Eyepiece Field of View (FOV)

Field of View

By: Sam Pitts 2001

What is my Field of View

This depends on your eyepiece’s magnification and apparent field of view, along with the telescope being used. The eyepiece has a focal length indicated in millimeters. The longer the focal length of an eyepiece (25mm-50mm) the lower the power (magnification) and the wider the field of view. This assumes we are using the same telescope with a fixed focal ratio (f/10) and length (fl/2000). The higher the power or magnification the shorter the focal length in millimeters, resulting in a smaller field of view (FOV).

To determine the magnification of an eyepiece, divide its focal length into the focal length of the telescope’s objective lens or mirror. 8″ f/10= 2000mm focal length (fl).

Telescope: 8″ f/10 – 2000mm fl (focal length)
Plossl Eyepiece 32mm with 50° FOV (apparent field of view)
Magnification 2000mm ÷ 32mm = 62.5x

The field of view with this setup is determined by eyepiece magnification and apparent field of view 50°. Hold an eyepiece and look through it. The circular view of light observed is its apparent field of view. The diameter of this circle is the apparent field of view measured in degrees. Below is a list of apparent field of views with different types of eyepieces.

Tele Vue ® Nagler™ 82°
Tele Vue® Radian™ 60°
Meade® Ultra Wide 84°
Erfle 60°
Orion® Ultrascopic™ 52°
Plossl 52°
Orthoscopic 45°
Kellner 40°

Find the Field of View

To find the actual field of view, divide the eyepiece’s apparent field of view by the magnification on a particular scope. Using the example above (8′ f/10 scope ).

50° ÷ 62.5 = 0.8°

The 32mm Plossl on an 8″ f/10 (2000mm) telescope will render a true field of view of 48 arc minutes or 8/10 of a degree. Remember the moon is approximately 1/2° (30′) in diameter. Wide field of view lenses may suffer from aberration near the edges due to astigmatism. The stars may be slightly distorted near the edge of the field of view.

A 32mm Tele Vue Nagler 82°, with the same telescope, would have a FOV of 1.312° or 1° 18′ 43.2″
A 32mm Kellner 40°, with the same telescope, would have a FOV of .64° or 38′ 24″

Eyepieces > 32mm are best used with 10″ or larger objectives/mirror and a 2″ diagonal.




Milky Way over Diamond Peak, Oregon





Taken from Wolf Mt. NW of Diamond Peak
Canon F1
Canon 50mm f/1.4 Stopped down to f/2.8
Kodak E200 puhed
8 min exposure
Piggyback Losmandy G11
2 image mosaic/composite/ 3rd image of foreground 
Telescope / Lens 50 f/1.4 Canon Camera Lens stopped down
Mount Type Piggyback on G11 (Stepper)
Camera Canon F-1 with Bright Screen & Angle B magnifier
 Filters 112mm UV
 Film  Kodak E200 (Slide Film)
 Exposure 10 minutes; manual guiding FS/78
 Processing Push 1, Scanned – 2400 dpi, Photoshop
 Date July 2003-Milky Way; Diamond Peak 8/2006
 Location Panther Creek, south of Veneta, Oregon
 Conditions 1200′ magnitude 6 Skies; Clear, steady

M33 NGC598 Triangulum Galaxy


Telescope / Lens TMB 80 mm f/ 6 with Tele-Vue .8 Reducer TRF2008 / reduced TMB to f/4.8   –  384 mm  Focal Length
Mount Type Astrophysics 1200
 Filters Astrodon LRGB e-series of balanced filters     (generation 1) 9nm HA
 Film  CCD
 Exposure 345 minutes (5.75 Hours) HaLRGB (Ha=50 min; L=145 min; RGB= 150 min.; (5 min. & 10 min. sub-images)
 Processing CCDSoft, CCDStack, AIP, Photoshop CS2
 Date  09/19/2009
 Location Snow Peak, S/E of Cottage Grove, Oregon                122° 52′ 35″ W, 43° 31′ 21″N
 Conditions 4658′ elevation, magnitude 6+ Skies; Clear ;

M 33 (NGC 598) Triangulum Galaxy
M33 is visible to the naked eye from a very dark sky site, Bortle 3 or better.  The Triangulum Galaxy is a very challenging naked eye object but it can be seen.   M33 is a Type SC galaxy belonging to the local group, 0.9 mpc or 3.1 Million Light Years away. This image was taken with a TMB triplet CNC APO 80mm f/6 with a TeleVue .8 reducer / flattener (TRF2008). The full resolution image actually reveals the hint of structure and stars within the many red nebula knots shown throughout its arms. These Nebula (Red areas) are star forming areas much like the Great Orion Nebula (M42) and Eagle Nebula (M16). I had taken a much shorter image years ago with an Orion ED 80 and wanted to really capture more detail. You can also see several background galaxies in this image.

The bright Red nebula (upper right portion) is NGC  604.  NGC form the largest known HII region currently known.  The nebula spans 1500 light years.  M33 itself is approximately 60,000 light years in diameter, home to 40 billion stars.  Our own Milky Way (100,000 LY diameter) is estimated to have 400 Billion stars.