Quote:
Originally Posted by Wavytone
Hmm.
He paired the mono with a Barlow, which defeats the point of using the mono. May as well have been looking through an ordinary 5mm such as a Plossl.
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Not really as it is often more about execution, especially when the group count is low. So as example, for accepted high performing planetary eyepieces like a ZAO, it is 4 elements in 2 groups. The Pentax XO is 5 elements in 3 groups. The Monocentric in question is 4 elements in 2 groups. No Plossl out there that I've had my hands on, and I've had many, has ever risen to even close to the likes of a ZAO, CZJ Ortho, Pentax Ortho, Monocentric, AP-SPL, XO. Execution and backscatter in the base design make big differences.
One of the contrast robbers in any design is the backscatter between the elements. The Abbe design as example is quite low in backscatter...much less than a Symmetrical design (remember that production "Plossls" are really Symmetricals which have much inferior spots to what the true Plossl is able to achieve). The 5XO design is actually very Plossl-like in the upper 4 elements, but Pentax went to some extremes taming the backscatter not just between the groups, but also between the upper cemented groups as well. The Monocentric design is about the lowest until you get to singlets. The very distant spacing of the Barlow's doublet from the Monocentric assembly really negates most of any backscatter between the Monocentric and the Barlow due to the extended distance (and assuming good baffling and a quality Barlow). If you've ever take some of the premium lines of planetaries like ZAOs, AP-SPLs, Pentax Orthos, etc, and Barlowed one of the longer focal lengths so it was equivalent to a shorter focal length of the same line, you would be hard pressed to see any impact at all, again assuming a high quality Barlow build and optics. I have done this on many occasions with these best-in-class planetaries. Plus when you look at specialty designs like the Vixen HR series, essentially the same concept with a 3 lens group up top followed by an internal doublet Barlow. So as long as you can keep that upper group minimized then adding and properly baffling and coating to lower doublet Barlow is always a winner.
https://image.jimcdn.com/app/cms/ima...2297/image.png
But all the theory aside field results always are a trump card. So where it counts, in the field, the combo of the Mono wit the Barlow was a planetary winner. Was easy to see at the eyepiece on Jupiter, Mars, and Saturn that the Barlowed Monocentric easily kept up with and in some areas beat the venerable XO. Proof for me was in the seeing.