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Televue 102 is a doublet, Tak TSA102 is a triplet. Per Roland Christen of AstroPhysics fame to make a true APO it needs to be a triplet. APO means 3 colours focus within a tight tolerance at the same spot.
What you typically find with these high quality doublets like Televue 102 or Tak FS series is one colour will be slightly out of focus. This is usually blue hence for imaging you will see an excess of blue hazy stars or star with bluish halos. Whereas the same object imaged with a true APO shows the stars as white. However labelling a doublet as APO is really misleading and should really be described as high quality semi APO.
When you use a real APO after using a doublet you will immediately see the considerable jump in performance. For me that was using a AP140 after using a Tak FS152 for some time. Also using a TEC180mm fluorite triplet. Whilst the FS152 was great for visual and sharp, for imaging stars often had the bluish halos that a doublet often gives and the AP whilst lower in magnification, left it for dead in a whole different league altogether. Still the absolute sharpest scope ever looked through. Brings new meaning to pinpoint stars.
The doublet will cool down faster most likely.
I have heard some really rave reports of the optics of the Tak TSA from a guy who'd owned them all including high end APs and I hope that Tak follows through and makes more triplets like that at larger sizes although I am sure they are in no hurry as they have their TOA 130 and 150.
Greg.
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