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Originally Posted by SkyWatch
Thanks for the comments John. I am interested in the idea that eyepieces can perform poorly in an f10 SCT because of the fast primary, but well in a refractor or newt. Do you have references to any studies about this? It would be an interesting read. I am guessing that it is much more an issue for wide-field eyepieces than the more traditional 40-50 degree fields of plossl's, orthos etc?
Empirically I haven't really noticed a major problem with relatively poorer performance in SCT's. For example I (and a couple of experienced friends) tried a 19mm Panoptic alongside a 20mm GSO SV in a range of scopes, including a 8" SCT, an f5 dob (with and without coma corrector), and an f8 refractor. The edge sharpness and useable (sharp) field of each eyepiece in each telescope was surprisingly similar, especially given the huge cost difference. I ended up returning the Panoptic and bought a 18mm Radian which was a huge improvement (giving sharp stars right across the field in the range of scopes), and my friend with the SCT kept using his GSO SV20 as he felt there was no justifiable reason to "upgrade" to the Panoptic in terms of sharpness of the view...
I suggested the GSO's because the IP said that he wanted thoughts about an "8" dob" of eyepieces. I interpreted this to mean low-price but still producing reasonable quality views. The GSO SV's I have used have performed surprisingly well given their price point, so I thought they might fit his requirement.
I hadn't noted the size of the secondary on his scope though: it does make a big difference. I agree that the Vixen LVW would be worth checking out if he wants to spend more. The 24mm Panoptic is also a nice eyepiece (I remember trying one of them at the same time as I checked out the 19mm version, and it performed much better).
All the best.
Dean
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Hi Dean,
I tried to put this into really simple terms and unfortunately it's not that simple. I was trying to get the point across that you can't just make a general blanket statement that because a telescope has a slow F-ratio an eyepiece should perform fine in it. There is a lot more involved than just the F-ratio of the telescope system. Some people presume that if a telescope has a slow F-ratio that most eyepieces, including cheap ones which will not perform in fast scopes, will perform well because of the slow F-ratio and this isn't always the case, particularly, when it comes to the various Cassegrain designs. The overall eyepiece performance in any given telescope system is based on 3 factors:-
1) The aberrations inherent in the telescope
2) The aberrations inherent in the eyepiece
3) The aberrations introduced or removed by the interaction of the particular telescope and eyepiece.
One of the most predominant aberrations is field curvature of both the eyepiece and the telescope. This can be reduced when each component has a different direction of curvature and compounded when they have the same direction of curvature.
Another common aberration is coma of the telescope and to a lesser extent internal coma from the eyepiece.
and another common aberration is off axis astigmatism from the eyepiece. This is always a predominant aberration with cheaper eyepieces and it sometimes swamps and masks all the other aberrations going on at the same time.
With a slow F-ratio telescope the off axis astigmatism is always reduced irrespective of telescope design. This occurs because the angle of incidence of the light rays entering the field lens of the eyepiece is reduced and the light rays enter the eyepiece in a much more parallel fashion, rather than at a steep angle. Internal eyepiece Coma is also reduced in slower telescope systems regardless of design.
An F10 Cassegrain Telescope (irrespective of design) still has the light rays entering the field lens of the eyepiece at the same angle of incidence as any other type of F10 telescope, so it will in fact offer reduced off axis astigmatism, compared to faster newtonians and refractors and cheaper eyepieces like GSO superviews will definately perform better in an F10 SCT than they will in say an uncorrected F5 newtonian, in terms of off axis astigmatism and internal eyepiece coma.
Where it gets complicated is that depending on the type of cassegrain design some of the telescopes aberrations correlate to the F-ratio or the focal length of the primary and not to the overall F-ratio of the system. For instance a traditional Schmidt Cassegrain exhibits Coma and Field curvature correlating to the focal length of the F2 primary and not the overall F10 system. Depending on the eyepiece interaction with the telescope, these can manifest as pretty strong aberrations. With a traditional Dall Kirkham (non corrected) the predominant aberration is off axis coma which again correlates to the focal length of the primary mirror. In slow F-ratio dall Kirkhams (F15 to F20) using a fairly slow primary mirror of say F4 to F5 then eyepiece performance will be very good as coma will not be severe. In say an F8 DK with an F2 primary coma will be severe and eyepiece performance will suffer.
I don't know where you would find any emperical testing data on this. I have done a bit of testing on this over the years and my findings have been consistent with historical optical theory. If you are interested in understanding a bit more about this and other aspects of telescopes, eyepieces and optical performance there are a couple of good books well worth reading. Historically the best resource to learn about this type of stuff was a book called
Telescope Optics - A Comprehensive Manual for Amateur Astronomers by Rutten and Van Venrooij. This was originally released in the late 1980's. My edition is about a 2002 reprint. Whilst this doesn't have information about the latest in eyepiece and telescope designs included it still does an excellent job of explaining a lot of the basic optical principles which still apply with todays modern eyepieces and telescopes. A much more recent release book which is really excellent on how it explains things and also the fact it incorporates all the latest optical designs is
Telescopes, Eyepieces and Astrographs - Design, Analysis and Performance of modern astronomical optics by by Gregory Hallock Smith, Roger Ceragioli and Richard Berry This is a 2012 publication and contains the latest of everything. I highly recommend it.
Cheers,
John B