Four common causes, in the order worth checking: the tube has not cooled, the air is turbulent, the telescope is out of collimation, or the optics are dewed. Only the third is a fault, and only the third costs anything to fix.
Almost every complaint of this kind traces to something free to fix, which is why the order of checking matters.
Cool-down
Warm glass sets up currents inside the tube that blur the image as effectively as bad seeing. A telescope brought out of a heated house and used immediately will not perform, and no adjustment compensates.
Seeing
The atmosphere sets a ceiling on most nights. If the image boils and shimmers, the sky is the limit and nothing bought will raise it. High magnification makes it worse rather than better.
Collimation
If the image is steadily soft rather than moving, and a defocused star shows rings that are offset rather than concentric, the mirrors are not aimed at each other. This is the one to fix with a tool.
Dew
A film on the corrector, the lens or a mirror scatters light and softens everything. It is invisible in the dark and obvious with a torch, and a dew shield prevents more of it than anything else.
The questions that keep arriving
How long does a telescope take to cool?
It depends on the mass of glass and the temperature difference, and a closed tube with a thick corrector takes considerably longer than an open reflector. Putting it outside before you need it costs nothing and fixes more first nights than any purchase.
How do I tell seeing from a focus problem?
Seeing makes the image boil and shimmer while remaining roughly the right size. A focus or collimation problem produces a steady, consistently wrong image that does not change moment to moment.
Last reviewed 17 September 2026