How to calculate EP's apparent (angular) FOV and true FOV from EP's field stop

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Bigzmey
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How to calculate EP's apparent (angular) FOV and true FOV from EP's field stop

#1

Post by Bigzmey »


EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.

Example: ES 68 deg 24mm EP has a field stop of 27.2mm and advertised apparent FOV =68 deg.

However, if we calculate AFV from the field stop it comes to 57.3 x (27.2/24) = 64.9 deg.

In 102mm F7 refractor ES 68 deg 24mm EP will produce TFV = 57.3 (27.2/714) = 2.18 deg.

Maximum possible true FOV in any scope is determined by the telescope focal length and max field stop of either 1.25" or 2" EP

1.25" EP max field stop is ~27mm
2" EP max field stop is ~46mm

Thus, max TFV for 102mm F7 refractor is ~2.2 deg for 1.25" EP and ~3.7 deg for 2" EP.
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
Mounts: SW: SkyTee2, AzGTi; iOptron: AZMP; ES: Twilight I; Bresser: EXOS2; UA: MicroStar.
Binos: APM: 100-90 APO; Canon: IS 15x50; Orion: Binoviewer, LG II 15x70, WV 10x50, Nikon: AE 16x50 & 8x40.
EPs: Pentax: XWs & XFs; TeleVue: Delites, Panoptic & Plossls; ES: 68, 62; Vixen: SLVs; Baader: BCOs, Aspherics, Mark IV; Meade: UWA, SWA.
Diagonals: Baader: BBHS mirror, Zeiss Spec T2 prism, T2 prism, Clicklock dielectric; TeleVue: Evebrite dielectric; AltairAstro: 2" prism.
Filters: Lumicon: DeepSky, UHC, OIII, H-beta; Baader: Moon & SkyGlow, Contrast Booster, UHC-S, 6-color set; Astronomik: UHC.

Observing: DSOs: 2823 (Completed: Messier, Herschel 1, 2, 3. In progress: H2,500: 2107, S110: 77). Doubles: 1863, Comets: 29, Asteroids: 168
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Re: How to calculate EP's apparent (angular) FOV and true FOV from EP's field stop

#2

Post by JayTee »


Now my brain hurts!

But good info nonetheless.
JT
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob & #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: Canon T3i (x2), ZWO ASI294MC Pro & 120MC, Orion SSAG, WYZE Cam3 (x2)
∞ Binos: 10X50,10.5X70,15X70², 25X100
∞ EPs: ES: 21 100° & 30mm 82° X-Cel LX: 9, 12, 18, 25mm 60° GSO SV: 15 & 20mm 68°
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Makuser
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Re: How to calculate EP's apparent (angular) FOV and true FOV from EP's field stop

#3

Post by Makuser »


Hi Andrey. Thanks for this excellent calculation. I have always used the old method that you mentioned above and is repeated below.
(M) magnification = (FLt) telescope focal length divided by (FLe) eyepiece focal length.
Next (True Field of View) TFOV = (Apparent Field of View) AFOV divided by (M) magnification.
You can algebraically combine these equations into one process (I used easy numbers to round out nicely).
For example on my 6" f/5 refractor (focal length 750mm) with a 15mm eyepiece and a 50 degree apparent field of view I get a magnification of 50x. My true field of view would be 50 degrees divided by 50x which results in a 1 degree true field of field.
Now we can combine the two equations:
TFOV=APOV/1 further divided by FLt/FLe is APOV/1 times FLe/FLt which algebraically finally results in the one step process of TFOV=AFOV times FLe/FLt. A check reveals that in my 6" refractor results in 50 degrees times 15mm divided by 750mm which comes out again in a 1 degree true field of view.
However, I must say that your calculations are probably much more accurate. Thanks for sharing this with us on here Andrey and the very best of regards.
Marshall
Sky-Watcher 90mm f/13.8 Maksutov-Cassegrain on motorized Multimount
Orion Astroview 120ST f/5 Refractor on EQ3 mount
Celestron Comet Catcher 140mm f/3.64 Schmidt-Newtonian on alt-az mount
Celestron Omni XLT150R f/5 Refractor on CG4 mount with dual axis drives.
Orion 180mm f/15 Maksutov-Cassegrain on CG5-GT Goto mount.
Orion XT12i 12" f/4.9 Dobsonian Intelliscope.
Kamakura 7x35 Binoculars and Celestron SkyMaster 15x70 Binoculars. ZWO ASI 120MC camera.
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Re: How to calculate EP's apparent (angular) FOV and true FOV from EP's field stop

#4

Post by notFritzArgelander »


Bigzmey wrote: Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.
This raises an interesting point that has a subtlety of optical design buried in it: How is this exaggeration done? In many cases to the observing eye at the exit pupil the AFOV is as advertised and specified. But the formula isn't strictly applicable since the wide apparent field of view is achieved by distorting the field of view using aberrations!

https://www.telescope-optics.net/eyepie ... tion_2.htm

Angular magnification distortion is used to increase the AFOV above what the formulas would give based on field stop size. So it's not so much advertising hype as it is using an optical trick to exaggerate the image of the TFOV at the exit pupil.
Last edited by notFritzArgelander on Fri Sep 23, 2022 12:24 am, edited 1 time in total.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Bigzmey
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Re: How to calculate EP's apparent (angular) FOV and true FOV from EP's field stop

#5

Post by Bigzmey »


notFritzArgelander wrote: Thu Sep 22, 2022 4:01 pm
Bigzmey wrote: Wed Sep 21, 2022 8:28 pm EP's apparent (angular) FOV = 57.3 x ( EP field stop /EP FL)

True FOV for EP/scope = 57.3 x (EP field stop/telescope FL)

If you don't know EP field stop, you can calculate TFV by dividing EP AFV by power it produces in the scope. This is not the most accurate, because many EP manufacturers exaggerate AFV.
This raises an interesting point that has a subtlety of optical design buried in it: How is this exaggeration done? In many case to the observing eye at the exit pupil the AFOV is as advertised and specified. But the formula isn't strictly applicable since the wide apparent field of view is achieved by distorting the field of view using aberrations!

https://www.telescope-optics.net/eyepie ... tion_2.htm

Angular magnification distortion is used to increase the AFOV above what the formulas would give based on field stop size. So it's not so much advertising hype as it is using an optical trick to exaggerate the image of the TFOV at the exit pupil.
Interesting point nAF!
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
Mounts: SW: SkyTee2, AzGTi; iOptron: AZMP; ES: Twilight I; Bresser: EXOS2; UA: MicroStar.
Binos: APM: 100-90 APO; Canon: IS 15x50; Orion: Binoviewer, LG II 15x70, WV 10x50, Nikon: AE 16x50 & 8x40.
EPs: Pentax: XWs & XFs; TeleVue: Delites, Panoptic & Plossls; ES: 68, 62; Vixen: SLVs; Baader: BCOs, Aspherics, Mark IV; Meade: UWA, SWA.
Diagonals: Baader: BBHS mirror, Zeiss Spec T2 prism, T2 prism, Clicklock dielectric; TeleVue: Evebrite dielectric; AltairAstro: 2" prism.
Filters: Lumicon: DeepSky, UHC, OIII, H-beta; Baader: Moon & SkyGlow, Contrast Booster, UHC-S, 6-color set; Astronomik: UHC.

Observing: DSOs: 2823 (Completed: Messier, Herschel 1, 2, 3. In progress: H2,500: 2107, S110: 77). Doubles: 1863, Comets: 29, Asteroids: 168
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