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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2695
Loc: 51°N 4°E
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In his review of the WO 7x50 ED, EdZ notes he measured this binocular's TFOV at 7.3 to 7.4 degrees i.e. 7°18' to 7°24'.
I believe my TS 7x50 to be an identical re-branded copy of the WO. Just now I noticed that I can just barely fit Wega and Sulaphat (gamma Lyrae) into the same FOV; they're both really at the very edge of view. The distance between these two stars is 7°33' i.e. 7.55 degrees.
Can anybody explain this difference?
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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KennyJ
Reged: 04/27/03
Posts: 10142
Loc: Lancashire UK
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Mark ,
What follows is nothing more than sheer speculation .
In his review of the W.O 7 x 50 , EdZ pointed out that at around 18mm , eye relief , was somewhat TOO long for him , noting that he suffered from blackouts if placing his eyes too close to the eyecups .
It is just possible that EdZ , being one who ALWAYS wears glasses while observing , may subconsciously or purposely hold his eyes ever so slightly BACK OFF , away from the eyepieces , to avoid any possibly contact between eyeglasses and oculars , and / or to avoid blackouts .
The difference between the reported TFOVs of roughly 7.4 to approximately 7.55 degrees is hardly GREAT , so this is just one POSSIBILITY .
Another possibility would be the accuracy of the information by which either , or indeed both of you , are basing your calculations .
For example , I recall in a thread on this forum two or three years ago that there was some discrepency between the assumed angle of distance between the outer Orion belt stars Alnitak and Alnilam , which from memory , varied from 2.7 degrees according to one correspondent , to 2.9 degrees according to another .
I've also noticed that the setting of the IPD on binoculars can slightly affect the area of field which can be seen .
As I said , this is merely guesswork on my part .
If someone else comes along with another explanation , I will be as grateful as yourself :-)
Clear Skies , Kenny
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Two eyes and a preference to use both
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2695
Loc: 51°N 4°E
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thanks Kenny, I'll let you know if I come up with anything.
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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Pinewood
professor emeritus
Reged: 12/07/04
Posts: 663
Loc: 40.77638º N 73.982652 W
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Quote:
In his review of the WO 7x50 ED, EdZ notes he measured this binocular's TFOV at 7.3 to 7.4 degrees i.e. 7°18' to 7°24'.
I believe my TS 7x50 to be an identical re-branded copy of the WO. Just now I noticed that I can just barely fit Wega and Sulaphat (gamma Lyrae) into the same FOV; they're both really at the very edge of view. The distance between these two stars is 7°33' i.e. 7.55 degrees.
Can anybody explain this difference?
Maybe it is not an identical binocular.
Clear skies,
Arthur
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Bread is not enough. We demand circuses!
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12565
Loc: Cumberland, R I , USA42N71.4W
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I suspect either
Quote:
Another possibility would be the accuracy of the information by which either , or indeed both of you , are basing your calculations .
or
Quote:
Maybe it is not an identical binocular.
as the most plausible answers.
First I'll tell you how I measure. In most cases I mount the binocular to either an Orion slo-motion control or to the Bogen 401 slo-motion head. I place a baseline star on one edge of the E/W field and use the N/S adjust to scan stars on the opposite edge of the field. That way I find stars at the extreme edges directly across the center of the field. Then I measure the stars in the largest format charts I have, either SA2000 or Uranometria. The large scale of Uranometria, 18mm per degree, would allow an accuracy of about 3 to 6 arcmin, or less than 1/10th degree.
Binoculars that are truly collimated and that show perfectly merged points and perfectly overlapped fields will not show any difference in the TFOV as the binocular is moved throughout the entire IPD range. If they do, the binocular is not truly collimated. Also if they do, the binocular is being thrown out of conditional alignment.
Binoculars that are conditionally aligned, and where the points may be merged BUT the fields of view are not perfectly matched, may show a wider field of view thru combined barrels than thru either barrel separately, especially if the mis-lapped spread is outward. This is a function of a slightly misaligned binocular, not of the field of view due to the field stop.
I recently read an article that stated you could gain a wider filed, as much as a half degree of field of view, by extending the IPD to a range where you could still just take it in (meaning the outside edges of your pupils are just catching the inside edges of the exit pupils. This is false. A clear understanding of collimation, conditional alignment and True field of view is needed to understand why this would be impossible.
One setting that can have an affect on the TFOV is by going from extreme close focus to infinity focus. Since this will have the affect of slightly changing the magnification, and since Afov cannot be changed, this will have an affect on the TFOV, by exactly the same percentage change as the delta magnification.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2695
Loc: 51°N 4°E
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Ed, for all your careful fine-tuned technique, I'm a bit surprised that the final step is measuring the angular distance between the two stars in an atlas. I'm sure you're aware of the distortions created by the chart projection.
If you know the stars' coordinates there's a simple formula that tells you the distance; I'll try to find it and will report back when I do. Alternatively, in Cartes du Ciel, clicking on both stars gives the angular distance just like that.
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12565
Loc: Cumberland, R I , USA42N71.4W
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I already have a spreadsheet that I can use to calc separation by coordinates. I find it generally unnecessary to use.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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