If you want to use a filter on a telescope, use only the filter supplied by the manufacturer or by a manufacturer who makes the filter specifically for the instrument you are using. However, if the manufacturer’s filter is the type that screws onto the eyepiece (pictured left), DO NOT USE IT! The manufacturers of some cheap refractors supply a welder’s glass filter that screws on to the eyepiece. It may heat up and crack as you are looking through the telescope. A proper solar filter always goes on the front end of the telescope, blocking the sunlight before it enters the optical system. Thanks to the Orion Telescope Center for the loan of this unsafe filter. Orion does not sell these! They just had one around as a bad example… They are good and knowledgable people.