Sherry D. Byers
Administrative Assistant

P: (540) 464-7225
F: (540) 464-7767

320 Mallory Hall
Virginia Military Institute
Lexington, VA 24450

Physics@vmi.edu

Refractor vs. Reflector?

Is a reflector better for astrophotography? Or a refractor? Well.... there are several key considerations:

  1. Are you interested in photographing a very faint object? Perhaps a faint Cepheid variable star? A large reflector will collect a lot more light, and the resulting small field of view is not a problem.
     
  2. Are you interested in a moderately wide field, say enough to capture the Orion nebula? Then a short focal length telescope (and correspondingly smaller plate scale) will get more of the sky onto film of a given size. Plate scale (in arcseconds/mm) and the amount of the sky shown on your image depends only on the focal length of the telescope.
     
  3. For photographs of nebulae and galaxies, the focal ratio of the telescope is very important! The brightness of stars (point sources) is proportional to D2 (the Diameter of the telescope, and D2 is proportional to the collecting area, pi*R2). However, the brightness of a nebula is proportional to D2/FL2 since the image of the nebula is also spread out over x and y dimensions by an amount proportional to the focal length (FL). 
     
  4. Reflecting telescopes, with their secondary mirror (generally with mirror supports), often produce "diffraction spikes" around star images. While not aesthetically objectionable, it is true that any obstructions can degrade the image a little. Refractors don't suffer from this problem.

In summary:

Refractors generally have the edge over reflectors for a quality image, both from the standpoint of lens/mirror quality and precision optical mountings, and lack of central obscurations.   Equally important in a choice of telescope type is the focal length and the resulting field of view.