A phone held to an eyepiece will photograph the moon well enough to surprise people, and an adapter turns that from a two minute wrestle into a repeatable arrangement. What it will not do is turn a visual telescope into an imaging one.
What the adapter does
It clamps the phone over the eyepiece with the camera lens centred and at the right distance, holding it there while you tap the shutter. Doing it by hand works and takes both hands, moves constantly and puts a phone above a hard floor. The adapter costs little and removes all three problems.
What it will photograph well
The moon, spectacularly, because it is bright and large. The bright planets, poorly, because a phone sensor is small and the exposure control fights you. Bright double stars and the Orion nebula, adequately from a dark site with a phone that allows long exposures. Anything faint, not at all.
Getting a usable image
Lock the exposure and the focus manually if the phone allows it, because automatic modes will expose for the black sky and blow out the moon completely. Use the timer or a Bluetooth remote so nothing is touched at the moment of exposure. Take many frames: most will be spoiled by vibration and by the atmosphere, and one will be sharp.
When to stop and buy a camera
If you find yourself wanting to photograph faint objects, the phone route ends quickly and a dedicated camera on a tracking mount begins. That is a different hobby with a different budget, and this index prices telescopes rather than cameras, so it says so rather than pretending the adapter is a step towards it.
Questions people ask about telescope
Do telescope phone adapters work?
Yes, well, for the moon and for bright objects. They hold the phone steady over the eyepiece, which is the whole difficulty of afocal photography.
Can I take photos through my telescope with a phone?
The moon easily, the bright planets with patience, and faint objects essentially not at all. Manual exposure and a timer make the difference.
What is a telescope smartphone adapter?
A clamp that holds a phone with its camera centred over the eyepiece at the right distance, so the phone photographs what your eye would see.