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"Smyth Lens" / "Internal Barlow"

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#1 matt

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Posted 31 May 2005 - 12:04 PM

This is not the first time the subject comes up, but a recent post by Lawrence Sayre had me jumping out of the proverbial chair:

"The Tak LE's in general are 5 element 2:1:2 pseudo Masuyama's, with only the 7.5mm and 5mm in the set having the extra 2 element "Smythe" groupings (often incorrectly assumed to be "built in barlows", as this is effectively what they wind up being). "

Very often, one hears about long eye relief eyepieces (Radian, Lanthanum, etc) having "built-in Barlow lenses"; only to read corrections by people who jump in to say these "barlows" actually are "Smythe lenses". Then Lawrence says that Smythe lenses are effectively built-in Barlows.


So...
Is there a difference? Is "Smythe lens/group" the proper term to describe the negative lens group which one uses in long-eye relief, short focal-length designs? Or is there a nuance in the design between an eyepiece using a Smythe lens and one where you just solder a 4x barlow and a 20mm Erfle to get a 5mm long-eye-relief wide field eyepiece? :question:

#2 Scanning4Comets

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Posted 31 May 2005 - 01:52 PM

I imagine that the eyepiece guru would know....Did I hear Mike Hosea ?...Mike will interject with his wits. Go get 'em Mike !!! ...lol
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#3 Brian Carter

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Posted 31 May 2005 - 03:49 PM

I believe that the Smyth Lens is a field flattener and not a barlow.

#4 David Knisely

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Posted 31 May 2005 - 03:49 PM

This is not the first time the subject comes up, but a recent post by Lawrence Sayre had me jumping out of the proverbial chair:

"The Tak LE's in general are 5 element 2:1:2 pseudo Masuyama's, with only the 7.5mm and 5mm in the set having the extra 2 element "Smythe" groupings (often incorrectly assumed to be "built in barlows", as this is effectively what they wind up being). "

Very often, one hears about long eye relief eyepieces (Radian, Lanthanum, etc) having "built-in Barlow lenses"; only to read corrections by people who jump in to say these "barlows" actually are "Smythe lenses". Then Lawrence says that Smythe lenses are effectively built-in Barlows.


So...
Is there a difference? Is "Smythe lens/group" the proper term to describe the negative lens group which one uses in long-eye relief, short focal-length designs? Or is there a nuance in the design between an eyepiece using a Smythe lens and one where you just solder a 4x barlow and a 20mm Erfle to get a 5mm long-eye-relief wide field eyepiece? :question:


Yes, there is a difference. A Barlow will make the angles of the light shallower (i.e. closer to normal incidence), so a less complex eyepiece might have a slightly easier time in dealing with the incoming light without significantly increasing the aberrations that they might show more strongly when used in a shorter f/ratio instrument. However, the main reason for doing this is to shorten the eyepiece's effective focal length without reducing its eye relief. Thus, if you take a cheap eyepiece design like a longer focal length Kellner and stick in a negative purly Barlowish lens in front of it, its effective focal length is shortened, the longer eye relief will be maintained (or slightly extended) and the combination *might* perform a bit better than the original eyepiece alone in areas away from the center of the field.

This is somewhat different than a true "Smyth" lens, a negative lens set which is used mainly in more complex wide-field eyepiece designs such as the Nagler. In optics, it is possible to come up with a set of elements to produce an eyepiece which has significantly reduced astigmatism, but this comes at the expense of introducing a lot of field curvature. To get rid of this, a "field flattener" is used (often called a Smyth lens) ahead of the other elements to get the final field of view flatter than it would be without it. It is designed specifically for the eyepiece rather than existing as some sort of inexpensive "add-on" which might be used in the case of the so-called "built-in Barlow". In fact, since the Smyth field flattener diverges the light rays somewhat, the follow-on elements of the wide-field eyepiece must be made significantly *larger* in diameter to catch that light, which is why many of these eyepieces are so big around. This can be seen with the older version of the Meade 14mm Ultrawide, where the Smyth lens set is only about an inch across, while the next element in the eyepiece is a whopping *two inches* in diameter. Thus, while the forward Smyth lens in wide-field eyepieces is a negative lens like a Barlow, it is not just a "built-in" Barlow, but really a field flattener. Clear skies to you.

