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PART IV: Observing the sky
4.4 Cooling the Telescope
• Telescopes require at least 10 to 30 minutes to cool down to outside air temperature. This
may take longer if there is a big difference between the temperature of the telescope and
the outside air. This minimizes heatwave distortion inside telescope tube (tube currents).
Allow a longer cooling time for larger optics.
4.5 Adapting Your Eyes
• Do not expose your eyes to anything except red light for 30 minutes prior to observing.
This allows your pupils to expand to their maximum diameter and build up the levels of
optical pigments, which are rapidly lost if exposed to bright light. It is important to observe
with both eyes open. This avoids fatigue at the eyepiece. If you nd this too distracting,
cover the non-used eye with your hand or an eye patch. Use averted vision on faint ob-
jects: The center of your eye is the least sensitive to low light levels. When viewing a faint
object, don’t look directly at it. Instead, look slightly to the side, and the object will appear
brighter.
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