What you are thinking of is known as friendly_name and it you can’t specify it via MQTT Discovery. It uses name for friendly_name and to create the sensor’s object_id (that’s the telescope_ambient_temperature in sensor.telescope_ambient_temperature.
After the sensor has been created via MQTT Discovery, you can edit its options by going to Configuration > Entities and clicking on the desired sensor.
Customizations can also be done using manual configuration in a file.