The most common and most likely answer to this crossword clue is the 9 letter word COELOSTAT
C
O
E
L
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T
A
T
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The straight part
We know that "light reflector" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case light reflector = coelostat.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does COELOSTAT mean?
COELOSTAT noun- A device that rotates a telescope so as to keep its orientation constant with relation to the stars.