Quote:
Originally Posted by Auditor
Hey, Ausastronomer, AKA John, I have the following question regarding your great write up. Is the Televue Paracorr needed with the 5mm Pentax XW, 7mm Pentax XW, and or 10mm Pentax XW? Maybe needed is not the right wording to use. Therefore, put another way does the Paracorr improve the views in the above listed eyepieces?
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Hi,
Let me answer by saying that my 2 larger newtonains (14" and 18") are both F4.5 focal ratio telescopes and I do not use the paracorr when I use the 5mm, 7mm and 10mm Pentax XW's in those scopes. Any eyepiece be it a Nagler, ETHOS, DELOS or Pentax XW, in my opinion benefits when used with a paracorr in telescopes faster than F4.5
The 14mm and 20mm Pentax XW's benefit from a paracorr when used in any newtonian telescope.
Let me explain a little bit about the optical theory on why this is. The two predominant aberrations from using a fast newtonian are field curvature and coma. This comes from the telescope itself. An eyepiece can also introduce additional field curvature and off axis astigmatism. The astigmatism gets worse as you go further off axis. Top quality widefield eyepieces like Pentax XW's and Naglers generally have minimal astigmatism. A newtonian telescope has inherent positive field curvature. The amount is solely dependent on the focal length of the primamry mirror. The shorter the focal length the more field curvature. It has
NOTHING to do with the F-ratio of the telescope which is solely a function of the focal length and the aperture. I have attached a graph showing the field curvature of all the Pentax XW eyepieces. As you can see the 3.5mm, 5mm, 7mm and 10mm Pentax XW's have inherent negative field curvature. When used in a newtonian the +ve field curvature of the telescope cancels out with the -ve field curvature of the eyepiece and you get a flat field view. The 14mm to 40mm Pentax XW's have inherent +ve field curvature and when used in a newtonian both field curvatures compound and the field curvature is noticeable to many people. Some peoples eyes have a better capacity to accomodate and adjust for field curvature so it will appear more noticeable to some people than others. Enter the paracorr. The paracorr does two things. It's main goal is to correct for coma. However, in order to achieve this, it necessarily has to flatten the field of a newtonian telescope. It does this by having inherent -ve field curvature built in, which cancels out the +ve field curvature of the telescope. Consequently when an eyepiece is used in combination with a newtonian telescope and a paracorr you are only left with the inherent field curvature of the eyepiece itself, which is usually very small and hardly noticeable. The compounding effect of two field curvatures in the same direction is eliminated.
I hope this explains the benefits of the paracorr and the Pentax XW's a little.
Cheers,
John B