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bish

"f5 or faster...."

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As the owner of a fast scope I tend to skip the first part of any  ep review  -  this because they will often go something like 'great ep, really cheap, long eye relief, sharp across entire fov, it will reveal all the secrets of the Universe.....unless your scope is f5 or faster, in which case it's a big pile of pants'.  If the review says at the end  ' and it was the same in my f4.5 scope', then I read the rest of the review (in which case it's probably a TV or Pentax etc)

I just wondered how much difference there is once you get down to f5? Is it uniformly incremental in performance degradation the faster you go, or are f5 mirrors really a 'line in the sand'?  My scope is f4.7 - would my mid range eps perform noticeably better (from  fairly casual observer) in an f5.3 scope? Would they appear much worse in a f4.4 scope? of course it's a very subjective thing, everybody's tolerance to what is 'acceptable' is different. Also I see coma at the edges, which I understand to be from a fast mirror. Presumably a better 'f5' ep would only help the astigmatism, field curvature etc? If your ep does show astigmatism, would you notice it anyway when you mirror shows coma?

 

Thanks

 

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I would love to know of eyepieces that work at even faster F ratios probably at least F4 and maybe F2.8.

Alan

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Think TV's are produced to work down to F3 

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I think the changes are gradual and perhaps where they become noticeable and bothersome a matter of what individuals find OK - we all seem to be a bit different in that respect as you point out.

I think you are also right that eyepiece astigmatism can often mix with scope generated coma with the latter becoming dominant with the best corrected eyepieces.

In the central, say, 50% of the FoV most eyepieces today do very well - even low cost ones seem quite useable in my F/5.3 dob, as long as you keep the target in that central area.

There are other issues such as glare and light scatter that really low cost eyepieces show more of than the better quality ones - those issues can make seeing fine detail and contrast somewhat harder and are probably more detrimental to viewing enjoyment than outer field distortions.

 

 

 

 

 

Edited by John
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Some aberrations rise with the 2nd or 3rd power of the cone angle of the light beam so can rise quite rapidly. My MaxVision 24mm 68 deg EP works fine at F/6, but becomes decidedly grotty at F/4.1 (at the edges, not centre)

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51 minutes ago, John said:

 

There are other issues such as glare and light scatter that really low cost eyepieces show more of than the better quality ones - those issues can make seeing fine detail and contrast somewhat harder and are probably more detrimental to viewing enjoyment than outer field distortions.

 

 

 

 

 

Yes I remember first looking through my old 11mm nag and being amazed at the view compared to really cheap ep. It was more than just a wider fov and sharper stars. 

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26 minutes ago, michael.h.f.wilkinson said:

Some aberrations rise with the 2nd or 3rd power of the cone angle of the light beam so can rise quite rapidly. 

I think I understand what you mean by this. I have heard other people say of eps that they are great at f6 but surprisingly awful at f4.5. I have the 24mm maxvision for my f4.7 dob. Not perfect by any means, but good enough for the cost ( its only really a finder for me)

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Bish,

Myself at F4.3 which I guess is pretty fast I can live with eyepieces like Panoptic's with 68 degree FOV's. I do have a Paracorr though as I just don't see the point of buying TeleVue eyepeiecs and then putting up with coma. That said I have used the Panoptics without it but only them and maybe Delos.

Where I can't live with the effect is when I use either Nagler of Ethos eyepieces which have much larger fields, both to me look much worse and strengthen the Paracorr case. My problem is for years I worked in Qualtiy Assurance and am still remarkable good at picking faults out on almost anything.

Alan

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4 hours ago, alan potts said:

Bish,

Myself at F4.3 which I guess is pretty fast I can live with eyepieces like Panoptic's with 68 degree FOV's. I do have a Paracorr though as I just don't see the point of buying TeleVue eyepeiecs and then putting up with coma. That said I have used the Panoptics without it but only them and maybe Delos.

Where I can't live with the effect is when I use either Nagler of Ethos eyepieces which have much larger fields, both to me look much worse and strengthen the Paracorr case. My problem is for years I worked in Qualtiy Assurance and am still remarkable good at picking faults out on almost anything.

