Re: I'm disappointed, why would Tom Back say this?

From: rander3127 (rander3127_at_rogers.com)
Date: 10/17/04 

Date: Sat, 16 Oct 2004 20:03:28 -0400

On 16 Oct 2004 22:47:05 GMT, chris1011@aol.com (Chris1011) wrote:

>>>s there some information to show how a triplet lens canpossibly keep off axis
>stars sharp in a normal telescope?
>>From f4-10, I really don't see how it's possible.Orthos can't, and they have an
>extra element and two extra
>surfaces.
>>>
>
>It's really quite simple to do in any of these simple eyepiece designs, Plossl,
>Ortho, Mono. Normally the designer tries to produce a flat field in the
>eyepiece design with the resultant astigmatism that all Orthos and Plossls
>show. However, if you allow the field to be curved, then astigmatism goes away.
>You get round stars, except that they will be more and more out of focus as you
>go from on-axis to the edge of the field.
>
>A curved field is less of a problem for young individuals because as they move
>their eyes across the field, their accomodation will adjust the focus to some
>degree. Older folks like me with no accomodation will see out-of-focus stars
>with this type of eyepiece. Also, the faster the optical system the more
>evident it becomes. You can minimize the problem by making the field small. You
>could design a common Ortho or Plossl to have no astigmatism over a 30 degree
>curved field.
>
>Roland Christen

Maybe then it's time to be practical about it and accept some off axis
aberration since the eyepieces were designed to image planets to
best advantage? I think some have become too used to 60 deg. plus
non-aberrated FOV's and can't reconcile the true goal of the planetary
eyepieces with their current tastes.

Relevant Pages

  • Re: ortho vs. plossl
    ... Since you are at f/10 then either the plossls or the Orthos would work. ... they are likley to be a simpler 4 element design ... plossl no matter how much Meade claims to the contrary. ... modern eyepiece designs are actually orthoscopic. ...
    (sci.astro.amateur)
  • Re: Get Me Started (again...)
    ... designed for visual use and the higher end models especially tend to come with low profile focusers designed for eyepiece use. ... In order to do imaging with one of these on a platform drive, you will need to push the focal plane further out of the focuser by either shortening the poles on a Truss design, or pushing the mirror forward in a solid tube design. ... For children this is especially difficult as you will need some means to get them up to the eyepiece of larger apertures. ... Give him as much aperture as possible and digital setting circles. ...
    (sci.astro.amateur)
  • Re: Building first dob - questions
    ... > - What's the optimum eyepiece height to design the dob at? ... > with my little Mak-Newt using a drummer's stool, but the dob is going ... So do you try to design it to do most of the ... way of observing at that position, even if the eyepiece is pointing ...
    (sci.astro.amateur)
  • Re: Eypiece quality
    ... Each design has its weaknesses and strengths. ... scopes similar to yours and with their eyepieces. ... just cannot look through an eyepiece with them on. ... If you are into wide field of view with good eye relief than the ...
    (sci.astro.amateur)
  • Re: 1.25" TV plossl vs other 2"
    ... diameter eyepiece barrel. ... While the Plossl is said to be a simple design with generally two ... While it looked nice, and bore some physical resemblence to the Leitz, ... Fast focal length scopes place a heavy demand on certain eyepiece ...
    (sci.astro.amateur)