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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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For two weeks in a row, I've done drift tests on two different stars that are almost exactly on the celestial equator. I used a stopwatch to time the interval between the appearance of the star on one side of the eyepiece field, and its disappearance on the other. I was careful to only count the tests where the stars actually drifted across the center of the FOV, and my individual tests were consequently pretty consistent.
The two stars I used were Struve 1757 in Virgo (a pretty double of 2 arcseconds separation, listed in Burnham as magnitudes 7.5 and 8.5) and a 6.5 magnitude star slightly south of Lambda Ophiuchi (a.k.a Marfik, another pretty pair with a 1.5" separation, curiously not mentioned by Burnham).
I tested 6 eyepieces, 5 of them also using a barlow lens. Part of the reason for the test was to get a more accurate calculation of the power of the barlow -- it's listed as 2x, I ended up measuring it as 2.1x.
The eyepieces were:
32mm TV Plossl
25mm No-name Kellner
12.5mm UO Orthoscopic, the "volcano top"
9mm "Astrola" Plossl
8mm Hyperion
2.5mm Nagler Type 6
I was planning on doing a fairly general review of my subjective impressions of each eyepiece, but instead, I got some unexpected results measuring the fields of view.
Using the drift test to measure the actual field of view, I then multiplied by the magnification to get the apparent field of view. This is simply the inverse of what is often recommended as a means of finding the actual field of view from the listed apparent one.
The Plossls, Kellner and Orthoscopic had measured apparent FOVs that were not too far off from those commonly listed. In fact, they measured slightly narrower.
The TV 32mm Plossl showed an AFOV of about 47.5 degrees, the Kellner about 38.2 degrees, the Orthoscopic about 39.7, and the Astrola plossl came in at 49.9 degrees.
However, the Hyperion in all tests far exceeded its rated 68 degree AFOV. In fact, my measurements clustered tightly at an average of 76 degrees! And optical aberrations were far smaller than they were in any of the narrower-field eyepieces.
When I first did the test last week, I simply didn't believe it. I waited until tonight and redid it, thinking the first one was a fluke. However, the measurements repeated. My 8mm Hyperion seems to be a 76-degree eyepiece.
And the Nagler? Strangely, it didn't make the advertised 82 degrees. In fact, I "only" got 78 degrees out of it! They were a very good 78 degrees -- I saw no change in the star image from edge to center. It simply didn't quite make the advertised field diameter. That was very surprising to me.
In fairness, the Nagler is a NEAF blem, I bought it at a huge discount, and it's more than worth what I paid for it. So I'm not complaining. However, I don't think the FOV difference is related to the blemish, which is nothing but a couple of cosmetic specks near the edge of the field stop.
I'm most surprised about the Hyperion. Not only was the field significantly larger than expected, it compares very well with the Orthoscopic for image quality and transmission. The TV eyepieces are, as has been noted by others, a bit on the "warm side", and the Astrola plossl, while sharp, also exhibits a slight haze. The Kellner is almost useless at my F-5.6 focal ratio, although barlowing improves its edge performance considerably.
I'm wondering if anyone else reports similar results, or if there are indeed big variations between individual eyepieces of a particular make and focal length?
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"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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Lamb0
Carpal Tunnel
Reged: 07/25/07
Posts: 1587
Loc: Fairbury, Nebraska
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 Your results are indeed typical. I'm a little surprised at the Hyperion, but not the Nagler. Using Al's field stop formula - the 2.5T6 with it's 3.4mm field stop, the eyepiece should have a useful A.F. of 77.92 degrees. This is typical of Naglers. 
The Hyperion would require a field stop of 10.6mm - but with so many people playing ring games, an actual measurement may be problematic, and determined by the lenses themselves as there may not even be a field stop, as such. 
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John, "Have eyepiece - will travel!"
8" f/5 Dob w/2.14" sec in a 12" alum tube "The Mortar" - w/PCorr 2.16° TFoV @ ~32.5X 70+% illum *Yes!*
24Pan, 5-8 SW, 3 Faworskis (16.8, 10.5, & 7mm), TMB 3.2mm, Hyp 36mm Aspheric, 20T5, & 14ES100
Other 2": Paracorr (Green Parrot), 2X PowerMate, Antares 1.6X Barlow, Astronomik Hß, (older) Lumicon UHC & OIII
60mm $50 Walmart Special in training - aka "Backpack Observatory"
Binoculars: Minolta Activa 12x50s & Steiner 15X80s
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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I hate to say it again, but it bears repeating: The old TRUE FIELD = APPARENT FIELD divided by the MAGNIFICATION is *only* an approximation (10 percent accuracy usually). There are numerous instances where it just plain falls apart. You *cannot* use the formula backwards to get a very accurate apparent field of view figure! The only way to get accurate figures for apparent fields of view is to actually measure them on the optical bench (there are a few threads on Cloudynights which outline the details, and it isn't terribly hard to do). The apparent field of view is the angle which your *eye* sees when looking in the eyepiece (whether it is in a telescope or not). It is what it is regardless of what any formula or calculation says it is. For true field figures, the drift method is the way to go for certain. For a good formula for true field of view, the field stop formula can give fairly decent results (within a percent or two), but again, you just can't work backwards to get an apparent field of view figure that means very much. You have measured the true field of view that your eyepieces produce in your scope, and that is really all that matters. The apparent field is just the "size of the window" you are looking through, so it often isn't nearly as important as some people make it out to be. Clear skies to you.
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Lamb0
Carpal Tunnel
Reged: 07/25/07
Posts: 1587
Loc: Fairbury, Nebraska
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And THAT is why I referred to a useful Apparent Field. 
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Your results seem to strongly vindicate the concept of the RFOV ("Real Field Of View", formerly known as eAFOV, which stood for "effective Apparent Field Of View"). They also verify that unlike virtually all other eyepieces which have AFOV enhancing and thereby TFOV robbing pincushion distortion, the Hyperions (some at least, and perhaps all of them) have AFOV robbing and TFOV enhancing barrel distortion.
Also, the field stops for the TeleVue eyepieces would indicate exactly the TFOV's that you have measured. A powerful vindication of the field stop method of measuring TFOV, and (as David has indicated already) strong proof that the TFOV = AFOV/Mag method is not very precise (as opposed to TFOV = RFOV/Mag being right on the money,and TFOV = FS/Telescope_FL * 180/Pi also being right on the money).
I'm the worlds first (and perhaps only) advocate of urging eyepiece manufacturers, distributors, and retailers to replace their reporting of the very misleading and generally impractical AFOV with the quite useful and practical RFOV.
Provided that the focal length of an eyepiece is exactly as indicated, the two following formulas are true, and they are also equivalents:
RFOV = FS/Eyepiece_FL * 180/Pi
RFOV = AFOV * (1 + FD)
[where FD = the fractional representation of the aggregate percentage of pincushion and/or barrel type field distortions present within the eyepiece, with pincushion being negative in value, and barrel being positive in value. For example, 4% pincushion distortion would mean that FD = -0.04]
Due to inherent field distortion, this statement is false:
TFOV = AFOV/mag
Whereas this statement is true:
TFOV = RFOV/Mag
So therefore this statement (a third definition of RFOV) is also true:
RFOV = TFOV * Mag
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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For the two Baader eyepieces which just happen to hail from the same manufacturer (specifically the Genuine Ortho's and the Eudiascopics) Baader reports the RFOV, but sadly they quite confusingly report it under the term AFOV.
As a result, where U/O reports the AFOV of their HD Ortho's as 43 degrees, Baader reports this as 40 degrees for their essentially identical Genuine Ortho's. Likewise where Orion reports the AFOV for their Ultrascopics as 52 degrees, Baader reports the AFOV for their essentially identical Eudiascopic's as 45 degrees or so.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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sabir
professor emeritus
Reged: 06/23/07
Posts: 731
Loc: Pune (India)
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This would be really interesting when applied to the 13mm Ethos .... 
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No one can attain good livelihood-even if he is so lucky - without passing through misfortunes. Days will usurp the opportunities of those who waste today's opportunity due to their expectation of tomorrow's opportunities. Day's custom is the usurpation of opportunities and the habit of time is wasting them.
Imam Ali (a.s)
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Actually the RFOV of the 13 mm Ethos can be calculated, since TeleVue gives us the field stop as being 22.3 mm.
