[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.