#5 Lawrence Sayre

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Posted 31 May 2005 - 03:54 PM

Wow! Perhaps I should have said that they are barlows and a whole lot more? :question: :jump: :roflmao:

#6 Mike Hosea

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Posted 31 May 2005 - 04:06 PM

Actually, I don't know all that much about it because it is a matter of history and semantics rather than something you can work out with math and physics.

My impression is that Charles Piazzi Smyth is attributed with the idea of an integrated field flattener in an eyepiece. Consequently, any pre-focal-plane field group that is designed to match curvature with the post-focal-plane part of the eyepiece is a "Smyth lens".

Although one can find various online biographies of Smyth, none that I have found mentions the Smyth lens. Lord has some mention of the matter in the Brayebrook article, which gives a date of 1874.

Peter Barlow, on the other hand, is attributed with the invention of the achromatic Barlow lens, a device for extending the focal length of a telescope. According to

http://chem.ch.huji....ory/barlow.html

the time of that invention would be somewhere between 1827 and 1834.

It seems proper to me that a negative achromat that is designed both to shorten the eyepiece focal length and to act as field flattener for the eyepiece should be called both a Smyth lens and a Barlow lens. However, one can imagine other field-flattening field group designs that are not based solely on the negative achromat and/or that do not extend the focal length of the telescope appreciably (possibly even reducing it). In such cases the honor to Smyth would still be functionally descriptive, while the honor to Barlow would not. Conversely, a negative achromat field group that is designed solely to shorten the eyepiece focal length without consideration to flattening the field (as would be the case with an eyepiece already having a flat focal plane without the field group) would be a Barlow lens but not a Smyth lens.

#7 David Knisely

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Posted 01 June 2005 - 12:39 AM

The negative achromat in a Smyth set of lenses is designed primarily to introduce field curvature that compensates for the opposite field curvature found in the rest of the eyepiece. The portions of the complete wide-field eyepiece must be designed in a "matching" fashion to function *together* as a unit. Without the Smyth lens, the eyepiece does not function well at all other than perhaps on-axis. I once removed the front Smyth lens of an old 13mm Type-1 Nagler, and the views off-axis got noticably worse off-axis. Clearly, the eyepiece was designed to function well only with the forward smaller negative achromat in the right location relative to the other elements, and with the correct internal field stop in place between the Smyth lens set and the rest of the eyepiece. Again, this is quite different from a "built-in" Barlow, which only shortens the effective focal length of the combination and helps preserve the eye relief while providing higher magnification. If the original simple design had a little field curvature and the built-in Barlow ended up helping it a little, so much the better, but the two were really not specifically designed specifically to *together* eliminate (or at least minimize) the field curvature. By contrast, the negative lens found in the 5-8mm Speers Waler eyepiece *does* act both like a field flattener *and* as a variable power Barlow lens. Clear skies to you.

#8 Mike Hosea

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Posted 01 June 2005 - 02:22 AM

Again, this is quite different from a "built-in" Barlow, which only shortens the effective focal length of the combination and helps preserve the eye relief while providing higher magnification.


While I of course do agree about the practical upshot of Smyth-based eyepiece systems (specifically about not using them without their integral field groups), you seem to be promoting a false dichotomy here. These terms "Barlow lens" and "Smyth lens" are based in part on functional intent, and there is no law of nature that limits the designer to a single intent. My understanding is that the former term is much more specific than the latter, which is quite general, but the main point is that, based on their historical origins, they are not parts of a mutually exclusive classification system. Indeed, the Smyth lens of a Type 1 Nagler also happens to be a built-in Barlow lens, and Al Nagler himself occasionally refers to them as such when the context of the discussion is the effect they have on focal length or some other matter they might have in common with other Barlow lenses. It is also true that the field doublet is a Smyth lens, and the importance of this fact in the academic context of studying eyepiece designs and their properties can hardly be over-emphasized. The practical upshot, however, is simply that the eyepiece, while incidentally having a longer focal length, just doesn't work very well without it.

