Field curvature in eyepieces

[Andrew*]

Just need a reality check here...

when checking for field curvature in an eyepiece, is it right that you can only be sure it's the fault of the eyepiece if you know that the scope is delivering a completely flat field in the first place? So testing it in a non-astrograph refractor or fast reflector would not be fair as the eyepiece, if completely free of field curvature, would show up the telescope's field curvature.

I'm wondering about the field flatness of the Skymax 127mm - would that be a suitable control telescope?

[Peter Drew]

Good point. In general, long FL telescopes will have a flatter field and would be a better control scope. Field curvature of an eyepiece is very much a design factor, not really fair to call it a fault. Its quite common to find that some eyepieces with opposite "faults" to those of the telescope, actually cancel them out significantly.

[Merlin66]

Picking up on Peter's point....

That's why Klee designed the Pretoria eyepiece for fast newtonians. To correct for coma etc.

[Andrew*]

Good point. In general, long FL telescopes will have a flatter field and would be a better control scope. Field curvature of an eyepiece is very much a design factor, not really fair to call it a fault. Its quite common to find that some eyepieces with opposite "faults" to those of the telescope, actually cancel them out significantly.

Very good point. I would imagine a few manufacturers try to hit a "sweet-spot" level of field curvature to compensate that found in more common observing telescopes.

I also wonder how to isolate the source of astigmatism in a system. I'm not sure if I can really rule out my own eye's astigmatism and whether the 15 minute test at the opticians gives a truly accurate assessment of my eyes, and consequently if my glasses are cancelling that...

There's much more to eyepiece assessment than (ahem!) meets the eye!

Andrew

[Andrew*]

On second thoughts, if the astigmatism is limited to off-axis, does that suggest that the fault is not with my eyes?

[great_bear]

I also wonder how to isolate the source of astigmatism in a system. I'm not sure if I can really rule out my own eye's astigmatism

Eyepiece astigmatism is different to eyeball astigmatism.

With eyepieces, the astigmatism is circular in direction, radiating out from the centre of the eyepiece. So, if you sweep the telescope to place the star at different points around the edge of the eyepiece, the distortion (mini seagull shape, etc. etc.) will rotate accordingly.

If the star's distortion is identical no matter where it is in the eyepiece, (and additionally does not change if the eyepiece is rotated in the focusser), you can be sure that your eye is the source of the issue.

[Andrew*]

I think our posts crossed, and it seems we are in agreement

[great_bear]

Yup