RFOV = FS/Eyepiece_FL * 180/Pi
RFOV = 22.3/13 * 57.296
RFOV = 98.28 degrees
This tremendous result would indicate that pincushion distortion (rectilinear distortion, or the bending of straight lines) is quite well controlled for the Ethos, and this means that by default it must have predominantly angular magnification distortion instead (whereby objects do not retain a common magnification [size] across the field if view). With a field that wide, one would likely never notice that Saturn for example is perhaps 5% or 6% different in size from center to edge, and since stars effectively do not change in size (having no effective size to magnify by virtue of being "point sources"), this effect would never be noticed for them.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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In addition to accounting for the rectilinear distortion present within the eyepiece, the RFOV also corrects for the field of view modifying effects of the "real" focal length of an eyepiece as opposed to the focal length under which it is marketed (and upon which it is stamped). I believe this is why Baader's AFOV (actually RFOV) figures for the Eudiascopics are so far off from Orion's 52 degree AFOV for the Ultrascopics. The Eudiascopic (and Ultrascopic) eyepieces not only have inherent pincushion, but their focal lengths are also likely not as indicated (being in all instances somewhat less than indicated). This also could be a significant factor for the Hyperions TFOV vs. AFOV. Ditto for the Ethos. If in fact the Ethos exhibits noticeable pincushion of greater than about 1.7%, then its real focal length is likely not 13 mm.
People have often wondered why the pseudo-Masuyama's from Celestron (the original Ultima's) were sold as for example 12.5 mm, 18 mm, and 24 mm, whereas the others all sell them as 15 mm, 20 mm, and 25 mm respectively. Hmmm???
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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sabir
professor emeritus
Reged: 06/23/07
Posts: 731
Loc: Pune (India)
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Thanks Lawrence 
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No one can attain good livelihood-even if he is so lucky - without passing through misfortunes. Days will usurp the opportunities of those who waste today's opportunity due to their expectation of tomorrow's opportunities. Day's custom is the usurpation of opportunities and the habit of time is wasting them.
Imam Ali (a.s)
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Here's a formula to calculate eyepiece field distortion (FD), given that the eyepieces field stop (FS) and focal length (FL) are reliably known and trusted:
FD = [(FS/FL * 57.296)/AFOV] - 1
To represent FD as a percentage, just multiply the result by 100 and take the absolute value (ABS) of the result.
For the example of the 24 mm Panoptic (FL = 24 mm, FS = 27 mm):
FD = [(27/24 * 57.296)/68] - 1
FD = -0.0521
The percentage of pincushion (plus any FL induced error, if present) is therefore:
ABS(100 * -0.0521) = 5.21%
And since this is pincushion distortion (FD is negative), we can conclude that the TFOV for this eyepiece will be 5.21% less than would be indicated by the old formula:
TFOV = AFOV/Mag
PS: For those who are confused by the term "distortion", the one thing that needs to be made apparent is that pincushion, barrel, and/or angular magnification distortions do not involve such things as sharpness, resolution, contrast, transmission, color purity, lateral color, ghosting, etc... so they are not generally what first comes to mind when one thinks of image "distortion".
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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This makes for an interesting discussion. I suppose the barrel distortion in the Hyperion could explain my results. What I'll need to do next is check out the drift at different distances from the center and look for variations.
It's not at all obvious to the casual eye that it's there, and certainly doesn't seem to visually distort any extended features. I'm also curious about what angular magnification distortion might be present in either of the Hyperion or Nagler. From my visual impressions so far, I think I'm willing to put up with a subtle barrel distortion in order to pick up a wider true field of view. I guess it wouldn't be such a good choice for astrometry, though.
What surprises me most is how people seem to think that apparent field of view is such an obvious and objective characteristic. It's not for me. Peripheral vision is a funky thing, and in wide field eyepieces, that comes into play at the edge of the field. It's actually hard for me, at times, to see a difference in the apparent surface area of the field between the Hyperion and the Nagler. I have to look closely and move my eye around to confirm that the Nagler is wider.
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"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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It just dawned on me that if manufacturers (distributors, retailers, ...) began reporting the RFOV for each of their eyepieces, in conjunction with the real focal length (to at least 1/10 mm), then we would immediately have the means whereby to compute the exact field stop diameter for the eyepiece, be it directly measurable, or only "effective" (due to the problem of the built in Barlow).
The precise field stop measurement for any eyepiece would be directly available from the equation:
FS = (RFOV * Eyepiece_FL)/57.296
The value of knowing the RFOV would simply be huge. No actual field stop measurement would ever again be required. Also there would be no need to ever again drift test for precise TFOV. In fact you could say that knowledge of an eyepieces RFOV would be "Majestic"!
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Lawrence Sayer posted:
Quote:
Your results seem to strongly vindicate the concept of the RFOV ("Real Field Of View", formerly known as eAFOV, which stood for "effective Apparent Field Of View").
Well, I am afraid that the definition of the apparent field of view is as I stated. There is no "Real Field of View", or "effective apparent field of view", as those are artificial concepts used to attempt to make the old formula for true field appear to be more accurate than it actually is. The only "real" fields are the actual angles measured for the angular span that the eye sees in the eyepiece (the Apparent Field of View), or that the observer measures on the actual "real" sky (the True Field of View). One should either use the old formula and accept its "ballpark" results, use the field stop formula, or just go ahead and measure the true field of view in a telescope and forget about formulae altogether. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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I don't think it's an artificial concept at all. It's a real characteristic of a specific eyepiece. When all distortions and aberrations are taken into account, the Hyperion 8mm will perform as a 76 degree eyepiece, and the UO Orthoscopic will perform as a 39 degree eyepiece.
Once that measurement is done, I can insert the eyepiece into a scope of a different focal length and predict what the true field will be.
Is that incorrect?
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"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
I don't think it's an artificial concept at all. It's a real characteristic of a specific eyepiece. When all distortions and aberrations are taken into account, the Hyperion 8mm will perform as a 76 degree eyepiece, and the UO Orthoscopic will perform as a 39 degree eyepiece.
Once that measurement is done, I can insert the eyepiece into a scope of a different focal length and predict what the true field will be.
Is that incorrect?
Well, it will not be a number that corresponds to something which is physical, well understood, and can actually be measured. More importantly, it will not be the angle which your eye sees when it looks into the eyepiece (the definition of "apparent field of view"). It will be some number which, by jumping through various mathematical hoops, may or may not be able to accurately predict the true field of view. This is why I like the field stop. It is a physical dimension of the eyepiece that can be measured and that gives reasonably accurate values for the true field (within two percent, which is more than accurate enough). Tele Vue cites it in their eyepiece data for this very reason. I measured the field stops and apparent fields for all 12 of my eyepieces. The true fields of view predicted by the field stop formula all came within about 1.2 percent of the actual true field produced when the eyepieces were in the telescope. The apparent field of view figures for the eyepieces yielded AFOV/Mag results which were up to 6.6 percent off of the actual true field values. In general, the field stop formula seems to be, on average, up to five times more accurate at predicting the true field of view than the old AFOV/Magnification formula is. With the "generic" or inaccurate AFOV figures given by some retailers, the accuracy of the old formula can be even worse.
The field stop diameter very well *should* be the standard for eyepieces, although the apparent field of view is something people might like to look at to see how much of a span they will see when they look in the eyepiece. I see little reason to "calculate" some odd-ball "real apparent field of view" that pulls in the field stop number to the equation, when the simple field stop formula gives great results to begin with. Introducing some "fudged" apparent field is just unnecessary and potentially confusing. Clear skies to you.
Edited by David Knisely (05/18/08 03:15 AM)
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jcjr
professor emeritus
Reged: 01/06/08
Posts: 563
Loc: TN, USA
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David, I haven't looked thru a lot of eyepieces, but found interesting that the Hyperion 8mm might have a wider view than advertised--
I have a 38mm Q70, supposedly 70 degree AFOV, but every time I look thru that thing, the AFOV seems huge. It gives the subjective impression of being much wider than some cheapie 80 degree AFOV UWA's I have, and that 38mm Q70 feels drastically wider than a 66 degree AFOV Expanse.
I might buy a 26mm Q70 to see if it gives the same 'huge' subjective impression as the 38mm Q70. Maybe the 38mm Q70 just subjectively seems wide because of the huge eyelens, but it is a very repeatable subjective impression. Every time I look thru the 38mm Q70, it just slaps me in the forehead, "Boy, that's an awfully wide view!"
Is the large eyelens the likely explanation? Or maybe the big exit pupil?
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CPC 1100, C102SLT, SV F80, Meade 70 & 60 AZT
Q70 38mm, Pan24, Meade 5K 18mm UW, Axiom LX 15mm, 10mm, 7mm, Nagler 13T6, Expanse 20mm, 9mm, 6mm, BO/TMB 5mm, 2.5mm
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Benjamin B
Pooh-Bah
Reged: 06/21/04
Posts: 1101
Loc: Sweden
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I have the 26 clone and the AFOV is very large. I also get that huge impression when I use it. By the way, its one of my favorite eyepieces, really sharp.
