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I had a TEC180 fluorite for several years. A lovely scope. Views though, whilst really good were not as memorable as from a Tak FS152 I had. But then I had a wider collection of eyepieces and spent more time using it visually. The TEC180 was only a few times and not for long. M42 though started to show some colour (a greenish colour in the neb).
I suspect there is a less improvement with size factor which is the seeing.
6 inch is often regarded as the sweet spot for APOs.
TEC used to make a 200 APO using FPL53 glass. They come up occasionally. They make a 250mm APO now as well. I see also APM make large APOs. 200mm is probably max for amateur as the scope ends up too long and too hard to handle. The TEC180 was close to my physical limit for length and handleability. Also simple aspects like how do you fit a long 200mm APO in the back of an SUV. The 180 took up most of the room in the back of a Prado with a little to spare.
Moment arm momentum means APOs will require a more sturdy mount faster than a mirrored compound scope for the same weight.
A Tak NJP mount handled AP140, BRC250, Tak FS152 but was really overloaded by the TE180. Everything had to be spot on for round stars. PMX would be the minimum in my opinion.
Greg.
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