NO! NO! NO!
These cheap “filters” are manufactured and marketed as being able to be inserted into the eyepiece. That means that by the time the Sun’s dangerous rays get to the filter, they have been magnified by your telescope, and are being focused directly on that filter. The cheap glass cannot take all that thermal energy, and it WILL crack! If you are looking through the scope when that happens, the result to your eyes will be devastating. You will not be able to look away or shut your eyes quickly enough, and your eye will almost certainly be permanently blinded.
If you have a cheap “solar filter” that came with a telescope you didn’t pay much money for, then THROW THAT “FILTER” AWAY!
This is a real solar filter, and it costs more than several of those cheap telescopes put together. These filters can be made of glass, or of a special film that is manufactured and certified for use as a filter which is safe for direct solar viewing.
See how it (well, one like it) is used over the END of the scope, so that all the Sun’s harmful energy and radiation are blocked BEFORE they enter the scope? This is what you want to use, if you are going to try to view the Sun safely with a telescope. But if you set this type of thing up, make sure to remove the finderscope from your telescope, and NEVER leave the setup unattended. If you are going to walk away from it, even for a second, point the scope away from the Sun so that no one tries to look through it unsupervised.
