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Deepsky Observer's Companion tutorial
Part 8: Selecting an observing site

In this section: criteria for an observing site, site selection and dew.

When selecting an observing site, various criteria should be considered. I've collected the points of view of a few authors, summarising the properties they think are important.

My feeling is that convenience and comfort should be carefully considered. The quicker and easier you can observe, the more often you'll do it. Also, as MacRobert notes, "your surroundings colour your experience of the Universe." I've spent hours observing alone in a nature reserve, with only the sound of small animals and the nearby stream to compliment the celestial views - a different level of enjoyment compared to my backyard, with its easy access to the CD player and coffee machine.

Hunter (1989) considers:

  1. transparency
  2. favourable weather
  3. height above sea level
  4. seeing
  5. amount of sky visible from the site
  6. cost of using the site
  7. cost of transport to the site
  8. set-up time

MacRobert (1991) presents his "list of six things that matter." They are:

  1. lights nearby
  2. light pollution in the sky itself
  3. how much sky is visible
  4. convenience
  5. privacy/safety
  6. the location's overall aesthetics.

From Sidgewick (reference incomplete):

  1. remoteness from road and rail traffic
  2. remoteness from any urban area, especially in the direction of the prevailing wind, and in inhabited buildings in the immediate vicinity,
  3. absence of all artificial lights not controllable from the site
  4. clear view of the whole stellar hemisphere
  5. protection from the wind
  6. surroundings planted with low vegetation, or at least grassed

Site selection and dew

To help eliminate dew, you can choose a site less prone to dewing. As it gets later in the evening, the air cools, gets heavier and sinks into low-lying areas. The moisture-laden air thus settles into valleys and low depressions, and dew is likely to form; to avoid this, select a site situated on the side of a gentle hill.

MacRobert's (1995) suggestions: "Geography is critical. Smooth, laminar airflow is the ideal sought by observatory siting committees worldwide. The best sites on Earth are mountaintops facing into prevailing winds that have crossed thousands of miles of flat, cool ocean. You don't want to be downwind of a mountain; the airstream breaks up into turbulent swirls after crossing the peak. Nor do you want to be downwind of varied terrain that absorbs solar heat differently from one spot to the next. Flat, uniform plains or gently rolling hills extending far upwind can be almost as good as an ocean for providing laminar airflow. You may learn to predict which wind direction brings you the smoothest air."

Where to go observe?

The "Observing Sites" section of this website describes observing locations across the world. Why not submit a report on your favourite spots?

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