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G11/Gemini
TMB 80/480 super apo
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Quote:
(1) Well, I am afraid that the definition of the apparent field of view is as I stated. (2) There is no "Real Field of View" ...
(3) Well, it will not be a number that corresponds to something which is physical, well understood, and can actually be measured.
Note: '(1)' and '(2)' and '(3)' have been added by me to the above quotes so I can reference them below.
(1) This is precisely why the RFOV must not be called the AFOV (which is the mistake that Baader seems to have made if my assumption regarding their reported values is correct).
(2) That Baader does "apparently" report the RFOV (albeit with the just stated caveat) for two of its eyepiece series indicates (by my admittedly broad and sweeping extension) that it is in fact a real quality of every eyepiece. The concept is practical and can be used to calculate highly correct results, so it is clearly not purely fictitious and arbitrary in the fashion of the popular "Majesty Factor".
I will however grant the concession that the reporting of an eyepieces RFOV in no way means that the eyepieces AFOV should not also be openly reported. In fact with both numbers reported, loads of detail can be calculated regarding the inherent distortions present in the design, so both must be reported.
(3) As to directly measurable being the end all criterion for being real, I give you string theory, dark energy, and dark matter. Their effects are perhaps measurable, though they themselves are directly undetectable. They can however be mathematically expressed to explain real world (OK, universe) phenomenon. The RFOV can likewise be mathematically expressed to explain real phenomenon. The only difference is that the former can not be precisely stated (the +/- 3-sigma being huge), but the latter can be precisely stated (the +/- 3-sigma being small).
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Another quite useful outcome of the reporting of RFOV would be that it could be used to actually quantify the presently somewhat loose and arbitrary term known as "orthoscopic". Eyepieces with some specific degree of close affinity between their AFOV and RFOV would by this means definitively BE orthoscopic, and eyepieces wherein the AFOV and RFOV deviate by more than this (as yet not pinned down) "orthoscopic affinity value" (to be called the eyepieces stated 'OAV' perhaps? ) would by definition NOT BE orthoscopic.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Thought experiment (with no correct answer expected, only my open philosophical pondering):
If TeleVue had come out with the reporting of the RFOV for all of its eyepieces, would they have met with the same level of resistance to the concept that I have experienced, or would it immediately be hailed as a great development?
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Greg K.
Reged: 12/11/03
Posts: 12019
Loc: Clifton Park, NY
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Quote:
I will however grant the concession that the reporting of an eyepieces RFOV in no way means that the eyepieces AFOV should not also be openly reported. In fact with both numbers reported, loads of detail can be calculated regarding the inherent distortions present in the design, so both must be reported.
You could perhaps get an idea of the linear distortion present in the eyepiece. However, it seems like this could unfairly stigmatize eyepieces that correct for, say, astigmatism by using a non-orthagonal field.
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Astro-Tech AT111EDT f/7 - Celestron CGEM
NexStar 11 GPS
Orion SkyView Pro 8EQ (w/ Autostar mod)
15x70 Celestron SkyMasters
Orion 90mm Mak
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Good point! I thought of this, but OTOH there is (I would hope at least) likely no one who is not by now aware of the highly non orthoscopic character of the Nagler's and Panoptics, and this certainly has not stigmatized their very well deserved reputations. I therefore rapidly concluded that there is simply no valid stigma associable with being either orthoscopic or not orthoscopic, so I completely discounted this potential.
Regarding the term orthoscopic (or the misnomer, Ortho with a capital 'O'), my belief is that a quite high percentage of eyepiece buyers who desire this feature do not even realize that the feature itself has little (to perhaps nothing) to do with what they think it does. In making this statement I am assuming that the average uninformed eyepiece buyer who desires an "Ortho" does so because they believe that the term orthoscopic implies sharpness, contrast, transmission, color purity, etc..., when in fact the term orthoscopic has zero relationship to these optical qualities. I.E. they haven't got a clue.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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andydj5xp
professor emeritus
Reged: 05/27/04
Posts: 661
Loc: 52.2693 N/10.5707 E
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Quote:
Good point! I thought of this, but OTOH there is (I would hope at least) likely no one who is not by now aware of the highly non orthoscopic character of the Nagler's and Panoptics, and this certainly has not stigmatized their very well deserved reputations. I therefore rapidly concluded that there is simply no valid stigma associable with being either orthoscopic or not orthoscopic, so I completely discounted this potential.
The term "highly non orthoscopic" sounds a bit negative and is kind of inappropriate for an ultra-widefield eyepiece. They are normally designed for small angular magnification distortion (AMD) which - as the apparent field gets much larger than about 60° - is by definition not orthoscopic. They are simply not ment to be orthoscopic. AMD means constant focal length from axis to the edge which will invariably bend outwards straight lines at the edge of field. For astronomical observations this will be no disadvantage while OTOH other advantages will be achieved. Orthoscopic means increasing focal length from axis to the edge which will keep the lines straight but will squeeze the outer part of the field.
Orthoscopic without distortion will give AFOVs as
AFOV = 2arctan(FS/(2*FL)) called also tan-formula
and AMD without distortion will give
AFOV = (FS/FL)*57.3° called also arcus-formula
with FS = field stop diameter and FL = paraxial focal length of the eyepiece.
Distortion will of course change the AFOVs more or less and since it is not normally known it is better to rely upon the FS which can be determined quite easily (either by star drift or by observing a tape measure).
Andreas
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TEC140 (#216)
Leica 17.8...8.9mm ASPH Zoom
also barlowed 1.5x/2.0x/2.5x with Baader VIP Modular Barlow
Zeiss AOII set with Zeiss Abbe 2x Barlow
WO UWAN 28mm
AYO AltAz mount on a Berlebach UNI19 tripod
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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Quote:
The field stop diameter very well *should* be the standard for eyepieces, although the apparent field of view is something people might like to look at to see how much of a span they will see when they look in the eyepiece. I see little reason to "calculate" some odd-ball "real apparent field of view" that pulls in the field stop number to the equation, when the simple field stop formula gives great results to begin with. Introducing some "fudged" apparent field is just unnecessary and potentially confusing. Clear skies to you.
OK, I get what you're saying here. Not all eyepieces have a directly measurable field stop, however. So what the drift test gives me is a kind of "virtual field stop" number that I can use to predict true field of view for a given eyepiece in any telescope.
That seems rather arbitrary too, but at least I see how it can be tied to something physical -- even if it's just imaginarily so.
--------------------
"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12486
Loc: Los Angeles
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Quote:
I don't think it's an artificial concept at all. It's a real characteristic of a specific eyepiece. When all distortions and aberrations are taken into account, the Hyperion 8mm will perform as a 76 degree eyepiece, and the UO Orthoscopic will perform as a 39 degree eyepiece.
Once that measurement is done, I can insert the eyepiece into a scope of a different focal length and predict what the true field will be.
Is that incorrect?
No, the Hyperion's apparent field will not be 76 degrees, i.e. the eye will not see an apparent field that subtends 76 degrees. Your true field measurement cannot be backed into apparent field without taking distortion into account.
The apparent field is probably the 68 degrees claimed, and the true field is what you measured. The compensatory factor required to reconcile the two is distortion.
An example of how timed passage of stars cannot derive an apparent field, for exaggeration purposes, is a sphere.
Let's say you're in space, watching the Earth turn. You time the passage of a city as it passes from one "edge" to the other. The timed passage (12 hrs) indicates the width of the Earth is wider than you see it. How? Well, during a lot of the time you watch the passage, the city is not moving directly sideways relative to your eye but away and toward your eye.
It moves slowly when it first appears, moves quickly as it passes the center of the globe, and then slows down again.
The passage of a star across the field of an eyepiece is a lot like that. It doesn't move at a uniform speed, but changes according to the amount of distortion. Thus, timing a star will give you the true field, but it will not give you the apparent field unless you know the distortion factor.
The way I look at it, true field is one thing, apparent field is another. And there's no easy way to derive one from the other.
DonP
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Quote:
The way I look at it, true field is one thing, apparent field is another. And there's no easy way to derive one from the other.
DonP
Don, have you read my posts in this thread? The RFOV is a quite easy way to bridge the gap between AFOV and TFOV. All we need is to encourage its adaptation by the manufacturers, distributors, and retailers. In fact, TeleVue has given us all of the requisite tools required already.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12486
Loc: Los Angeles
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Larry,
True, but I don't see that happening when the "convention" is so entrenched.
