occulter
/əˈkʌltə/ /ˈɒkʌltə/noun
Something that occults; specifically (a) a device for temporarily cutting off a lighthouse light; (b) a device in a telescope or other instrument for cutting off the direct light of the sun or other bright object.
Origin
Early 20th century; earliest use found in Encyclopaedia Britannica. From occult + -er.