Almost always the coin cell, and almost always because the switch was left on. Carry a spare in the case: a dead finder ends a session faster than cloud does, because aiming without one is genuinely difficult.
This is the most predictable failure on the whole site, and the one that most reliably ends an evening.
Check the obvious first
The cell, the switch position, and whether the cell is the right way up. A cell inserted reversed is a common assembly error and looks identical from outside.
Check the contacts
Coin cell holders in inexpensive finders corrode, and a dew-soaked finder left in a shed will do it faster. A contact cleaned with a dry cloth often revives a finder declared dead.
Prevent the recurrence
A spare cell taped inside the case, and the habit of switching off at the end of a session rather than at the end of a week.
While it is out of action
An optical finder, if you own one, or aiming along the tube for bright objects. Neither is pleasant, which is exactly why the spare cell is worth the trouble.
The questions that keep arriving
Why does the cell go flat so fast?
Because the switch on most of these is easy to leave on, and a coin cell has very little capacity to begin with. Cold weather shortens it further, which is why failures cluster in winter.
Is the dot supposed to be that bright?
Not under a dark sky. A dot bright enough for a suburban garden will hide the faint star you are trying to point at in the countryside, which is why a working dimmer matters more than maximum brightness.
Last reviewed 17 September 2026