.

Thank you

enter image description here

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    $\begingroup$ Can you describe your setup? It may help to identify the right answer, and a way to avoid it in the future. For example, did you hold a camera or a phone up to an eyepiece of a telescope. or use a camera body attached to a telescope directly? $\endgroup$
    – uhoh
    Commented Apr 10, 2017 at 5:49
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    $\begingroup$ It is a Celestron AstroMaster 76 ESQ. The smudge was there no matter what eyepiece I have used. I've seen it even without a camera. Though in real it is not that prominent as on the picture. It was faded out and misty. $\endgroup$
    – bakua
    Commented Apr 10, 2017 at 6:03
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    $\begingroup$ OK that sounds like very helpful information. You can consider also including it within the description in your question, since comments are not considered permanent. Also, did you take more than one image? If so, can you check them to see if it moves in the opposite direction or same direction? Did you use a camera in front of an eyepiece, or did you remove the eyepiece and use a camera body with no lens? (best to answer by editing the question and completely explaining the setup there). $\endgroup$
    – uhoh
    Commented Apr 10, 2017 at 6:05
  • $\begingroup$ Great! Oh, by moving, I mean if the camera moved with respect to the eyepiece, for example if it was hand held, or if for example the guiding is not perfect, if jupiter moves one direction within the FOV, does the smudge move the opposite direction. $\endgroup$
    – uhoh
    Commented Apr 10, 2017 at 6:48
  • $\begingroup$ Also just to correct myself about prominence, I've just looked at the picture and it expresses the real display pretty accurately. Sorry I've got confused. $\endgroup$
    – bakua
    Commented Apr 10, 2017 at 6:48

2 Answers 2

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The faint green smudge looks to me like a flare (that is, an optical defect called a flare).

However I'd expect a flare to be symmetric with the bright object it relates to about the center of the field of view. This does imply either the image is off center (e.g. the camera is off center or the image is a crop off center) or there's a misalignment of your optics.

This question on Physics SE is about correcting flare in a telescope and may be relevant.

The similar question Does this smartphone photo show Mars just below the Sun? and its answers contain some other examples.

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  • $\begingroup$ If the image is from a camera or phone held up to an eyepiece, there's all kinds of off-centered-ness possible. $\endgroup$
    – uhoh
    Commented Apr 10, 2017 at 5:48
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    $\begingroup$ 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. $\endgroup$
    – bakua
    Commented Apr 10, 2017 at 6:46
  • $\begingroup$ related, and some helpful examples: Does this smartphone photo show Mars just below the Sun? $\endgroup$
    – uhoh
    Commented Dec 24, 2019 at 1:48
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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.

OP's image of a smudge near Jupiter, cropped, brightened and annotated

enter image description here click for full size view

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