Frankly, though, I'd settle for the posting of apparent field and eye relief which the majority of low-priced eyepiece sellers somehow neglect to mention on their websites.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
Quote:
The field stop diameter very well *should* be the standard for eyepieces, although the apparent field of view is something people might like to look at to see how much of a span they will see when they look in the eyepiece. I see little reason to "calculate" some odd-ball "real apparent field of view" that pulls in the field stop number to the equation, when the simple field stop formula gives great results to begin with. Introducing some "fudged" apparent field is just unnecessary and potentially confusing. Clear skies to you.
OK, I get what you're saying here. Not all eyepieces have a directly measurable field stop, however. So what the drift test gives me is a kind of "virtual field stop" number that I can use to predict true field of view for a given eyepiece in any telescope.
That seems rather arbitrary too, but at least I see how it can be tied to something physical -- even if it's just imaginarily so.
For one eyepiece in particular, I have had to use this approach: the 5-8mm Speers Waler. Its variable effective focal length due to the changing location of the Smyth "field flattener" lens out in front meant that there was no way to actually measure any field stop. The only thing I could do was to measure the true fields at the ends of the focal length numbers and then work "backwards" to arrive at two "effective field stop" figures for the shortest and longest focal lengths of the eyepiece. However, this was far better than what the *company* actually did. They worked backward using the AFOV/Mag formula and got apparent fields that varied and were just plain silly to begin with (well beyond what the eyepiece actually provides). The apparent field of view of that eyepiece was actually fixed at a constant 79 degrees no matter how the eyepiece was set in focal length, so the company's claims were just plain ridiculous. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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I found a field stop listed as 10.7mm for the Hyperion 8mm on a pdf linked from the Alpine Astronomical page. This is close to what I calculated using a derivation of Lawrence's forumla RFOV(nonexistent as it may be) * fl / 57.3 = Field Stop.
Using his formula for distortion, there is a 12.7% positive distortion for the Hyperion, and a 4.9% negative distortion for the 2.5mm Nagler. Knowing this, the drift test results I got aren't "strange" at all. I do, however see the utility of the "RFOV" concept. it allows you find an equivalent field stop and predict the true field of view of an eyepiece in any Telescope, even if you don't have access to the written specifications. And you don't have to disassemble the eyepiece to do it, you only have to use it.
If you generalize this, you realize that a 68d Hyperion and a 82d Nagler of the same focal length should share almost the same total field of view!
Looking at it this way, I gain extra respect for both the Hyperion AND the Nagler: The Hyperion shows sharp detail and excellent transmission over a very wide field, as does a Nagler, but does it at 1/3 the price.
So what makes the Nagler superior? Two things:
1. There is less overall distortion in the field of view of the Nagler. Less than half as much. Views of extended objects are likely to be more acurate.
2. The nature of the distortion is such that in the Hyperion, objects would appear to drift more slowly near the edge of the field, and speed up as they pass through the center. It would be just the opposite in the Nagler. Objects would drift more quickly into and out of the field at the edges, but would spend more time near the central area of the field where they are most easily seen.
This thread has been quite the learning experience!
--------------------
"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12486
Loc: Los Angeles
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Just a note,
With Lawrence Sayre's help, I've added a calculated field stop for every eyepiece to my latest version of the Eyepiece Buyer's guide. I also added a column that will automatically calculate the magnification for every one of the over 1500 eyepieces if you merely type the focal length of your scope in mm into the column header for that column. If you change scopes, merely type in a new focal length and the chart will recalculate every eyepiece.
If anyone would like a copy of that spreadsheet before it's posted here on CN, send your e-mail address to me in a private message. Give me a day to respond since I work long hours.
Don
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 4350
Loc: "Metrowest" Boston
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Quote:
I do, however see the utility of the "RFOV" concept. it allows you find an equivalent field stop and predict the true field of view of an eyepiece in any Telescope, even if you don't have access to the written specifications.
Here are calculated "RFOVs" for the various Nagler eyepieces: 77.6, 77.1, 81.0, 78.5, 81.9, 79.1, 77.6, 81.6, 77.6, 78.9, 79.4, 80.2, 78.6, 77.9. How are these numbers easier to use in the field or elsewhere than the effective field stop diameters that Tele Vue quotes?
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Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 36mm QX, 20mm 5K SWA, 13mm Ethos, 9mm BGO, 6mm ZAO-II, 5.1mm XO, 2x TV Barlow
- Filters: Baader M&S, 6-piece color set, ND.6, ND.9
- 120mm f/8.3 home-built grab-n-go Newt with 25mm Tak Ortho + GSO 3x Barlow
- Binoculars: 15x50 image stabilized, 12x50 roofs
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
Quote:
I do, however see the utility of the "RFOV" concept. it allows you find an equivalent field stop and predict the true field of view of an eyepiece in any Telescope, even if you don't have access to the written specifications.
Here are calculated "RFOVs" for the various Nagler eyepieces: 77.6, 77.1, 81.0, 78.5, 81.9, 79.1, 77.6, 81.6, 77.6, 78.9, 79.4, 80.2, 78.6, 77.9. How are these numbers easier to use in the field or elsewhere than the effective field stop diameters that Tele Vue quotes?
As far as I know, they are not easier. Maybe it is because people almost always see the old TFOV = AFOV/Mag formula and then get hung up on using that particular equation rather than the somewhat lesser-known but more accurate TFOV = 57.3*EFSD/Fl formula (EFSD is eyepiece field stop diameter and Fl is telescope focal length). Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
David, I haven't looked thru a lot of eyepieces, but found interesting that the Hyperion 8mm might have a wider view than advertised--
I have a 38mm Q70, supposedly 70 degree AFOV, but every time I look thru that thing, the AFOV seems huge. It gives the subjective impression of being much wider than some cheapie 80 degree AFOV UWA's I have, and that 38mm Q70 feels drastically wider than a 66 degree AFOV Expanse.
I might buy a 26mm Q70 to see if it gives the same 'huge' subjective impression as the 38mm Q70. Maybe the 38mm Q70 just subjectively seems wide because of the huge eyelens, but it is a very repeatable subjective impression. Every time I look thru the 38mm Q70, it just slaps me in the forehead, "Boy, that's an awfully wide view!"
Is the large eyelens the likely explanation? Or maybe the big exit pupil?
Hi there. Well, visual appearance does have a lot to view with the impression one gets about the apparent field. I think the only way to know for certain whether the eyepiece apparent field of view figures are accurate is to just go ahead and measure the apparent field of view of the eyepiece. Here is how it is done:
MEASURING THE APPROXIMATE
APPARENT FIELD OF VIEW OF AN EYEPIECE
MATERIALS: 1. A Meterstick, Yardstick, or other linear device whose length is accurately known, which can be hung vertically on a wall, and whose exact middle or center is accurately marked. This could also be a narrow strip of paper of known length with its exact middle and ends marked clearly. This object will be known as the observing "target".
2. A method of holding and properly supporting an eyepiece rigidly in a horzontal position (like a bracket attached to a camera tripod), but which can be manually moved towards or away from a measuring target.
3. A tape measure.
STEP #1: Mount the vertical "target" (ie: the Yardstick or its substitute) on the wall so that its exact middle is will be about same height above the floor as the center of the eyepiece. For a meter stick, the midpoint will be the 50cm mark, and for a yardstick, it will be the 18 inch mark. Mark this midpoint with a visible marking like a small piece of tape or a black felt tip marker, so the middle can be easily seen from a distance.
STEP #2: Mount the eyepiece at a height above the floor which is exactly the same as the mid-point of the target, so that the observer can look into the eye lens with the eyepiece optic axis or barrel horizontal and parallel to the floor. Make certain the eyepiece is as horizontal as possible, and that it can be easily moved towards or away from a nearby wall from as little as two feet from the wall to as much as six feet away.
STEP #3: place the eyepiece straight out from the wall from where the observing "target" is located. Look into the eyepiece with *both* eyes open and merge the images of the eyepiece field of view and the target. Make the center of the superimposed eyepiece field centered on the mid-point mark of the observing target as closely as possible, and keep your head level with the floor (ie: keep your eyes at the same height above the floor).
