WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES.
NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!
Etalon from inside the housing. A negative and positive lens ensure parallel light passes through the etalon in the middle. It is the power of these lenses which demands a 20cm etalon position inside the focus of an f/10 objective. A 'shorter' or 'faster' focal ratio will stop down the objective to f/10.