Wow, lots to unpack here.
Takeaways so far:
Can use simpler, less expensive eyepieces.
Easier to achieve higher magnification
Easier to focus
Don't suffer field curvature to the extent faster ones do
Smaller field of view
Larger and harder to handle
Is it practical (for visual) to use a reducer to shorten focal length?
Large field of view
Smaller and easier to handle
Can take high magnification without field curvature
Require more complex eyepieces to work optimally
Require higher power eyepieces or Barlows to reach high magnifications
Suffer field curvature
Is a field flattener a practical option for fast refractors for visual? This would address the field curvature issue.
With quality Barlows up to 5X available, why not use lower power eyepieces for high magnifications? By adding another optical element, do Barlows noticeably compromise image quality?
Observation: The TV NP101 seems to strike a balance between fast and slow, which begs the question: If it is the Goldilocks optical design, why don't all premium scope makers offer one?
I’ve had both fast triplets and slow triplets. My fastest triplet was a LZOS 102mm F6.25 in a SV tube with a 2-speed FT focuser using OK4 glass. It had a killer optic! My slowest triplet was a 1989 152mm F9 Astro-Physics triplet with a single speed focuser that used no ED glass but used special flint glass. What a wonderful scope!
IMO, when using either a fast or slower high quality triplet, most of the comments in this thread about disadvantages are over-blown.
I never had an issue finding exact focus with my fast LZOS F6.2 triplet with the 2-speed, FT focuser. If the optic is of high quality it will snap to focus. The 2-speed focuser was a help but you could tell when focus was achieved even without it. Color correction was superb. More color was introduced at the edges with wide field eyepieces and the atmosphere than anything from the main optic.
For the planets, field curvature at high power was not an issue. And to be honest, field curvature was never an issue on visual deep sky. I really don’t spend much time observing the “extreme edge” of the field but usually center the object of interest. And, of course, slower refractors might not have field curvature but they also have smaller fields to start with.
Today’s top quality Powermates and Barlows are of very high quality and will not impact the quality of the optic and can be used for high power observations with fast triplets. The same goes for reducers and field flatteners. Eyepieces like TV Delites (which actually have something like a Barlow built into the design) with their high ER and flat fields and “excellent ergonomics” are very comfortable to use with fast triplets, even more so than the short, simple eyepieces with low, uncomfortable ER used with long FL refractors of yesteryear. Astro-Physics would not produce a Barlow, reducer or flattener that compromised the quality of their main optic. How much “harder” is to use a Barlow anyway, I mean really.
My wonderful, 1989 AP 152 F9 “slow” triplet produced superb, basically color-free visual images, but it was rather long. My later AP 155 F7 did the same but was surprisingly short and compact for a 6” class refractor (without the dew shield it was “shockingly” short), and of course was better for imaging. My 155 F7 had a single speed focuser that also presented zero issues finding focus.
My current Takahashi TSA 120 at F7.5 has basically zero noticeable field curvature and basically zero-color, even in the star test. No need to go longer. Tak’s TOA lenses use 2 ED elements so there is also little need to go longer.
I don’t remember having any issues finding focus even with my original TV Genesis F5 either.
A slower triplet (like from CFF) will most likely cost less but will also be less conducive for imaging. But the actual quality of a faster triplet lens from CFF and a slower triplet lens from CFF will be “the same” when it leaves the factory. The easier to make argument does “not” mean you get a lesser quality lens if it’s “faster” but only applies to “price”.
Most F5 apos use more than three lenses to correct for aberrations and they are used mostly for imaging. With mid FL apo triplets, any disadvantages listed in this thread will be non-issues for the visual, planetary or high power observer.
Marcus Ludes of APM has been quoted as saying that their LZOS 130 F6 outsells their LZOS 130 F9.2 by about 10 to 1. The market moved to faster lenses when the new glasses allowed manufacturers to make faster lenses. And the market embraced the advantages of these faster apo refractors.
With a high quality triplet, get the FL that is better suited to your visual or imaging and portability goals/needs. The more important concern is the quality of the main optic, which will have a much greater impact on any visual or imaging pursuits. The portability of the scope and mount will also impact use.
As far as why all apos are not Petzvals. The F5 TV Petzvals (and most all fast quadruplets) basically have a permanent reducer/flattener in the optical train whereas a triplet lets the user add the “additional expense” of a reducer or a flattener if so desired.
All IMO and based on my experience of course.