Telescope Reviews: Field curvature with 19mm Panoptic ?

I think in fast Newts there can be complexities in the combination, scope and eyepiece. The Newt has residual aberrations. It seems fashionable nowadays to point out the least significant--slight field curvature, but of course there is coma and some astigmatism as well. You have to understand first of all that the so-called focal plane of a telescope (or eyepiece) isn't usually a "real" thing. The focal plane is construed to be where, on the whole, residual aberrations least affect the image. Astigmatism in eyepiece and telescope can combine in complex ways, reinforcing or canceling in whole or in part. The result can bias the shape of the focal plane of the entire system. That is to say, you can have a flat-field telescope and a flat-field eyepiece, and the result may not have a flat field if both eyepiece and telescope have astigmatism off-axis. We're not talking about large amounts of astigmatism here, but we're also not talking just astigmatism. There's coma, some curvature, maybe a little collimation error thrown in, and it's small wonder that some eyepieces work better than others just because of how they match up. We haven't even begun to factor in the way exit pupil affects the perception of curvature, nor any complexities arising from figuring errors in the primary or secondary of the Newtonian.

So, I don't know whether a given sample of a 19mm Panoptic might have curvature its designer never intended, but even without that the complexity of the situation does admit the possibility of a 13T6 working great and a 19Pan not working as well with a particular telescope, as seems to have occurred here.

One thing I am increasingly sure of is that facile attempts at blaming coma or curvature of the Newt are never going to answer the questions. Unless Tele Vue (the guys with all the information on Tele Vue designs) wants to do some kind of dissertation on fast Newts without Paracorrs and Tele Vue eyepieces, I think we have no alternative but to respect the empirical results that people get. If they are making their first foray into wider apparent fields, then coma is a prime suspect, since it is more apparent at larger apparent field angles, regardless of magnification. If they are saying that some other eyepiece is unaffected and yet it should be similarly or more greatly affected, then that explanation is significantly weakened. Bear in mind that an experienced observer with an un-Paracorred fast Newt knows what that coma looks like when they are happy with an eyepiece. It's the same coma when they are unhappy but something must be making it worse.

Tales of wondrous performance in dramatically different telescopes can be useful to a point, but more useful would be reports of different performance in a very similar type of scope, as this raises the spectre of sample variation. Likewise, similar performance in similar scopes diminishes that.

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Mike

7" f/6.7 home-built planetary Newt
35mm Panoptic
13mm Ethos
5mm Tak LE
2x TV Barlow
Canon 10x30IS Binoculars