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Equipment buying guides

Refractors — lenses and contrast

A lens objective forms the image without a secondary-mirror obstruction. Small refractors are portable and quick to set up.

An achromat costs less; a well-corrected ED or APO is usually better for imaging. The APO label alone does not describe the entire image field.

Light from the object · Objective lens · Focus / camera or eyepiece123
  1. 1Light from the object
  2. 2Objective lens
  3. 3Focus / 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?

Wide fields, large nebulae and clusters; with sufficient aperture, also the Moon, planets and double stars.

Limitations and trade-offs

Large apertures quickly increase cost and weight. Achromats may show colour fringes; an ordinary doublet or triplet does not necessarily have a flat field.

Check before buying

  • Colour correction and sample raw frames
  • Required flattener and sensor spacing
  • Focuser load and total assembly length

For imaging, compare corner stars on a sensor similar in size to yours.

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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