noun
guiding telescope
A small telescope mounted on a larger one, used to keep the main telescope accurately pointed at a target during observation or photography.
天体写真を撮るときは、案内望遠鏡で星を追跡する。
When taking astrophotographs, I track the star with a guiding telescope.
Compound of 案内 (guidance) and 望遠鏡 (telescope), literally 'guiding telescope'.