Quote:
Originally Posted by Wavytone
Suggest you check http://www.telescope-optics.net/curvature.htm for the telescope types in question the aberrations are analytic to 3rd order at least from the Petzval sum.
Field curvature for refractors and 1 or 2 mirror systems is shown on the above page. The figure in the middle of the page quite nicely shows the situation for refractors vs Newtonians.
There is also a simple reason why this is the case - a simple doublet lens has a focal length that is almost constant as a point object (ie a star) as the object moves off axis the image of it must move in an arc - if it followed a flat plane the focal length must increase with the off-axis angle - which it doesn't - not for a doublet. The whole need for image flatteners arises from this.
For a Newtonian the simplest case is a spherical mirror - lets take a 6" f/8 for example which is near enough to parabolic to not matter. The spherical surface is symmetric about the centre of the sphere which means that if the primary is rotated about the centre to one side, the image MUST lie on a sphere of radius R/2 concentric with the centre of the primary spherical surface, ie concave towards an eyepiece and exactly as shown in the figure.
NOTE: Refractor = convex toward eyepiece, Newtonian = concave towards eyepiece. So in other words an eyepiece that suits one is not a great match for the other. |
I am pleased you quote telescope-optics.net in your attempt at refutation of my claim, I was considering using it as a
supporting reference but thought the technical treatment might put people off.
Yes indeed the Petzval surface of a Newtonian is opposite to that of refractors and SCT's,
however the actual field curvature only coincides with the the Petzval in the absence of astigmatism.
If you read further down the page you refer to, the author says: (my emphasis)
"As mentioned, the presence of astigmatism significantly alters the image field curvature properties. The Petzval surface becomes fictitious, and the actual best image surface becomes the one containing best astigmatic foci. Due to the longitudinal extension of astigmatism, this image is split into sagittal, tangential and best, or median image surface sandwiched in between the first two."
reading still further: (my emphasis)
"For a mirror of radius of curvature R with the stop at the surface, sagittal and tangential image curvatures, respectively, are given by:
which, with RP=R/2, can be written as 1/Rs=0 (implying Rs= ∞), and 1/Rt=-4/R. The median image surface, given by Eq. 33 as one half of the sum of sagittal and tangential curvatures, is 1/Rm=-2/R, equaling mirror focal length (as absolute value; the plus sign implies that it is concave toward mirror, for mirror oriented to the left). In other words, the median surface has the same curvature radius as the Petzval, but of opposite sign." .
All this confirms my contention (and every statement about this aspect of telescope objective field curvature by a reputable optician) : the actual focal surface for a Newtonian is convex towards the eyepiece like refractors and SCT's. This is nicely illustrated in figure 67 on the same page - the actual focal surface is m, between the tangential and saggittal surfaces.
If any further support is needed see http://www.telescope-optics.net/eyep...rrations_1.htm . This page gives an extremely good but rather technical treatment of the interaction between eyepiece and objective. Under the sub-heading Eyepiece Field Curvature the author states: (my emphasis)
"Most objectives generate curvature concave toward objective, and most eyepieces nowadays have near flat field, in which case the combined visual field has curvature similar to that of the objective ...." - which in my opinion sums up this subject.
My last words on this subject are: the best eyepiece is the one you find works best with your telescope. Disregard blanket statements that a particular eyepiece only works well with a certain type of objective.