Telescopy - Choosing Your Configuration

When building or buying a telescope, there are seemingly endless configurations,
each with their benefits and drawbacks, their evangelists and detractors. Trying
to choose between one or another can be a difficult task; a task eased by a
simple but thorough comparison.

There are three optical designs available, each with advantages
and disadvantages:

  1. Refractive (dioptric) telescopes uses a lens as the objective to form an
    image.
  2. Reflective (catoptric) telescopes use one or more curved mirrors to
    reflect light to form an image.
  3. Catadioptric telescopes use a combination of mirrors and lenses to focus
    and form an image.

Refractive/Dioptric Telescopes

Refractive telescopes use one or more lenses to bring an image into focus and
were the earliest type of telescope to be built, the earliest being built around

  1. Refractive optics are used in binoculars, some long camera lenses,
    microscopes, and other such devices. Despite their ubuiquity, there are a number
    of issues with refractive telescopes which do not affect reflective telescopes,
    or which affect reflective telescopes to a smaller degree:

An issue affecting larger apertures is that of lens sagging, the means by which
gravity deforms a large lens that is held by the edges. Yerkes Observatory in
Williams Bay, Wisconsin houses the largest refractive telescope used for
scientific research. The 100cm lens has experienced enough sag to cause small
optical distortions. Lenses may only be supported around the edges, which means
the weight of the lens can cause deformation of the lens itself. Mirrors in
reflective telescopes can suffer the same issue if they’re not adequately
supported, but you have the opportunity to support the mirror on the entire
opposing face, leading to increased strenght and allowing for very large
mirrors.

Chromatic and spherical aberrations tend to affect shorter focal length
refractors more than longer focal length refractors, thus an f/4 is far more
likely to display image halos than an f/10 or an f/12, but the longer focal
length devices will still demonstrate these issues. These issues can be
corrected only to a limited extent. Reducing the number of dioptric elements
in the system is the best way to reduce chromatic aberration. Spherical
aberration may be overcome by using non-spherical elements, a task made far
easier in the construction of reflective elements as opposed to refractive
ones.

Dioptric elements absorb some energy wavelengths more than others. Typically,
visible light passes quite well through a well-built lens, but if you’re
attempting to capture images using wavelengths other than those of the
visible spectrum, this behaviour can be a limiting factor. Additionally, a
lens’s entire volume must be free of imperfections in order for the image to
be transmitted correctly. This is a difficult proposition, and it is far
simpler and more effective to use a mirror which requires that only one
surface be perfectly polished.

Beneficially, one can construct very small reflective devices much more
easily than reflective devices of equal size. This is especially true depending
on the configuration of the devices. Consider microscopes and binoculars, which
are largely refractive devices. Some of these devices use mirrors for reflection,
but rarely are they used for magnification. More often to redirect an image to
another location within the device.

In no way should this explanation be considered an exhaustive overview of
the benefits and pitfalls of refractive telescopes. I think it’s quite
obvious that I’m more interested in reflective telescopes than refractive
ones, and that’s fine. It’s okay to be more interested in refractive
telescopes too. But as I’m not interested in attempting any lens-making, I’ll
be sticking with reflective telescopy design in the future.

Reflective/Catoptric Telescopes

Nearly every telescope built for scientific purposes is a reflective
telescope. These telescopes use one or more mirrors to bring an image into
focus.

An immediate benefit of purely reflective systems are the complete lack of
chromatic aberration. Light that passes through a glass lens is dispersed,
but the different wavelengths cannot be made to converge at the same focal
point, resulting in “fringes” of colour in the resulting image.

Given that the system’s reflective surfaces can be supported on the entire
opposing face, it is much more practical to build large aperture devices.
The technologial limit for monolithic mirrors is roughly 8 metres, though
there are a few examples, such as the Large Binocular Telescope, that have
monolithic mirrors larger than 8 metres. Beyond that size, we enter the
realm of segmented mirrors, where the mirror in question is composed of
two or more specially shaped and designed mirrors that are aligned in
order to provide a larger effective aperture. The increase in light-
gathering power of the device - the greater range of focal ratios -
increases the flexibility of the configuration. You can find scopes that
are great for imaging deep sky objects, or planetary bodies, or celestial
bodies. Compared to refractive telescopes, the available options are
exceedingly great.

Additionally, reflective devices have the opportunity to work with a wider
range of the energy spectrum. Dioptric elements absorb certain wavelengths
of energy as it passes through. Under most circumstances, reflective
elements won’t absorb visible light, nor light in the infrared and
ultraviolent range. Reflective devices can also capture radio signals, and
in some cases mirror coatings other than aluminum/silver may be used. Gold
coatings can be used to enhance the reflection of radio signals for example.
The extreme thinness of the coatings results in remarkably low cost,
regardless of the material used.

Reflective telescopes suffer certain defects more often than refractive
telescopes. Defects such as coma, field curvature, astigmatism, and
distortion are often simple to overcome but it is difficult to overcome
each simultaneously. Modified/alternate mirror configurations may be
used to correct for some of these issues.

Combination/Catadioptric Telescopes