STEP #4: Look at the top and bottom of the target, again with both eyes open. Try to make the top and bottom edges of the eyepiece field match the top and bottom edges of the target on the wall by carefully moving the eyepiece towards or away from the wall. Make certain when moving the eyepiece that it remains pointed exactly towards the center of the observing target, and that its height above the floor does not change. Once the edges of the eyepiece field match the top and bottom of the target, take the tape measure and measure the distance from the back of the eyepiece just beyond the eye lens (ie: where your eye was sitting when you were looking through the eyepiece) to the middle of the target on the wall. If the target has a length of "2Y" and the distance to the wall you measured is "D", then the apparent field of view of the eyepiece is then AFOV = 2*ATAN (Y/D), where Y is *half* the total length of the target and ATAN the arc-tangent (or inverse tangent) function. For example, if you were using a yardstick (36 inches in length, or Y = 18.0 inches) and your eyepiece field matched its length at a distance of 37.0 inches from the center of the target, the apparent field of view of the eyepiece would be about 51.9 degrees. Measure the apparent field several times and take an average of the measurements for the most accurate results.
Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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jcjr
professor emeritus
Reged: 01/06/08
Posts: 563
Loc: TN, USA
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Thanks David. That is an excellent straightforward method, though it wouldn't directly work for me since I'm blind in one eye (grin). Think I understand the principle of the procedure though-- Use one eye direct-viewing to match the unfocused illuminated field stop seen thru the other eye, and use the ruler and trig to calc the subtended angle.
If I REALLY want to measure it, could probably kludge up some half-silvered mirror or prism gadget that would give the same result with one eye...
Apologies, my original question was probably drifting the thread too much. Psych perceptions are so dependent on set and setting, and I'm willing to believe that a Q70 AFOV really is in the ballpark of 70 degrees. If it were substantially greater, then they would likely have taken marketing advantage by naming it a Q80 or Q90 (grin).
I love science, and make a living on technology, but my reason for looking thru scopes is to see the pretty pictures. Since the 38mm Q70 seems to make a prettier, more immersive picture than some other eyepieces which have similar or bigger published AFOV-- I was just curious what eyepiece specs one would look for, since the AFOV spec doesn't seem to be the one that matters. If that makes sense. I don't know much about it.
Maybe there is something similar about the hyperions which makes them popular-- Some subjective pleasant immersive experience that isn't described by the AFOV alone. I've not looked thru hyperions, but there must be something special that makes them more gratifying than other budget eyepieces which have similar AFOV?
There are quite a few EP lines with 65 degree or better AFOV, but some of them just feel like you are seeing a wide view thru a tiny peephole.
Thanks
jcjr
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CPC 1100, C102SLT, SV F80, Meade 70 & 60 AZT
Q70 38mm, Pan24, Meade 5K 18mm UW, Axiom LX 15mm, 10mm, 7mm, Nagler 13T6, Expanse 20mm, 9mm, 6mm, BO/TMB 5mm, 2.5mm
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Quote:
Here are calculated "RFOVs" for the various Nagler eyepieces: 77.6, 77.1, 81.0, 78.5, 81.9, 79.1, 77.6, 81.6, 77.6, 78.9, 79.4, 80.2, 78.6, 77.9. How are these numbers easier to use in the field or elsewhere than the effective field stop diameters that Tele Vue quotes?
They do have a significant advantage in that they speak directly to the observer, and they are directly valid and true in the equation:
TFOV = RFOV/Mag
The simplicity and intuitiveness of this trumps calculations and comparisons involving field stop numbers. Mike, you and I are in professions where complex calculations are a daily happening. Most others are not. We must attempt to see the world though their glasses.
Back to how the RFOV speaks to the observer directly and intuitively. I offer an example using the findings of the first poster to this thread. He observed that the RFOV of the 68 degree AFOV Hyperion and the RFOV of the 82 degree Nagler were virtually one and the same. He thereby intuitively knows that despite the markings on the eyepieces, they will in fact show him the exact same amount of the night sky, assuming only that their FL's are the same. Is this not immediately intuitive and simple?
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 4350
Loc: "Metrowest" Boston
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Quote:
Is this not immediately intuitive and simple?
No, not really. It's constructed to be simple only when the focal lengths match. How does the TFoV in a 12mm eyepiece with an RFOV of 80 degrees compare to a 15mm eyepiece with an RFOV of 65? But what if I had told you instead that the 12mm has an EFSD of 16.8mm and the 15 has an EFSD of 17.0mm? You know immediately which will have the larger true field, and that required no arithmetic at all.
Now suppose I want to calculate the TFoV in my spreadsheet and I know my scope has a focal length of 1200mm. All the field stop formula says is that true field is proportional to EFSD. That proportionality constant is (180/pi)/TFL, usually approximated by 57.3/TFL. So I can calculate that once and for all, in my example it's 57.3/1200 = 0.04775 . Any time I want to convert an EFSD to a TFOV for that scope, I just multiply by 0.04775.
Now contrast that with the way you use the RFOV. With the RFOV you have to divide by magnification, which means that I first have to divide the telescope focal length by the eyepiece focal length, and then I have to divide the RFOV by that. That's not less complicated.
Remember the whole point here was calculating TFOV. Whereas EFSDs are superior for that, the analogous RFOV advantage would be in comparing angular magnification distortion. However, for that purpose it would be even easier to supply a quantity to represent that directly, e.g. 1 - RFOV/AFOV.
A long time ago, Jon Isaacs and I were pushing this idea of RFOV, only we called it something different, but the more I think about it, the more I realize that Al Nagler was right all along--just report EFSDs. That's what every manufacturer should do, and when they don't, we should use our drift-test data to do it for them.
--------------------
Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 36mm QX, 20mm 5K SWA, 13mm Ethos, 9mm BGO, 6mm ZAO-II, 5.1mm XO, 2x TV Barlow
- Filters: Baader M&S, 6-piece color set, ND.6, ND.9
- 120mm f/8.3 home-built grab-n-go Newt with 25mm Tak Ortho + GSO 3x Barlow
- Binoculars: 15x50 image stabilized, 12x50 roofs
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frisianstar
member
Reged: 01/29/06
Posts: 58
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Hi, I have a 31 nagler and and just received a Ethos 13mm.
with a laserpointer you can easy measure the FOV by pointing the laser through the eyepiece on to a board,
After some playing with the laser you will notice the edge of the field on the board, mark this edge of the fieldstop (of the eyepiece) ON the board,
slide the laser to the opposite of the fiedstop and mark it also.
Measure the distance between the eyepiece focal point and the board.
Measure also the distance between the markings.
With basic mathematics (and a calculator with tangent) I calculate for the 31 Nagler (first measure): 84 degree
(second measure): 83 degree.
For the Ethos (first measure): 99.96 degree
(second measure): 100.2 degree
Simply a 31 Nagler HAS a 82 degree field, and a Ethos HAS a 100 degree field. 
Holland from the Netherlands
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
Hi, I have a 31 nagler and and just received a Ethos 13mm.
with a laserpointer you can easy measure the FOV by pointing the laser through the eyepiece on to a board,
After some playing with the laser you will notice the edge of the field on the board, mark this edge of the fieldstop (of the eyepiece) ON the board,
slide the laser to the opposite of the fiedstop and mark it also.
Measure the distance between the eyepiece focal point and the board.
Measure also the distance between the markings.
With basic mathematics (and a calculator with tangent) I calculate for the 31 Nagler (first measure): 84 degree
(second measure): 83 degree.
For the Ethos (first measure): 99.96 degree
(second measure): 100.2 degree
Simply a 31 Nagler HAS a 82 degree field, and a Ethos HAS a 100 degree field.
Holland from the Netherlands
I have edited things here. This is a *very* good method, but does not require a laser. All you need is a well-collimated flashlight of sufficient size and a fairly parallel output. "Lambo" and I did this tonight. We set things up and moved the flashlight around a bit until we could actually see the edges of the field stop projected on the wall. Usually, you can't see both edges at the same time, but for a number of eyepieces, the whole disk from field stop edge to field stop edge was visible. Particularly amusing was his little reticle eyepiece, where we saw the reticle projected as well. To get the precise location of the focal point, we put the flaslight several meters away from the eyepiece so as to approximate a distant source. The fields of view we got were nearly identical to that using my method, but for the wider field eyepieces, they ended up closer to the figures given by the manufacturer. Indeed, I suspect that because of the difficulty in viewing both side of the field stop at the same time in the eyepiece, my "both eyes open" method may occasionally be somewhat less accurate than this nice projection method. When we measured the 24mm Panoptic, we got *exactly* 68.0 degrees rather than the 69.5 degrees I had with the "both eyes open" technique. The only eyepiece which gave us a few headaches was my old Meade 14mm Ultrawide, as the flashlight had to be held very steady to get the field edges to be projected with any repeatability. My 40mm Mk-70 Konig ended up having an apparent field of 68.8 degrees rather than the nearly 70 degrees I had gotten earlier. Johns old 32mm Burgess was 70.9 degrees, while his 20mm Type 5 Nagler was 80.5 degrees. This is a simple yet quite elegant way of measuring the apparent field of view, so again, thanks a bunch for bringing it to light. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
Edited by David Knisely (05/21/08 05:00 AM)
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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No one doubts that the AFOV's are as reported. They simply don't matter. All that matters is the RFOV and/or the field stop diameter. The AFOV will not tell you how much sky you will see. The RFOV and the field stop will tell you how much sky you will see. And the RFOV (or the field stop) will tell you without measurements of any nature, provided that the manufacturers get on the stick and accurately report these values.
