A glow, which is a very small amount of scatter and almost coincident with the target, can have many causes:
1) Dust. The most common. Add a "haze" on the optics (micro-dust or organics).
2) water vapor deposition. Even before dew is visible to the eye on optics, a small amount of water droplets condensed on the optics will cause this scatter.
3) rough optical surfaces on either the scope optics or the eyepieces. Scope optics are usually more likely to blame than the eyepieces, though eyepieces are not immune.
4) light scatter in the eye--the cornea (roughness), the lens (incipient or forming cataracts), floaters (though out of focus, can cause scattering), and the retina (light intensity activating
the firing of adjacent rod cells not in the actual image.
5) old mirror coatings.
Some questions concerning the above:
1) Have the mirrors been recently cleaned, like in the last month? Did it make a noticeable difference? I clean my mirrors quarterly and ALWAYS see a difference in light scatter.
Dust is not your friend. And organic haze is even worse.
2) How do you keep the secondary and eyepieces free from moisture? In the winter, removing the eyecups may allow more circulation between warm, moist, eye and the cold lens.
Moisture won't condense on surfaces that are warm or under a breeze. One of the side advantages of a fan pushing up.
3) Do you see this with all eyepieces? Do you see it with the same eyepiece in another scope? If it's different in another scope, the fault isn't the eyepiece. If it's different with another
eyepiece, it isn't your scope. Experiment to see.
4) Can't do much here. An eye exam may give you a clue. If you try the other eye is the issue exactly the same? That doesn't prove it's not in your eye, but if there is a difference between
your eyes, it points to the eye as a possible problem.
5) I've seen many a mirror's figure magically improve with a new coating, and light scatter virtually disappear. Coating more than 8-10 years old? Get the mirror re-coated.
Edited by Starman1, 12 March 2017 - 04:02 PM.