Because a rack-and-pinion or a light Crayford is being asked to hold a load it was not designed for, pointed upwards. Most have a tension adjustment and a lock, and using both is the fix before any upgrade is considered.
A focuser that holds an eyepiece perfectly can sag under a camera, and the reason is the direction the load pulls rather than its weight.
What changes with a camera
An eyepiece weighs little and sits close in. A camera hangs further back, which multiplies its effect, and the whole assembly pulls downwards whenever the telescope is pointed anywhere but straight up.
The two adjustments most focusers have
A tension screw, which sets how firmly the draw tube is gripped, and a lock, which fixes it once focus is found. Many owners never discover either, and the first is usually set light from the factory.
What to check next
Whether the draw tube itself is tilting rather than slipping. A tube that moves sideways under load produces stars that are worse on one side of the frame, and that is a different problem from focus drift.
When an upgrade is genuinely the answer
When the focuser is a light rack-and-pinion on a telescope now carrying a camera, a filter wheel and a guide setup. At that point the load has outgrown the part, and no adjustment recovers it.
The questions that keep arriving
Does a focuser lock hurt anything?
Not if it is a proper lock rather than a screw bearing on the draw tube. On some designs the lock shifts the focus slightly as it tightens, which is worth discovering before an imaging run rather than during one.
Is a motor focuser worth it?
For imaging, yes eventually, because it removes the hand from the telescope entirely and allows refocusing as the temperature drops. It is not a fix for a focuser that slips under load.
Last reviewed 17 September 2026