Why Stars and Planets Look Blurry Through a Telescope
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A telescope that shows fuzzy, blurry or shimmering stars can make owners think the optics are faulty. In most cases the telescope is fine. Blurry views usually come from focus, atmospheric turbulence, a telescope that has not cooled to outdoor temperature, collimation problems or too much magnification. Learning to identify the cause helps you get sharp views on the right nights.
Focus carefully
Stars should focus to tiny, sharp points. Turn the focuser slowly back and forth through the point of best focus, since it is easy to stop slightly short. A bright star makes a good target for focusing before moving to faint objects.
If you wear glasses for astigmatism, you may see better with them on. Nearsighted and farsighted observers can usually remove glasses and simply adjust focus.
Understand atmospheric seeing
Turbulence in the atmosphere causes stars to twinkle and planets to shimmer or boil. On poor seeing nights, no telescope shows crisp planetary detail at high magnification, even if skies look clear.
Seeing is often better when stars twinkle less and objects are high in the sky rather than near the horizon. Avoid observing over rooftops, roads and parking lots, which release heat after sunset.
Let the telescope cool
A telescope brought from a warm house into cold air has warm air currents inside the tube, which blur the image. Reflectors and Schmidt-Cassegrains with large mirrors can take 30 minutes to over an hour to cool.
Place the telescope outside early, and use cooling fans if your reflector has them. Refractors usually cool faster.
Check collimation
Reflectors and Schmidt-Cassegrains need their optics aligned, called collimation. When collimation is off, stars look like comets or flares rather than points, even at best focus.
Perform a star test by slightly defocusing a bright star. The rings should look concentric. If they are off-centre, collimate using a collimation cap, laser collimator or Cheshire eyepiece.
Use sensible magnification
High magnification enlarges atmospheric turbulence and any optical imperfections. Many observers find the best planetary views at moderate powers on typical nights.
A useful rule is that maximum practical magnification is about 50x per inch of aperture under excellent seeing, but average nights support much less. Drop to lower power if the image looks soft or dim.
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