Usually too much magnification on a night the air will not support it, a tube that has not cooled, or a reflector out of collimation. Reducing the power and waiting are free, and they fix it more often than any purchase would.
Planetary detail is the hardest thing a small telescope does, and it is limited by the air far more often than by the optics.
Drop the magnification
Counterintuitive and almost always right. A smaller, brighter, steadier image shows more detail than a large dim wobbling one, and the difference is immediate.
Wait for the tube
Warm optics produce currents that blur exactly the detail you are looking for, and the image improves steadily through the first hour. If it is getting better as the evening goes on, that was the cause.
Check the collimation
A reflector out of true shows soft detail and a bloated image that does not change moment to moment. Defocus a bright star slightly: concentric rings mean aligned, offset rings mean not.
Look at where the planet is
Low in the sky means looking through much more atmosphere. The same planet at a higher altitude, in a different month, is a different observation entirely — and no equipment substitutes for waiting for it.
The questions that keep arriving
How high is too high?
A common rule of thumb puts the ceiling at roughly twice the aperture in millimetres, and the atmosphere usually caps it well below that. Past the ceiling the image gets larger and no more detailed.
How low is the planet in the sky?
It matters a great deal. Near the horizon you are looking through far more atmosphere, and a planet low in the south will never show what the same planet shows high up a few months later.
Last reviewed 17 September 2026