Application Note:
Introduction to Solar Eclipse Science & Viewing

Overview

On Monday, August 21, 2017, those in parts of North America from Lincoln Beach, Oregon to Charleston, South Carolina will be treated to a rare total solar eclipse– where the Moon, passing in alignment between the Sun and the Earth will briefly and completely block out the Sun. Those located in other parts of North America will also experience views of a partial solar eclipse. In this brief application note we'll define some basic eclipse terminology and go through how to construct an optical beam expander setup to safely view a solar eclipse.

Solar Eclipse Phases Sequence

Solar Eclipse Terms & Definitions

Safely observing the Sun requires careful attention and precaution to avoid dangerous eye damage. Simple pinhole shadow viewers and optical light expansion setups are great ways to safely enjoy this rare event while learning about basic optical principles and light management techniques. Here we'll explore solar eclipse science and terminology along with a list of additional resources and reference links to learn more.

  • Antumbra - Area of shadow beyond the umbra.
  • Baily's Beads - Named in honor of English astronomer Francis Baily, Baily's Beads are the effect of slinlight shining through valleys and surface elevation variances on the Moon's surface appearing as bright beads along the outer disk of the Sun.
  • Corona – Extending millions of kilometres into space, the Sun's corona is a visible aura of plasma surrounding the sun. The corona is most easily seen during a tlital solar eclipse, during the period of totality (see below).
  • Penumbra - Shadow cast by the moon over an area experiencing a partial eclipse.
  • Syzygy - The straight-line configuration of three celestial bodies in a gravitational system– Sun, Moon and Earth in the case of a solar eclipse.
  • Totality — Period when the Moon completely obscures the disk of the Sun leaving only its solar corona visible.
  • Umbra - The innermost and darkest part of the Moon's shadow.
Solar Eclipse Geometry
Basic terminology and geometry of a total solar eclipse. (Image credit: wikipedia.org)
Solar Eclipse Visualisation
Visualisation of a solar eclipse from different positions. Each icon shows the view from the centre of its black spot, representing the moon. (Image credit: wikipedia.org)

More Eclipse Related Resources & Links

NASA.gov - Total Eclipse 2017 Eclipse Across America  
NASA.gov – Eclipse: Who? What? Where? When? and How?  

Basic Light Beam Expansion

Beam expansion or reduction is a common application requirement in most labs using lasers or light sources and optics. There are many ready made beam expanders available on the market, but often they are not available in the required expansion ratio or spectral range. And, for students, or those working within a tight budget, the plug and play solution may not be the answer. Luckily, building a beam expander from off-the-shelf components is easily accomplished and can be fit into most budgets. The quality of the output is dependant only on the input beam and the component optics used.

Simple beam expanders, sometimes referred to as telescopes, in their most basic forms generally consist of two lenses. The first lens should have a diameter larger than the maximum expected input diameter of the incoming light source. For example, if the diameter of the incoming beam is 10mm, a 12mm diameter lens will do a nice job with a little room to spare. The input beam is assumed to be collimated.

The diameter of the output lens should be larger than the desired exit beam diameter. For a 3x expansion ratio with a 10mm maximum input beam diameter, an output lens of greater than 30mm in diameter is required. The useable diameter of a lens is usually specified at approximately 80% of the actual lens diameter, a minimum lens diameter of 37.5mm would be required. If you are using off-the-shelf lenses, this will translate to 38-50.8mm in diameter. The magnification of a 2 lens system is equal to the ratio of the focal lengths of the lenses, which is also equal to the ratio of the radii of curvatures of the lenses.

For more in-depth technical explanations, formulas and optical arrangement examples see Optics Technical Note: How to Build a Beam Expander (PDF, 300 KB) and Optics Technical Note: Focusing and Collimating.

Galilean Beam Expander Optics and Components List

For Monday's Total Solar Eclipse, we'll be constructing a basic two lens Galilean Beam Expander setup including one small 0.5" (12.7 mm) diameter bi-concave lens (KBC028AR.14) for gathering and condensing the Sun's light during the eclipse, and a second large 3" (76.2 mm) diameter plano-convex lens (KPX223AR.14) for collimating and projecting the Sun's light onto a basic white paper backdrop screen. A makeshift foil disk pinhole will also be placed over the first bi-concave lens to limit the total gathered light intensity. All lenses and mounts have been mounted and aligned using Newport's A-Line™ Lens Alignment System components including notched guide rails and specialty rail lens mounts. Enclosing the area between the two lenses helps avoid unwanted light leaks and stray aberrations in the collimated light beam.

Galilean Beam Expander Setup

Related Product Categories

A-Line Lens Alignment System

A-Line™ Lens Alignment System

A-Line Fixed Lens Mounts

A-Line™ Fixed Lens Mounts

Plano-Convex Lenses

Plano-Convex Lenses

Bi-Concave Lenses

Bi-Concave Lenses