Quote:
Originally Posted by Profiler
I am hoping this is a fairly easy question to answer.
From what I have read the 14mm Pentax XW eyepiece seems to suffer from one reasonably significant design flaw in terms of FC when it is used in a fast Newt (f5 or less) without some type of corrector such as a Televue Parracor etc.
Consequently, my question is (which I hope is simple to answer):-
Does the 14mm Televue Delos also feature the same problems with FC??
OR
Did the folks at Televue utilise some type of different design so the 14mm Delos does not demonstrate the same problems with FC akin to the 14mm Pentax XW
My thanks in advance to whomever can offer some answers on this point.
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Hi Profiler,
It's not a design flaw. It is simply not possible to correct for all aberrations at the same time with any eyepiece design and it comes back to what aberrations the eyepiece designer has chosen to correct best and which aberrations he has compromised. In other words you can't correct for eyepiece coma (different to coma from the telescope), field curvature, astigmatism, angular magnification distortion and rectilinear distortion at the same time. What becomes more difficult to correct and how bad it is then becomes a balancing act and this is further affected by the eyepiece focal length and the desired eye relief. Generally Televue eyepieces are generally well corrected for angular magnification distortion and not so well corrected for rectilinear distortion (the Nagler type 4's are the exception). Pentax eyepieces because of their intended use in spotting scopes are well corrected for rectilinear distortion. If you use a Televue eyepiece (not a type 4) in a spotting scope you will find that the walls of buildings are curved not straight and the waterline of a ship at sea has a significant "bow" in it. Pentax XW's on the other hand will give straight walls and a straight waterline.
The truth is and the easy answer is in any fast newtonian you should be using a paracorr. When combined with a paracorr in a fast newtonian the 14mm and 20mm Pentax XW's perform beautifully and give excellent flat field views. The type 4 Naglers because of their design paramaters also benefit immensely by being used with a paracorr in any newtonian faster than F6.
In answer to your question the 14mm Delos will give a flatter field view than the 14mm Pentax XW when used in a newtonian faster than F5.5 or so. However you should be using a paracorr in that telescope as the view in both eyepieces will be noticeable improved by its use. When used with a paracorr both eyepieces perform as well as each other.
Note that the 14mm Delos "may"
require a special adaptor to reach focus, as it needs a lot of focuser in travel, as does the 17mm Delos.
Cheers,
John B