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Equipment buying guides

Newtonians — aperture and simple optics

The primary mirror collects light and a flat secondary sends it to a side-mounted focuser. This is a popular way to obtain substantial aperture at a reasonable cost.

Visual models and astrographs may differ in focal position, focuser and secondary size. Not every Newtonian reaches focus with a camera straight away.

Light from the object · Primary mirror · Flat secondary mirror · Focus / camera or eyepiece1234
  1. 1Light from the object
  2. 2Primary mirror
  3. 3Flat secondary mirror
  4. 4Focus / camera or eyepiece
Functional schematic, not to scale. Rays and component shapes are simplified; this is not a particular model’s optical prescription.

What is it good for?

Deep-sky and planetary observing; fast astrographs also suit imaging of extended targets.

Limitations and trade-offs

Collimation and thermal equilibration are needed. Off-axis coma is particularly visible in fast systems; a long tube places substantial demands on the mount.

Check before buying

  • Ability to reach focus with the camera
  • Coma corrector matched to focal ratio
  • Focuser rigidity and collimation procedure

Dobsonian describes the mounting arrangement, not a different Newtonian optical design.

Matching your setup

Start with targets and observing method. Large nebulae need a sufficiently wide field; small objects need image scale and adequate aperture. More focal length does not by itself mean a better telescope.

Focal ratio is focal length divided by aperture: 400 mm / 80 mm gives f/5. Camera field also depends on sensor dimensions; compare systems with the same sensor rather than f-numbers alone.

Budget for mount, field correction, power, dew control and transport. Check focuser load and dovetail fit. Atmospheric conditions and setup quality also limit the final image.

Documentation and further reading
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