I've got a 15cm aperture newtonian reflector on a computerised eq-mount. (It took me forever to save up for it!
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I haven't heard about being a cadet observer before. Is that spending however long at an observatory and learning how to be an astronomer?
Siding Springs would be cool to get into, but I'm not sure if they do any work experience (or cadet observing). I've emailed them, but they said they don't have any programs running for high schoolers any more.