I think what you wrote above would be fine if you would draw the distinction between Smyth lenses and lenses which are JUST built-in Barlows.

#9 David Knisely

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Posted 01 June 2005 - 03:50 PM

Again, this is quite different from a "built-in" Barlow, which only shortens the effective focal length of the combination and helps preserve the eye relief while providing higher magnification.


While I of course do agree about the practical upshot of Smyth-based eyepiece systems (specifically about not using them without their integral field groups), you seem to be promoting a false dichotomy here. These terms "Barlow lens" and "Smyth lens" are based in part on functional intent, and there is no law of nature that limits the designer to a single intent. My understanding is that the former term is much more specific than the latter, which is quite general, but the main point is that, based on their historical origins, they are not parts of a mutually exclusive classification system. Indeed, the Smyth lens of a Type 1 Nagler also happens to be a built-in Barlow lens, and Al Nagler himself occasionally refers to them as such when the context of the discussion is the effect they have on focal length or some other matter they might have in common with other Barlow lenses. It is also true that the field doublet is a Smyth lens, and the importance of this fact in the academic context of studying eyepiece designs and their properties can hardly be over-emphasized. The practical upshot, however, is simply that the eyepiece, while incidentally having a longer focal length, just doesn't work very well without it.

I think what you wrote above would be fine if you would draw the distinction between Smyth lenses and lenses which are JUST built-in Barlows.


I am not promoting any dichotomy other than to note that if a negative lens is just placed in front of another eyepiece without regard to the system as a whole, its not exactly a Smyth lens. I see this in some of the shorter Ultrascopics which are not wide-field eyepieces but have negative lenses inserted in front to shorten their effective focal length while maintaining the eye relief. In that case, the term "built-in Barlow" might be somewhat applicable. However, even if Al has referred to the forward lens in a Nagler as a built-in Barlow, it does not exactly function like one. It is clearly acting primarily as a field flattener which diverges the light well beyond its original barrel size, which again requires that the follow-on lenses in the eyepiece be larger in order to catch that light. Hence, the eyepiece and the forward negative lens are designed together to work as a unit to get a nice flat field and an eyepiece that is well corrected for off-axis astigmatism. If you take it out, the eyepiece does not perform well due to the field curvature, so in that case, it is functioning mainly as a Smyth lens and not really as a Barlow. Barlows are primarily external devices designed to work with almost any eyepiece, so perhaps calling any lens ahead of the rest of an eyepiece a Barlow (built-in or not) is inaccurate. Clear skies to you.

#10 Mike Hosea

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Posted 01 June 2005 - 06:30 PM

However, even if Al has referred to the forward lens in a Nagler as a built-in Barlow, it does not exactly function like one. It is clearly acting primarily as a field flattener which diverges the light well beyond its original barrel size, which again requires that the follow-on lenses in the eyepiece be larger in order to catch that light.


Your definitions are a little different from mine. I view these appellations, Smyth lens and Barlow lens, as homages to the inventions and ideas of C.P. Smyth and P. Barlow, respectively. I do not care about the modern incidental properties and occurrences--these things are distractions. If you want to decide whether it is appropriate to call a lens by the name of Barlow or by the name of Smyth, you just need to consider whether it is an embodiment of the contributions that these men made. Clearly the field doublet of a Type 1 Nagler is a Smyth lens because it is operating to correct significant field curvature in the upper part of the eyepiece, but it is also a negative achromat with a 2.3x Barlow effect (in the original Nagler eyepiece). I just don't think it's inaccurate to call a negative achromat with a 2.3x effect on telescope focal length by the name of "Barlow" if one wants to. The only cogent argument I can devise to the contrary is that one of Barlow's aims was not to add unacceptable aberration to the the telescope, and in the case of a negative achromat Smyth lens of a type 1 Nagler with a 2.3x effect we are adding a lot of field curvature and doing it on purpose. It's a good point, but it isn't enough, IMO, to take Barlow's name off such a lens and give everything to Smyth.

