Crossword Genius

Telescope on, characteristic rings appear, ultimately (9)

Ross

I believe the answer is:

refractor

'telescope' is the definition.
(thesaurus)

'on characteristic rings appear ultimately' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'characteristic' becomes 'factor'.
'rings' indicates putting letters inside.
'ultimately' says to take the final letters.
The final letter of 'appear' is 'r'.
'factor' going around 'r' is 'fractor'.
're'+'fractor'='REFRACTOR'

I've seen this clue in The Guardian.
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