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GC's will be disappointing in 90mm, a 90mm scope will be decent on planets & show the bright dso (M objects)..if you want more dso than a 5-6" mak would be the way to go, on alt/az I reckon
The 25mm ep is a bit long, 9-13mm is better for dso I reckon (except bigger ones) in shortish f/l scopes (<1000mm)
To calculate what objects takes up of the fov first get the size of the object eg 30 a.sec by 20 a.sec. You will need at least half a degree or 30 a.sec to fit in in. The use scope focal length to work out view size...eg 1000mm / 10mm ep = 100x at 50 deg gives 0.5deg at ep
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