I would most certainly like to know more about your method though!!!
--------------------
My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
No one doubts that the AFOV's are as reported. They simply don't matter. All that matters is the RFOV and/or the field stop diameter. The AFOV will not tell you how much sky you will see. The RFOV and the field stop will tell you how much sky you will see. And the RFOV (or the field stop) will tell you without measurements of any nature, provided that the manufacturers get on the stick and accurately report these values.
I would most certainly like to know more about your method though!!!
The Apparent Fields *do* matter, especially to those who care how much they see. Indeed, with a few of my eyepieces, the apparent field used in the old AFOV/Mag calculation gave results that were *very* close to the true fields I actually measured. The problem is that the formula just isn't consistently accurate, which is why the field stop formula tends to be a bit better. The apparent field is real, as it can be seen and measured. The Field stop is real and can be measured (usually). The true field of view of a telescope/eyepiece combination can be measured. The RFOV is not a real characteristic of the eyepiece. It is an artificial construct to get an old approximate equation for true field to seem to work better. It already involves the field stop in its calculation, which is redundant. All that really matters is that if someone wants to know *exactly* how much true field an eyepiece will give in a telescope, it needs to be *measured* (star drift method, or an accurately-known field for comparison).
If the amateur wants to know roughly how much true field one can get with a given eyepiece with known physical parameters, there are *two* formulae which work. The old formula TFOV = AFOV/Magnification will give at least an approximate result (within 10 %) and the field stop formula will give somewhat closer results (within 2%). Introducing another concept that has less of a clear connection to reality might be confusing and is unnecessary, considering we already have two workable formulae for true field of view. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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What possible harm would there be to anyone in having the manufacturers report the RFOV? Why does Baader feel it is important to report the RFOV? It is a simple, practical, and quite useful tool.
And as the original poster ascertained, AFOV's do not tell anyone what they will actually see, but rather only how far the eye will have to glance from side to side to see the REAL field that the eyepiece and telescope are presenting to the viewer. In his actual example, as regards the true field that he expected to see based on the apparent field, the AFOV was off by 12.7% in one direction for one eyepiece, and 4.9% in the other direction for another eyepiece, for a combined error of 17.6% in his simple yet futile attempt to use AFOV's to compare one eyepiece against another. With RFOV this error would be reduced to near zero.
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
--------------------
My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 4350
Loc: "Metrowest" Boston
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Quote:
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
You would rather have -11.7% angular magnification distortion than +6.1%? OK, I guess maybe I can see that, but where do you draw the line? How about an RFoV of 400 degrees in an AFoV of, say, 50. Of course there's a teensy bit of negative angular magnification distortion there, only about -700%, but it fits a lot of sky in 50 degrees! How much is too much?
--------------------
Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 36mm QX, 20mm 5K SWA, 13mm Ethos, 9mm BGO, 6mm ZAO-II, 5.1mm XO, 2x TV Barlow
- Filters: Baader M&S, 6-piece color set, ND.6, ND.9
- 120mm f/8.3 home-built grab-n-go Newt with 25mm Tak Ortho + GSO 3x Barlow
- Binoculars: 15x50 image stabilized, 12x50 roofs
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 13776
Loc: Amargosa Valley, NV, USA
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Quote:
Quote:
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
You would rather have -11.7% angular magnification distortion than +6.1%? OK, I guess maybe I can see that, but where do you draw the line? How about an RFoV of 400 degrees in an AFoV of, say, 50. Of course there's a teensy bit of negative angular magnification distortion there, only about -700%, but it fits a lot of sky in 50 degrees! How much is too much?
That's where "Majesty Factor" comes in! 
The answer to your question is subjective, of course. For general viewing, even the larger distortion is not going to be noticeable. But if you want to do imaging or sketching, then it can be a problem.
I would say it gets to be a problem when you can see the stars appear to follow curved paths across the eyepiece field, or when the image of an extended object is visibly distorted.
Or, when you're trying to take accurate measurements. Or star hopping.
I didn't realize the Hyperion had so much distortion until I used the formula that Lawrence provided. By that token, I would think it would not be a problem for visual observing.
For a budget widefield eyepiece with great views in a convenient image scale, the Hyperion is hard to beat. If you want the least distortion possible in the wide field, and can afford it, get the Nagler or Ethos. If you want the most distortion-free field possible, period, then get -- an Orthoscopic? A Brandon? A Plossl?
--------------------
"Since the process of science generates more mysteries than it solves, we can never learn everything, we'll simply generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Lawrence Taylor wrote:
Quote:
What possible harm would there be to anyone in having the manufacturers report the RFOV? Why does Baader feel it is important to report the RFOV? It is a simple, practical, and quite useful tool.
In the Baader literature, I do not see anything except the apparent field of view listed (68 degrees for the Hyperion). As for harm, as I said, it can sow the seeds of confusion (especially for the beginner).
Quote:
And as the original poster ascertained, AFOV's do not tell anyone what they will actually see, but rather only how far the eye will have to glance from side to side to see the REAL field that the eyepiece and telescope are presenting to the viewer. In his actual example, as regards the true field that he expected to see based on the apparent field, the AFOV was off by 12.7% in one direction for one eyepiece, and 4.9% in the other direction for another eyepiece, for a combined error of 17.6% in his simple yet futile attempt to use AFOV's to compare one eyepiece against another. With RFOV this error would be reduced to near zero.
Ah, not quite. He didn't get the true field expected because he used the approximation TFOV = AFOV/Mag and did not understand that it *is* an approximation. Had he used the field stop method, he would have been considerably closer to reality. As for apparent field, it shows the angular width that the eye sees when looking into the eyepiece (from field stop edge to field stop edge usually). Thus, it *does* show what the observer will see, not in terms of true field of view, but in the width of what is presented to the eye. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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You must go back through this thread, where when I speak of Baader using the RFOV I specifically reference this to only the Eudiascopics and the Genuine Orthos. Two eyepieces which hail from the same Japanese manufacturer (who I quite strongly suspect is the former manufacturer of the Masuyama eyepiece line).
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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frisianstar
member
Reged: 01/29/06
Posts: 58
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Thanks, I did my measurements with a 50 mW green laserpointer, just on the floor! the laser on a couple of books to get the right hight...the board is a piece a plywood...
the eyepiece was held in place with a counterweight...
when you held everything in place and square this method is very accurate.
With 50 mW laserpower you can easy see the various lensses inside a optical system, some are cemented, others absorbs
more light, some lensses are thin or very thick.
Holland (real name: Gerrit  )
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 4350
Loc: "Metrowest" Boston
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Quote:
If you want the least distortion possible in the wide field, and can afford it, get the Nagler or Ethos. If you want the most distortion-free field possible, period, then get -- an Orthoscopic? A Brandon? A Plossl?
As regards angular magnification distortion, the Type 4 Naglers and the Ethos are tops. Perhaps some other ultrawides may offer less angular magnification distortion than corresponding Naglers of other types. I'm not sure, but the Pentax XWs might be pretty good in this regard. I think they publish EFSDs, so I could check. Have to get back to you on that one.
As for the most distortion-free field possible, it really depends, and of course there is rectilinear distortion to think about also. Tele Vue Plossls have some angular magnification distortion, a few percentage points, IIRC, and I was most disappointed by an 18mm UO ortho (classic) in this regard. It's field stop was a lot smaller than I expected based on its AFoV, to the point that I didn't want to use it (as I'd bought it to be an eyepiece for a small finderscope). If that eyepiece is "orthoscopic" in any sense, it must be because rectilinear distortion is low. Contrast that with the 18mm Tak ortho I had--its field stop was pretty much dead-on for the zero angular magnification distortion. Unlike the Tele Vue Plossls, which show some stretching near the edge, I couldn't actuall see distortion in any of the so-called orthoscopics, though.
