Quote:
Originally Posted by Wavytone
In short:
- if you are using a fast dob, buy Televue.
- if you are using an SCT, Mak or refractor, Pentax, Nikon or Vixen eyepieces may be better.
- if you have a real flat-field scope (i.e. quadruplet ED APO or other esoteric expensive beast) then yes, you should buy flat-field eyepieces if you intend to use this visually.
As for the ES, GSO or cheapie clone eyepieces don't ask I have no idea and you will be taking pot luck IMHO.
Unfortunately eyepiece manufacturers won't state is the field curvature of the eyepiece, and whether it has a modest amount of coma built-in. However, over the years a few points have become abundantly clear:
Most japanese eyepieces (including Pentax, Vixen, Nikon, Masuyama and circle-V) are designed to match refractors, the most popular scopes sold in Japan being around 100mm f/7 give or take 20mm aperture. This means the eyepieces have curved focal planes around 1-2m radius, CONCAVE towards the sky.
Televue officially sits on the fence and says nothing either way. But personal testing suggests all Televue eyepieces include field curvature and some degree of modest coma correction to suit fast newtonians f/4...f/5 typical of the big dobsonians popular in the USA.
Note that the field curvature of refractors is opposite to that of newtonians. Hence eyepieces that suit one won't be a great match for the other.
A few eyepieces appear to have some compensation for coma by design (i.e. negative coma). Vixen has used careful wording which indicates some of its recent eyepieces have some form of compensation for coma, and the behaviour of their LVW eyepieces in fast newtonians does indicate the LVWs certainly do.
There is one exception to the above: The recent advent of fast flat-field refractors used for "imaging" (dreadful word, the correct word is "photography", I suggest) has resulted in the production of a a few flat-field eyepieces to match. These are invariably very clearly marked FLAT FIELD to distinguish them from the rest (which aren't). And yes, I am an owner of a flat-field APO scope.
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Quote:
Originally Posted by Wavytone
In short:
- if you are using a fast dob, buy Televue.
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That's not necessarily correct. I have been using large fast newtonians for a very long time and there are a lot of eyepieces that perform exceptionally well in these scopes besides Televue. For instance the 3.5mm, 5mm, 7mm and 10mm Pentax XW's. These all have inherent negative field curvature which off sets the very small amount of positive field curvature of a large fast newtonian telescope.
The 12.5mm Docter and all of the Nikon NAV HW's work very well in large fast Newtonians. There are plenty of other eyepieces that also work very well, including the 14mm Denkmeier.
The field curvature in a large fast newtonian (from the telescope itself) is very minimal and it's almost flat field, as the field curvature is a function of the radius of curvature (focal length) of the primary optic which is long. Usually much longer than the radius of curvature of a refractor, or catadioptic scope. Irrespective of the sign of the field curvature with a newtonian it doesn't matter all that much because it's small. Where it can matter is when the field curvature of the eyepiece is the same sign and compounds the observed field curvature, as is the case with the 14mm and 20mm Pentax XW's. Both of which I own. Both of which work beautifully in a fast newtonian when used with a paracorr, which is what you should be doing in any newtonian faster than F5.
The field curvature of the telescope is not related to the F-ratio of the telescope. Coma is directly related to the F-ratio of the telescope. For newtonians F5 and faster and maybe even F5.5 and faster, you should use a paracorr. A lot of people don't notice coma with F5.5 to F5 telescopes when not using a Paracorr and think the view is great. However, as soon as you insert a paracorr, the improvement becomes blatantly obvious. In many cases dim threshhold stars near the edge of the field become much more visible due to the reduction in spot size and the fact the light is concentrated into a smaller point.
Quote:
Originally Posted by Wavytone
Unfortunately eyepiece manufacturers won't state is the field curvature of the eyepiece
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The field curvature and astigmatism graphs and data have been readily available for all of the Pentax XW eyepieces for well over a decade. As have the lens configuration diagrams and the light transmission data. See attachments below. You just need to know where to look.
Quote:
Originally Posted by Wavytone
Most japanese eyepieces (including Pentax, Vixen, Nikon, Masuyama and circle-V) are designed to match refractors
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If this statement was true then all of the Pentax XW eyepieces would have field curvature of the same sign. But they don't. As per the graph below 4 of the Pentax XW's 3.5mm, 5mm, 7mm and 10mm all have negative field curvature and the other four, 14mm, 20mm, 30mm and 40mm have positive field curvature.
Quote:
Originally Posted by Wavytone
Televue officially sits on the fence and says nothing either way. But personal testing suggests all Televue eyepieces include field curvature and some degree of modest coma correction to suit fast newtonians f/4...f/5 typical of the big dobsonians popular in the USA.
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If this statement was correct from what you have alluded to right throughout your entire post, then Televue eyepieces wouldn't work well in their own refractors. But the truth is all of their eyepieces work very well in their own refractors. Do you seriously think Televue are going to design and sell eyepieces that don't work well in their own telescopes?
Quote:
Originally Posted by Wavytone
Note that the field curvature of refractors is opposite to that of newtonians. Hence eyepieces that suit one won't be a great match for the other.
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This is not correct. The short focal length Pentax XW's (3.5mm, 5mm, 7mm and 10mm) and just about every single Televue eyepiece and a large number of others including the 12.5mm Docter and both of the Nikon NAV HW's (17mm/14mm and the 12.5mm/10mm) work very well in both fast newtonians and fast refractors. You can also add the 14mm Denkmeier to that list. There are also plenty of others.
To be honest I think you have it wrong in your assessment of the design objectives and parameters of the eyepiece manufacturers. For certain if your assumption was correct all of the Pentax XW's would have field curvature of the same sign and they don't. Half have +ve field curvature and half have -ve field curvature.
Cheers,
John B