Quote:
Originally Posted by Wavytone
Don,
I'll again call you disingenuous.
I am absolutely *sure* you know Plossls have negative coma and field curvature which just happens to match short focal-length newtonians.This is why such a simple eyepiece has survived so long, and continues today as a very good budget eyepiece among the users of short Newtonians.
I simply do not accept the makers of premium eyepieces - and I put Teleview, Explore, Pentax, Vixen and Nikon in this group - are not aware of the tradeoffs to be had by designing eyepieces to match about 50-60% of the field curvature and coma of the average light-bucket (a fast newtonian say 30cm f/4.5) on the basis that this will be equally a match in a smaller refractor (10-15cm f/7) or the average 20cm SCT.
Aberrations as seen by the observer are the sum of all the elements - scope + eyepiece - not eyepiece alone. I suggest you are well aware of Peztval's contribution in this area.
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Well, we may have to agree to disagree.
Plossls do NOT have negative coma and the field curvature that exists runs the wrong direction for newtonians (both positive). But typical dobsonians have such long focal lengths they essentially have no field curvature to speak of. Figure it out. In a 42mm field, your 30cm scope would have such a tiny amount of field curvature that designing an eyepiece to correct it would be very difficult. Flat fields make the most sense. And I have used far too many eyepieces (e.g.35 Panoptic, all 4 longer focal lengths of Pentax XWs, Nagler T4s, etc.) with field curvatures that should be the opposite direction to have any cancellation occur.
And I have certainly seen my fair share of coma in Plossls in short f/ratio dobs. All you have to do to test the hypothesis is to stick one in a Paracorr and watch the eyepiece get sharp to the edge. If they corrected coma, they would be worse in a Paracorr, not better.
A manufacturer would have to be an idiot to design an eyepiece that would only work in newtonians given that roughly 2/3 of the market is other kinds of scopes (SCTs, MCTs, Refractors, etc.). And so they don't.
Sometimes other corrections leave in some curvature, or astigmatism, in the attempt to create a flat Petzval surface.
The Plossl has survived as long as it has because the modern variants (Brandon, Clave, TeleVue, et.al.) were designed so that the oblique angles of f/4-f/5 scopes wouldn't induce extra astigmatism in the field.
Better correction is possible, though, and there are some eyepieces coming out now that are better corrected at such short f/ratios--even 25 degrees off axis.
The Plossl eyepiece pre-dates the popularity of short focal length newtonians. So does the Abbe orthoscopic. Yet both are popular for narrow field eyepieces in dobs and other scopes because they are fairly well corrected.
There have been eyepieces designed for dobs (the Praetoria design by Klee comes to mind, as well as his barlows), but they are the rare exception.
Look, I've seen the ray traces and spot diagrams for many of the modern eyepieces. I've seen the tests measuring astigmatism and wavefront error.
Have you?
I am NEVER disingenuous, and I resent your calling me that.
You know not whereof you speak.