#11 David Knisely

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Posted 01 June 2005 - 10:00 PM

Mike Hosea wrote:

The only cogent argument I can devise to the contrary is that one of Barlow's aims was not to add unacceptable aberration to the the telescope, and in the case of a negative achromat Smyth lens of a type 1 Nagler with a 2.3x effect we are adding a lot of field curvature and doing it on purpose. It's a good point, but it isn't enough, IMO, to take Barlow's name off such a lens and give everything to Smyth.


I was not at all trying to "give everything to Smyth". I was making the point that a Barlow is mainly an image amplifier and to just stick one internally on some eyepiece without designing its characteristics to compensate for the problems in the follow-on optical elements is not putting in a true Smyth lens. If it is only designed to extend the focal length of the telescope (or to shorten the effective focal length of the eyepiece while not reducing eye relief), then it has more in common with a Barlow than a Smyth lens. If it is specifically designed to compensate for the strong field curvature created by another lens set that is attempting to minimize astigmatism, then it is acting as a field flattener and can be appropriately called a Smyth lens and not a Barlow. Again, I mention the Speers Waler eyepieces, as they use the negative field lens *both* as a Smyth-type field flattener *and* as a way to shorten the effective focal length of the eyepiece without reducing the eye relief (the name W.A.L.E.R. supposedly stands for "Wide Angle Long Eye Relief", although for most Speers Walers, the eye relief is modest rather than long). Clear skies to you.

#12 Mike Hosea

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Posted 01 June 2005 - 11:35 PM

What you're describing fits the whole Nagler design approach as well as a slew of quality eyepieces to follow, and I wonder whether you are giving Smyth too much credit. My impression is that Smyth's idea was in the spirit of prescribing a corrective lens for an existing eyepiece. The early uses of Smyth lenses that I know about through the Brayebrook article, like Clark's, probably don't meet with your modern notion of what constitutes a "true" Smyth lens.

#13 matt

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Posted 02 June 2005 - 01:36 AM

Thanks guys. So I guess the answer is "a bit of both", and thatyou could say that both are correct, as well as say that both are incorrect!

So I guess that "built in Barlow" is not 100% accurate, but not worth banning someone from the forum.

I'll also try editing "Smythe" back to "Smyth"!

#14 Mike Hosea

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Posted 02 June 2005 - 12:49 PM

So I guess that "built in Barlow" is not 100% accurate, but not worth banning someone from the forum.


I think David and I agree at least that "built-in Barlow" carries the wrong connotations if it is a Smyth lens, and it would the wrong denotation as well if the lens weren't a negative achromat with significant power. For example, I cannot imagine that the bulbous field groups of a Type 5 Naglers and the 17mm and 22mm Type 4 Naglers are in any sense "built-in Barlows" even if they have significant negative power. Similarly, and unfortunately for the poor amateur that is just searching for a short way of describing the field group of an eyepiece with an internal focal plane, the label of Smyth lens is unwarranted if it happens to be just a built-in Barlow. This was a point that David was making strongly, and I agree with it.

In a nutshell, it's actually too complicated to work out just by looking. We have good indications, for example, that the threaded in field group of a 5mm Ultrascopic is just a built-in Barlow, but it isn't at all clear whether the field group of a 7.5mm Tak LE, which seems to have about 1.2x or 1.3x magnification factor, is actually a Smyth lens or if it is just a weak Barlow introduced to make a 7.5mm eyepiece of longer eye relief. How are people generally supposed to figure this out? The intrepid can remove the field group and test the modified eyepiece for field curvature, but I don't think people should generally be doing that, and consequently, I think we should be longsuffering on the terminology and get to the point. The point is not usually what the thing should be called, but rather something along the lines of what it does, can it be removed, and can it be combined with a (regular) Barlow.


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