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Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 36mm QX, 20mm 5K SWA, 13mm Ethos, 9mm BGO, 6mm ZAO-II, 5.1mm XO, 2x TV Barlow
- Filters: Baader M&S, 6-piece color set, ND.6, ND.9
- 120mm f/8.3 home-built grab-n-go Newt with 25mm Tak Ortho + GSO 3x Barlow
- Binoculars: 15x50 image stabilized, 12x50 roofs
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Rob S
professor emeritus
Reged: 03/16/07
Posts: 518
Loc: NZ
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I was thinking about this this morning. Assuming the figures we see on the Tele Vue website for field stops are real dimensions, and not effective/virtual, when they decided to make the 8mm Ethos, would they have to start with the requesite sized hole in lets say a piece of metal, that gives a 100 degree AFOV (lets assume no distortion) for an 8mm focal length - and then build around that?
Regards,
Rob.
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8" f5 Dob with Feathertouch
Brandon: 16
Pentax: 7, 10 & 20XW
TMB: 30 Paragon
TV: 8 & 13 Ethos;
5 & 13 NaglerT6;
24 Panoptic
Leica Trinovid 8x50 BN
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Because the Ethos has a built in Barlow, its reported field stop is only apparent.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 4350
Loc: "Metrowest" Boston
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Well, the field stops are "effective" whenever there are any lenses in front of the focal plane, and that includes everything except for the Plossls and Panoptics. To answer your question, however, I suspect they did essentially what you said in a "virtual" design environment (to say a computer program), and set initial guess at the effective field stop diameter to be about 8*100/57.3, which would be about right for no angular magnification distortion, 8mm focal length, and 100 degrees of AFoV. The effective field stop diameter would be the physical field stop diameter adjusted for the effect of the eyepiece lenses between the telescope and the field stop. Consequently, this would be a calculated constraint for any particular lens configuration. There are lots of constraints to juggle, however, and so it is not surprising that the final value is allowed to deviate slightly, as it might be considerably more important to control something else a little more tightly and to ease up slightly on the idea of dead-on zero angular magnification distortion. The general tenor of this thread notwithstanding, there isn't much practical benefit to hitting that particular mark exactly.
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Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 36mm QX, 20mm 5K SWA, 13mm Ethos, 9mm BGO, 6mm ZAO-II, 5.1mm XO, 2x TV Barlow
- Filters: Baader M&S, 6-piece color set, ND.6, ND.9
- 120mm f/8.3 home-built grab-n-go Newt with 25mm Tak Ortho + GSO 3x Barlow
- Binoculars: 15x50 image stabilized, 12x50 roofs
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
Because the Ethos has a built in Barlow, its reported field stop is only apparent.
I wish people would not call the Smyth field flattener in some wider-field eyepieces a "built-in Barlow". Barlows act to increase the magnification of an eyepiece and may or may not help with flattening of the field. The negative Smyth lens set in the front of these eyepieces is an integral part of the eyepiece as a whole and cannot be removed without compromising the eyepiece performance. They are designed to compensate for the field curvature produced by a set of eyepiece optics that are correcting off-axis astigmatism. Also, these field stops for eyepieces such as these are *internal*, and the figures given by Tele Vue will result in true field figures which are dead-on accurate. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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There is absolutely no evidence at all that anyone named Smyth was ever involved in designing a negative lens grouping for a telescope eyepiece. As you are well aware, I ran an exhaustive thread here covering this very subject perhaps a year or two ago. The only possible person to honestly credit with being first with this accomplishment is Peter Barlow. Ask Al Nagler if his negative grouping in the Nagler is a Smyth and tell us what his answer is. Actually someone who reported it here has already done just this, and his answer was that if anything the negative grouping is a "Nagler" (meaning a purely Nagler concept and invention). Patents are generally required to reference the prior works leading up to any new invention. Show me an eyepiece patent whereby the negative lens grouping honors by reference anyone named Smyth. For that matter, show us a patent for a negative grouping specifically in a telescope eyepiece by anyone named Smyth. Somehow, about 90 or so years after his life, someone twisted his camera acomplishments and equated them to eyepieces, and the misplaced credit has stuck like glue. If such a stretch is to be permitted, perhaps we should call it a Petzval (just kidding of course). Telescope eyepieces were used in conjunction with Barlows for roughly 90 years or more through which no one ever credited Smyth (the father who was a close friend and associate of Peter Barlow, or the son who made strides in camera lens technology) with this accomplishment. Although admittedly this does not always happen (witness the Masuyama), it seems that within his lifetime someone would have given Smyth (meaning either of the Smyth's) credit if any was due.
That said, I feel that to continue this discussion within the context of a thread for which it has no specific contextual relevance would be inappropriate.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Ronin
super member
Reged: 10/18/04
Posts: 111
Loc: Phoenix AZ
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Perhaps a search for Piazzi-Smyth might be more fruitfull. Scottish Astronomer he developed a negative lens group field flattener.
--------------------
Martin "Ronin" Thompson
Observatory Manager
Gilbert Rotary Centennial Observatory
16" LX200R on Paramount ME
Personal Gear
Nexstar 8 GPS
Nexstar 11 CGE
Celestron Onyx 80
Canon Rebel XTi (EOS 400D)
Canon Xsi (EOS 450D)(Yippeee New Toy)
5 Baader Hyperions (Just Love Em)
All things are possible, they just may not be probable
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jcjr
professor emeritus
Reged: 01/06/08
Posts: 563
Loc: TN, USA
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Quote:
Quote:
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
You would rather have -11.7% angular magnification distortion than +6.1%? OK, I guess maybe I can see that, but where do you draw the line? How about an RFoV of 400 degrees in an AFoV of, say, 50. Of course there's a teensy bit of negative angular magnification distortion there, only about -700%, but it fits a lot of sky in 50 degrees! How much is too much?
Mike, there is an application where maybe that would be very desirable. I wonder if you have ideas if it has been done or could be kludged together (it would probably require more than just the eyepiece)--
I think it would be great to have a largish crosshair finder scope with good light-gathering, and maybe a 20 or 30 degree 'quasi-fisheye' true FOV. The eyepiece view would be close to rectilinear in perhaps the middle 20 degrees of the apparent field of view, where the crosshairs meet. But the view would get progressively wildly distorted/compressed toward the edges.
Why? Well, with a TFOV that wide, you could point the scope 'in the ballpark' of the target, then easily find the target in the distorted periphery, and then easily guide it into the crosshairs.
True fisheye's are near 180 degrees, which would be unnecessarily wide and make it difficult to see the tiny points of light. But a 20 or 30 degree telescopic fisheye might be VERY useful as a Finder?
I got a nice SV F80 Finder because it hurt the bones groveling on the ground to sight thru a Red Dot. But even with about 5 degrees of FOV in the F80, I STILL have to grovel on the ground to sight along the tube, because 5 degrees is often too narrow a capture range to 'just aim in the general direction' and find the object in the eyepiece.
A TFOV of 20 or 30 degrees should eliminate groveling on the ground to sight along the tube, even if most of the view is extremely distorted.
Anybody ever made a Finder like that? Any ideas how to make one?
--------------------
CPC 1100, C102SLT, SV F80, Meade 70 & 60 AZT
Q70 38mm, Pan24, Meade 5K 18mm UW, Axiom LX 15mm, 10mm, 7mm, Nagler 13T6, Expanse 20mm, 9mm, 6mm, BO/TMB 5mm, 2.5mm
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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Quote:
Perhaps a search for Piazzi-Smyth might be more fruitfull. Scottish Astronomer he developed a negative lens group field flattener.
He is the "son". Already been there.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 4350
Loc: "Metrowest" Boston
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Quote:
Mike, there is an application where maybe that would be very desirable. I wonder if you have ideas if it has been done or could be kludged together (it would probably require more than just the eyepiece)--
I only worked out the mathematical possibility of it. I have no idea how to put lenses together to make something like that.
--------------------
Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 36mm QX, 20mm 5K SWA, 13mm Ethos, 9mm BGO, 6mm ZAO-II, 5.1mm XO, 2x TV Barlow
- Filters: Baader M&S, 6-piece color set, ND.6, ND.9
- 120mm f/8.3 home-built grab-n-go Newt with 25mm Tak Ortho + GSO 3x Barlow
- Binoculars: 15x50 image stabilized, 12x50 roofs
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Ronin
super member
Reged: 10/18/04
Posts: 111
Loc: Phoenix AZ
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Indeed he is the son but he is credited with the development of a dispersive lens group to correct abberations in his work in Photographic lenses... and of course the name stuck to any such grouping in a lens/ eyepiece design
--------------------
Martin "Ronin" Thompson
Observatory Manager
Gilbert Rotary Centennial Observatory
16" LX200R on Paramount ME
Personal Gear
Nexstar 8 GPS
Nexstar 11 CGE
Celestron Onyx 80
Canon Rebel XTi (EOS 400D)
Canon Xsi (EOS 450D)(Yippeee New Toy)
5 Baader Hyperions (Just Love Em)
All things are possible, they just may not be probable
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Quote:
Perhaps a search for Piazzi-Smyth might be more fruitfull. Scottish Astronomer he developed a negative lens group field flattener.
