Imaging Methods
Quick Contents
(1) Prime Focus
  • Camera lens must be removed.
  • Telescope used like telephoto lens
  • Field Angle = (Image Diameter ÷ Telescope Focal Length)  x 57.3°
  • For Dobsonian/Newtonian, use Paracorr to correct coma. Telescope focal length increases 15%. Optimum back focus distance (T-Ring shoulder to focal plane) is 56mm.
(2) Focal Length Shortened by Using Reducer
  • Camera lens must be removed.
  • Focal length reduced (e.g.: Tele Vue 0.8X Reducer)
  • f/# faster by reduction in magnification
  • Example: 600mm focal length, f/7 (TV-85 telescope) with 0.8X Reducer = 480mm focal length at f/5.6
  • Tele Vue reducers are optimized for 56mm back focus distance (T-Ring shoulder to focal plane).
(3) Focal Length Extended by Using Barlow/Powermate
  • Camera lens must be removed.
  • Focal length extended (e.g.: Tele Vue Powermate 2x, 2.5x, 4x and 5x)
  • f/# slowed by Barlow or Powermate magnification
  • Example: 600mm focal length, f/7 (TV-85 telescope) with 4x Powermate = 2400mm focal length at f/28
  • Here are links to charts giving magnification vs. distance of Tele Vue Barlows and Powermates.
(4) Focal Length Extended by Using Eyepiece Projection
  • Camera lens must be removed.
  • Focal length extended by eyepiece acting as a relay lens
  • f/# slowed by projection magnification
  • Magnification of focal length determined by Relay Ratio (see diagram left):
    Relay Ratio  = A B
  • focal length = Telescope Focal Length x (A B)
    Where A and B are determined from the formula:

    Where F = eyepiece focal length
  • Example with 600mm focal length (TV-85 telescope) using the Tele Vue Eyepiece Projection Adapter with Tele Vue 11mm Plössl eyepiece: if the Plössl is 12mm from Telescope focal plane and 150mm from Film or CCD Plane, the relay ratio is 150/12 = 12.5. 600mm x 12.5 yields 7500mm focal length.
(5) Afocal Imaging (Also Called "Digiscoping")
  • Camera lens remains attached.
  • Focal length of camera lens extended by telescope magnification where:
    magnification of
    Telescope = objective Focal Length eyepiece focal length
  • Example: 600mm focal length telescope (TV-85 telescope) with 10mm eyepiece = 600/10 = 60x. If camera uses 50mm lens, above equation yields 3000mm focal length.
  • An alternative way of determining effective focal length considers the camera lens and eyepiece combination acting as a relay system. Using the example above, the 50mm camera lens with the 10mm eyepiece gives 5x relay magnification. 5x600mm telescope objective focal length yields 3000mm focal length.

  • Couple camera lens thread or filter thread adapter to Tele Vue digital adapter ring DEC-0028 (28mm), DEC-0037 (37mm), or DEC-0049 (49mm) directly (or if necessary with a step-ring) and digital adapter ring attaching to eyepiece in telescope focuser.
Choosing an Imaging Method

In general, for the widest fields, using a telescope, try (1) Prime Focus. Note (2) Reducers can increase fields but may introduce vignetting in large CCD chip sizes. This is usually compensated by "flat-fielding" during processing. (Flat-fielding in processing has no relation to optical systems such as reducers with field flattening correction for the curved fields produced by two- or three-element refractors.)

For smaller fields, imaging portions of the moon, sun or planetary imaging, (3) imaging with Powermates is very effective. (Some of the best published images use Powermate amplifiers).

(5) Afocal Imaging is ideal for nature photography as well as planetary imaging when long eye-relief eyepieces are used (to prevent vignetting). A wide power range is easily achieved using Delos, Panoptic, Plössl and Nagler eyepieces with at least 17mm eye-relief. See eyepieces recommended for digiscoping in Eyepiece Specifications table. Check out Tele Vue afocal adapters.

(4) Eyepiece Projection using Tele Vue Plössls is also capable of great magnification, but we believe this technique has been superseded by (3)  imaging with Powermates (in quality) and (5) Afocal Imaging (in convenience).

T-Ring Shoulder to Focal Plane Distance is illustrated below.

T-Ring Shoulder to F...
T-Ring Shoulder to Focal Plane Distance