Setting Up a Nighttime Artificial Source

You can arrange the same light reflection at night by providing your own illumination. Light no longer comes from a constant angle of the Sun, so care is required in arranging a lamp. Attach a flashlight on a photographic tripod or other adjustable support and direct it towards your sphere. The flashlight performs best when stopped down to about 1 cm and placed about 1 meter away. This procedure will approximate the 72° angular diameter of the Sun, so the tables above will work the same. You may achieve a tighter beam if you stop the flashlight down off-axis, so that the filament of the lamp is not directly visible. Use a flat sheet of aluminum foil to make the mask. The exit portal need not be perfectly round. The flashlight should be placed as near to the line of sight as possible.

You might also wish to color balance the light more closely to the output of real stars. Coloring the source lamp is particularly useful when testing refractors. Most incandescent filaments have a black body color temperature of around 2500°C, or as red as Betelgeuse. The spectrum should be filtered either with a pale blue (Wratten 80A) eyepiece filter or by filtering the output of the flashlight with a similar light blue filter (Berry 1992). An ideal filter corrects indoor lighting (i.e., "tungsten" light bulbs) for outdoor slide film. For most testing, though, you will prefer a single color, and the reddish tinge of the flashlight is welcome. In fact, you may prefer to test in gold-colored spheres, which are also available as tree ornaments.

Choose a site without stray sources of light. A daytime test can be conducted nearly anywhere because the Sun is the brightest light source, but a nighttime test can be compromised by the presence of street lights nearby. Your flashlight is still likely to be the most prominent source reflected in the sphere. After all, it's close and is directed right at the sphere. However, nearby interfering street lights provide secondary glitter points, and other glints may make interpretation difficult—particularly at very small defocus aberrations.

Excluding stray sources of light is easy. For example, you could construct an "accordion" box to shade the sphere from all but lights in the direction of the telescope. This is a little cardboard box blackened on the inside and having a black poster board accordion-folded at the back. This folding acts as a non-glossy beam dump. Most often, though, all that is really required is a carefully-placed hood. Easier yet is to go where there aren't any spurious lights. Your usual observing site supposedly is a fairly dark locale. Just take the test equipment along.

You have one luxury that daytime testers do not have: you can move the source closer to the sphere until the pinhole begins to exceed the Airy disk. In fact, this process is made easier if you form a square hole in the flashlight mask. Stop and draw the flashlight back a small distance when this squareness appears.

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