Ep's Fast or Slow scopes, myth?

[Rogering1]

Hi guys,

you see a lot of discussions stating that that a particular ep is good for slow or fast scopes.

One example is the W.O Swan, it doesn't state what type of scope it is good for, but people confidently comment that it's for slower scopes, is this a myth? or is there a specific piece of information to look for, please educate me !

http://www.williamoptics.com/eyepiecesDCL/swan25_feedback.php

[Moonshane]

I once owned a 33mm SWAN. it was superb in my f8.3 refractor but the view went mushy from about halfway out of centre in the f5.3 dob I had at the time.I'd suggest not less than f6 for the eyepiece. One reason I buy Televue is that they work in all speeds of scope very well - even the plossls.

[JamesF]

Effectively, "fast" scopes alter the light path more than "slow" scopes do and light can reach the focal plane at a much steeper angle to the optical axis. To present a clear image across the field of view is much more demanding of optics in such a situation than it is if the light is not a such a steep angle. Lower quality (or often just lower-cost) eyepieces don't tend to be up to the job in that instance. So, whilst I see no reason why an ep suitable for a fast scope shouldn't work equally well in a slow scope, the converse does not follow.

James

[ronin]

Somewhere it says that WO SWAN's are best on f/6 and slower. May even think it is/was on the WO site.

Know I have read it somewhere and I have the set of WO SWAN's which is why I recall it.

Read the WO site - Eyepieces - SWANS:

They can be used for all types of observing, with all telescope types, but they are recommended for use with focal ratios up to f/6 and above.

[michael.h.f.wilkinson]

An EP capable of correcting the wide light cone of a fast scope is indeed suitable for use in a slow scope, as James states. Some people go further, stating that high cost EPs such as the Naglers are wasted on a slow scope such as my F/10 SCT. I beg to differ. Whilst it is true the difference in performance may be slight, highly corrected EPs do offer better views even in slow scopes. Whether you want to pay for that slight difference is another matter entirely. I only contemplate changing EPs (or any other piece of kit) when I notice the shortcomings. The other consideration I had was that by buying top-notch EPs, I could migrate to faster scopes (such as the F6 80mm, or the Dob I am contemplating) without having to buy new EPs.