I do believe that this is where the term "Smyth Lens" very probably originated. Indeed, the first time I ran into the term was in reading Harry Rutten and Martin van Venrooij's book TELESCOPE OPTICS (c. 1988 Willmann-Bell). On page 175 of the 1993 edition, the authors state:
"As mentioned above, it is possible to reduce astigmatism in a positive eyepiece system at the expense of introducing field curvature. By placing a negative lens between the objective and the positive eyepiece, however, we can compensate this field curvature. The reason is that the negative lens introduces curvature in the intermediate focal surface F2. If this is chosen to match that of the positive lens, the resulting combination of lenses can be free of both astigmatism and field curvature in the imaginary focal surface F2.
The design principle of the Nagler is not totally new. The combination was proposed long ago by Smyth; hence, the negative lens is called a Smyth lens (see ref. 16.1)."
Now some eyepiece manufacturers just slap in a negative lens in front of a simpler eyepiece design to try and make it work a little better at what amounts to a longer focal ratio. This would be a "built-in" Barlow. The Smyth lens, however, would be specifically designed so as to compensate as much as possible for extensive field curvature produced by the positive lens set that has already dealt with the astigmatism problem. There are a few eyepieces where the Smyth lens also serves as a sort of Barlow (the 5-8mm Speers Waler eyepiece for example). However, the overall design and intent of the negative field flattener is for field flattening and not just to bump up the f/ratio to a point where a simpler eyepiece can handle the narrower light cone. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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I got these results with my own eyepieces, but it is tough to format it for Cloudynights to get every column to line up.
Code:
To compare both the old AFOV/mag formula and the Field Stop Formula
with reality, I did some measurements for my own eyepieces and telescope.
The telescope is a 10" f/5.55 Newtonian (1410 mm actual focal length), and
the eyepieces tested were the following: (EFSD is eyepiece field stop
diameter in millimeters).
____Eyepiece_________Apparent Field____True Field (on sky)__EFSD__
40mm Mk-70 Konig 68.8 deg. 1.883 deg. 46.00mm
30mm Widescan III 84.0 deg. 1.798 deg. 44.00mm
30mm Ultrascopic (Orion) 52.2 deg. 1.063 deg. 26.08mm
27mm Kellner (Jaegers) 52.5 deg. 1.042 deg. 25.38mm
24mm Panoptic (Tele Vue) 68.0 deg. 1.113 deg. 27.00mm
24mm Koenig (University Opt.) 58.9 deg. 0.981 deg. 24.04mm
20mm Celestron Plossl 51.0 deg. 0.705 deg. 17.23mm
15mm Ultrascopic (Orion) 58.3 deg. 0.590 deg. 14.40mm
14mm Meade Ultrawide 80.5 deg 0.820 deg. 20.3mm*
10mm Ultrascopic (Orion) 49.8 deg. 0.339 deg. 8.32mm
6.4mm Super Plossl (Meade) 49.4 deg. 0.217 deg. 5.31mm
6mm Orthoscopic (Brandon) 44.2 deg. 0.188 deg. 4.62mm
*14mm Ultrawide's field stop is inside of the eyepiece inside the negative
doublet of the field lens, and thus its size is *approximate only*.
Each eyepiece had its apparent field of view measured on the optical bench,
rather than depending on the listed manufacturer values, which tend to be a
little optimistic. The focal lengths used in the calculations are those
given by the manufacturer on the eyepiece barrel, as, due to some of their
construction and short focal length, they could not be accurately measured.
The true field of view (TF) for each eyepiece was measured at the telescope
using the star-drift method. Below are the calculated values for true field
and their percentage deviations from the measured true field for each
formula. All angles are in degrees. FS-FORMULA is the eyepiece field stop
formula TFOV = (180/Pi)*EFSD/TFL, and AF/Mag is the Apparent field divided
by the magnification.
EYEPIECE Mag True Field | AF/Mag % Dev. | FS-FORMULA % Dev.
-----------------------------------|--------------------|--------------------
40mm Konig 35.3x 1.883 deg | 1.95 deg +3.5% | 1.869 deg -0.7%
30mm WSIII 47.0x 1.793 deg | 1.79 deg +0.0% | 1.789 deg -0.2%
30mm Ultras. 47.0x 1.063 deg | 1.11 deg +4.7% | 1.060 deg -0.3%
27mm Kellner. 52.2x 1.042 deg | 1.01 deg -2.8% | 1.031 deg -1.1%
24mm Panoptic 58.8x 1.113 deg | 1.16 deg +4.5% | 1.097 deg -1.4%
24mm Koenig 58.8x 0.981 deg | 1.00 deg +2.0% | 0.977 deg -0.4%
20mm Plossl 70.5x 0.705 deg | 0.72 deg +1.4% | 0.700 deg -0.7%
15mm Ultras. 94.0x 0.590 deg | 0.62 deg +5.1% | 0.585 deg -0.8%
14mm Ultrawide. 101x 0.820 deg | 0.80 deg -2.4% | 0.825 deg +0.6%
10mm Ultrasco. 141x 0.339 deg | 0.35 deg +2.9% | 0.338 deg -0.3%
6.4mm S. Plos. 220x 0.217 deg | 0.22 deg +0.0% | 0.216 deg +0.5%
6mm Ortho. 235x 0.188 deg | 0.19 deg +0.0% | 0.188 deg 0.0%
The AF/Mag standard form gives values which deviate from the actual true field
by an average of about +1.6 percent (maximum deviation: 5.1%). Although the
apparent field could be measured with some accuracy, one has to depend on the
manufacturer's eyepiece focal length numbers, and this undoubtedly is at
least partly the reason for the errors with the AF/Mag method. The Eyepiece
Field-Stop formula is generally more accurate for calculating the true field
of view with a given eyepiece. The mean deviation with this formula is
-0.4 percent, with the maximum deviation of -1.4%. NOTE: filters for 1.25"
eyepieces, (LPR's, Nebula Filters, ect.) will often have a clear aperture of
about 24.5mm. This diameter may become the maximum field stop size available
when these filters are being used, thus limiting the field of view for 1.25"
eyepieces which have field stops larger than 24.5mm.
The chief difficulties with using the Field Stop formula are when you
cannot get direct access to the field stop to measure it (the case when some
eyepieces have a field stop just behind the field lens), or when you do not
have the ability to measure it with much accuracy for the smaller diameter
field stops (as in some very short focal length eyepieces). In addition,
many eyepiece manufacturers do not routinely provide the eyepiece field stop
diameters for their products (only the focal lengths and the approximate
apparent fields). Still, either the classic AFOV/Mag formula or the Eyepiece
Field Stop formula should be able to give the amateur astronomer an idea of
what size of true field of view to expect when viewing with a given eyepiece.
Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4912
Loc: N.E. Ohio
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As stated before, it took nearly 100 years before anyone equated the negative field flattener in a telescope eyepiece with Smyth. People like to make associations. Does this make all such associations correct? If so, then I state that Petzval likewise invented the idea (not really).
Per the testimony of someone here who asked him this directly, Al Nagler scoffed at the idea that his negative field group was a "Smyth". With this being the case, does it matter what Rooten and vanRooij thought it was in reference to Al's design?
Rooten and vanRooij likewise equate the Barlow directly with eyepiece field flattening. See page 155. Why do you scoff at the idea that a Barlow can be used in relation to eyepiece field flattening when your very source clearly states that it does?
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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David Knisely
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Reged: 04/19/04
Posts: 8799
Loc: Beatrice, Nebraska
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Lawrence Sayre wrote:
Quote:
Rooten and vanRooij likewise equate the Barlow directly with eyepiece field flattening. See page 155. Why do you scoff at the idea that a Barlow can be used in relation to eyepiece field flattening when your very source clearly states that it does?
I don't scoff at the idea. A Barlow can produce some flattening of the field, but the idea again is intent. The Barlow may not be able to completely compensate for a severely curved field. This is where *deliberately* designing a field flattener out in front comes in, as it is set up to *match* the field curvature of the positive lens set of the eyepiece, thereby canceling it out. It isn't a "built-in" Barlow used so that cheap eyepiece designs might be used at shorter f/ratios; its intent is for field flattening from the word "go". Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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