This WEB page assist you in designing your own Newtonian reflecting telescope.

This information is also applicable to other types of telescopes.

Parameter Scope Value Typical Values
[1] Objective Diameter (D): mm 50 mm to 20+ in
[2] Objective Focal Ratio (f): f/ 4 to 12
[3] Eyepiece Focal Length (Fe): mm 6 to 50
[4] Eyepiece Apparent Field (AF) deg 30 to 82 deg
[5] Observer's Fully Dilated Pupil Size (Deye): mm 3, 3.4, 4, up to 8.5
7 average, 5 if older


  1. ^ Objective Diameter (D)
    Bigger objectives gather more light, cost more, weigh more
  2. ^ Focal Ratio (f):
    Bigger ('slower') implies narrower views, higher magnification, longer scope.
  3. ^ Eyepiece Focal Length (Fe):
    Longer yields lower maginification and wider views.
  4. ^ Eyepiece Apparent Field (AF):
    Bigger is usually better (and more costly and heavier).
  5. ^ Observer's Pupil Diameter (Deye)
    (Fully dark adapted) bigger pupil admits more light, you can see fainter objectse.
  6. ^ Focal Ratio (f):
    Lower f gives wider views.
  7. ^ Focal Length (F):
    Longer F: longer, less portable 'scope, narrower views.
  8. ^ Objective Radius (r):
    Objective size; don't confuse with mirror Radius of Curvature below.
  9. ^ Objective Area (A):
    More area gathers more light, higher resolutions.
  10. ^ Radius of Curvature (R):
    Setup Foucault tester at Radius of Curvature.
  11. ^ Sagitta (s):
    Depth of spherical curve at center of mirror.
  12. ^ Glass to Remove, Spherical (Vs):
    Bigger and/or faster mirrors are more work to grind....
  13. ^ Glass to Remove, Parabolic (Vp):
  14. ^ Glass to Remove for Parabolization (Vsp)
  15. ^ Telescope Performance Parameters:
    Even a small scope gathers much more light than eye.
  16. ^ Angular Resolution (SA):
    Larger objectives have better angular resolution w in nm; D in mm .
  17. ^ Linear Resolution (SL):
    Faster (smaller) focal ratios have higher linear resolution w in nm; f is unitless.
  18. ^ Minimum Magnification (Mmin):
    Lower magnification "wastes" light that can't enter your eye.
  19. ^ Maximum Magnification (Mmax)
    Old rule of thumb, can be exceed with good seeing and good optics.
  20. ^ Limiting Magnitude (ML):
    Limiting Magnitude of the eye (Meye) = " + String(Meye) + "
    Telescope Transmission Factor (t) = " + String(t * 100) + "%.
  21. ^ Magnification (m):
    Longer focal length eyepieces produce lower magnifications.
  22. ^ True Field (TF):
    Field of view of the sky.