Alan

Hi Alan. I was always very fussy with camera lenses  performance in sharpness, CA, bokeh, vignette get, barrel or pincushion etc, etc. I am a bit more forgiving with eps, probably because it's not s permanent record. I like to use the best I can though - but can overlook small imperfections to an extent. Having said that, my next buy for my f4.7 with no paracor will be a 14mm Delos or 10mm XW. 

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As the owner of an F/4 Rich-Field Newtonian, I've come to think of the eyepiece problems as being similar to the rules for cooking Squid (Cuttlefish): 3, or 30 minutes / lens-elements. Otherwise you'll be eating rubberbands for dinner, in the case of squid.....

In other words, an EP with fewer lens-elements like Orthoscopics and Plössls - tend to be fine in very fast scopes. Or EP's with lots of lens-elements like a TeleVue®. Of course I'm rather over-generalizing the TV's, but you likely know what I mean. My Ortho's work great in my F/4, as does my TV 24mm Panoptic 68°. For the rest of what works and I wish - I'll be needing to perform major-surgery on the thing. But that's another story!

We could use a good list of eyepieces that work the best in F/5 on down.

Dave

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5 hours ago, bish said:

Hi Alan. I was always very fussy with camera lenses  performance in sharpness, CA, bokeh, vignette get, barrel or pincushion etc, etc. I am a bit more forgiving with eps, probably because it's not s permanent record. I like to use the best I can though - but can overlook small imperfections to an extent. Having said that, my next buy for my f4.7 with no paracor will be a 14mm Delos or 10mm XW. 

Bish,

I feel you will be more than happy with either the 14mm Delos which I have owned or the 10mm XW which I have not had my paws on but there are too many good reports for it to be not be good. I too like top camera gear once owning a mind blowing array of Canon L lenses, now I only have a couple but I know just what you mean.

Alan

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18 hours ago, estwing said:

Think TV's are produced to work down to F3 

I've read that too, that Someone tried TV eyepieces with paracorr in his F3 scope, and Al commented that paracorr works. But I'd like to think that the aberration correction is still not up to Al's taste, therefore Televue only states that their eyepieces are verified in f/4 here.

 

19 hours ago, Alien 13 said:

I would love to know of eyepieces that work at even faster F ratios probably at least F4 and maybe F2.8.

Alan

I'd think that F2.8 is very tough. not only astigmatism will be much more, spherical aberration, which affects the ON-AXIS quality, will gone too hgih, since it's propotional to the quadrupple of inverse focal ratio, i.e. spherical aberration in a f3 scope will be 3 times more than in f4. Have a look at the graph in the middle of this page:

http://www.telescope-optics.net/eyepiece_aberrations_1.htm

An eyepiece with best correction of spherical aberration of 1/10 wave P-V at f4, will have more than 3/10 wave P-V at f3, that's worse than diffraction limit.

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35 minutes ago, YKSE said:

I'd think that F2.8 is very tough. not only astigmatism will be much more, spherical aberration, which affects the ON-AXIS quality, will gone too hgih, since it's propotional to the quadrupple of inverse focal ratio, i.e. spherical aberration in a f3 scope will be 3 times more than in f4. Have a look at the graph in the middle of this page:

http://www.telescope-optics.net/eyepiece_aberrations_1.htm

An eyepiece with best correction of spherical aberration of 1/10 wave P-V at f4, will have more than 3/10 wave P-V at f3, that's worse than diffraction limit.

I was thinking more of the use of an eyepiece with a camera lens but you would need a barlow or similar additional elements if you wanted to add a diagonal so the effective F ratio might end up higher.

Alan

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I'm not so sure about the theory thata  barlow extends a scope's focal length, yes, a barlow does diverge the light ray, thus make the eyepiece "to think" that the cope is of longer focal length, a barlow does clean up some edge aberrations, that's because the diverged ray throws away the light in outer edges(with most aberrations), same as possible vignetting introduced by a barlow.

My thoughts, based on reading the link about spherical aberration(SA) above, is simply adding up the SA in each components, i.e. the f3 mirror, the barlow and the eyepiece. Assuming zero SA in the barlow, it'll be the sum of mirror and eyepiece. I'd be glad to be proven wrong if someone has a barlowed f3 scope to compare with a f6 scope.:smiley:

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