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# Eyepiece True Field Of View

5 replies to this topic

### #1 ras

ras

Lift Off

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Posted 07 December 2019 - 11:35 AM

As my question indicates I am a beginner.  If true field of view = eyepiece field stop diameter/telescope focal length times 57.3, does this mean that that a 1.25" eyepiece cannot have a tfov greater than approximately 1.25 degrees and a 2' eyepiece a tfov greater than approximately 2.10 degrees?

### #2 dan_h

dan_h

Mercury-Atlas

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Posted 07 December 2019 - 11:42 AM

No.  Telescope focal length is the changing parameter. A longer length reduces the field. A shorter scope has a wider field.

dan

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### #3 junomike

junomike

Hubble

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Posted 07 December 2019 - 11:42 AM

This is somewhat true although there are exceptions where the Field Stop is up inside the housing.  For the most part a 1.25" Field Stop limits is 29mm and a 2" is 46mm.

Also, based on your FOV I'm guessing you're using a 1200mm - 1250mm F/L OTA?

### #4 MitchAlsup

MitchAlsup

Aurora

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Posted 07 December 2019 - 12:11 PM

As my question indicates I am a beginner.  If true field of view = eyepiece field stop diameter/telescope focal length times 57.3, does this mean that that a 1.25" eyepiece cannot have a tfov greater than approximately 1.25 degrees and a 2' eyepiece a tfov greater than approximately 2.10 degrees?

A telescope with a 1 m focal length will have 2× the TFoV as a telescope with a 2 m focal length (using the same EP in both). My 13" F/3 has a TFoV of about 1.84º using a 21E

A telescope with a 0.5 m focal length will have 2× the TFoV as a telescope with a 1 m focal length...

A 6.5" F/3 would have a TFoV of about 3.69º using a 21E

### #5 Ernest_SPB

Ernest_SPB

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Posted 08 December 2019 - 09:37 AM

TFOV = 57.3*D/FL,

where

TFOV - field of view of the scope in degree

D - diameter of effective field stop of the eyepiece in mm

FL - focal length of the scope's objective lens in mm

Note: effective diameter of FS equal to diameter of physical FS (hole in diaphragm) only for eyepiece of classic design with frontal location of the FS and without reduser/amplifier (Barlow).

e.g.

D = 27 mm (maximum available for 1.25'eyepiece, like in 24 mm Panoptic)

FL = 1500 mm (like in 6" SCT)

TFOV = 57.3*27/1500 = 1.03 deg.

Edited by Ernest_SPB, 08 December 2019 - 09:44 AM.

### #6 Starman1

Starman1

Vendor (EyepiecesEtc.com)

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Posted 08 December 2019 - 05:25 PM

As my question indicates I am a beginner.  If true field of view = eyepiece field stop diameter/telescope focal length times 57.3, does this mean that that a 1.25" eyepiece cannot have a tfov greater than approximately 1.25 degrees and a 2' eyepiece a tfov greater than approximately 2.10 degrees?

The maximum field stop possible in a 1.25" or 2" design depends on where the eyepiece designer draws the line for vignetting (darkening at the edge of the field).

Where most 1.25" eyepieces max out at 27.0-27.2mm field stops, there are some that have been as large as 29mm.

Likewise, most 2" eyepieces max out at 46mm, but a few have gone to 46.5 or 47mm.

Note that when the field stops are larger than the inside diameter of the barrels, however, they must be placed very high in the eyepieces, above the barrels, and this necessitates a large

amount of in-travel at the focuser to achieve focus.

But you have put your finger on the WHY of 2" eyepieces--it is to get a wider true field of view than can be achieved with 1.25".

In general, if the focal length of the eyepiece exceeds "X" in 1.25", it must be a 2" eyepiece:

X = 40mm in 40° apparent field

X = 32mm in 50°  "

X = 24mm in 68°  "

X = 16mm in 82°  "

X = 13mm in 100°  "

Anything under the focal lengths listed can provide the same apparent fields in 1.25".  There is no reason to be a 2" eyepiece.

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