Site Selection and Observation Planning

The success or failure of an observing session is largely decided before you ever set out. No matter how capable your equipment is, it cannot offset the effect of clouds, moonlight, and light pollution on imaging and visual observing. This page systematically covers two topics: the criteria for choosing an observing site, and how to build a plan for an entire night, including assessing light pollution, forecasting weather and seeing, checking visibility and transit times, and arranging a target list.

Site Selection Essentials

An ideal observing site requires balancing several mutually constraining conditions; a single location can rarely be optimal across all metrics at once, so your actual choice should be weighted toward the priorities of your observing goals. The table below summarizes the main site selection factors.

Factor	Meaning	How to assess
Darkness	Low artificial light pollution, faint night-sky glow, visible Milky Way	Light pollution maps, Bortle scale, SQM measurements
Seeing	Weak atmospheric turbulence, steady star images, high resolution	Smooth high-altitude airflow, distance from surface heat sources, Clear Sky Chart forecasts
Clear	High proportion of clear nights, low cloud cover	Climate statistics, satellite imagery, numerical weather prediction
Transparency	Weak atmospheric extinction and scattering, low precipitable water	Clear Sky Chart transparency row, relative humidity
Open horizon	No obstruction from mountains, trees, or buildings	On-site survey, horizon azimuth–altitude map
Altitude	High elevation reduces atmospheric thickness and water vapor	Topographic maps, elevation data
Accessibility and safety	Road conditions, parking, overnight safety	On-site confirmation; never sacrifice safety for dark skies

A model site: Paranal in Chile sits at about 2,635 m in the Atacama Desert. Its climate is extremely dry and far from urban light pollution, excelling in darkness, seeing, and proportion of clear nights alike. 图源 Rivi · CC BY-SA 3.0

Professional observatory sites are almost all located in a few types of region—high-altitude, dry mountainous areas typified by Chile’s Atacama Desert and Mauna Kea in Hawaii. Mauna Kea has more than 300 clear nights per year, with median seeing often better than 0.6 arcseconds; some Atacama sites have usable nights amounting to nearly 90% of the year. These places simultaneously satisfy high altitude, dryness, low cloud cover, dark skies, and a stable atmosphere—precisely the result of stacking the factors above. Amateur sites cannot reach this level, but the dimensions of assessment are the same.

Different goals weight the factors differently

Deep-sky imaging values darkness and transparency most, since darkness sets the ceiling on signal-to-noise ratio; planetary and lunar imaging, along with high-magnification visual observing, depend more on seeing, because it directly limits the resolvable detail. Narrowband imaging is relatively insensitive to light pollution and can work under a brighter sky; wide-field Milky Way work and faint diffuse nebulae, by contrast, almost always require traveling to a dark site. When choosing a site, first decide on the main goal for the night.

Assessing Light Pollution

There are three complementary tools for judging how “dark” a site is: the qualitative Bortle classification, quantitative sky brightness measurement, and globally covering light pollution maps.

The Bortle Dark-Sky Scale

The Bortle dark-sky scale was proposed by amateur astronomer John E. Bortle in 2001 in Sky & Telescope. It divides the night sky into classes 1 through 9 based on naked-eye sky phenomena: class 1 is pristine dark sky, class 9 is an inner-city sky—the smaller the number, the darker the sky. It is a qualitative scale calibrated against the observing experience.

1 Excellent dark sky

2 Typical truly dark sky

3 Rural sky

4 Rural/suburban transition

5 Suburban sky

6 Bright suburban

7 Suburban/urban transition

8 City sky

9 Inner-city sky

Bortle classes 1–9: the smaller the number, the darker the sky. Deep-sky imaging is generally recommended at class 4 or darker, while visually discerning Milky Way structure requires class 3 or darker.

Sky Quality Measurement (SQM)

The Sky Quality Meter (SQM) quantitatively measures the zenith sky brightness in units of “magnitudes per square arcsecond (mag/arcsec²).” A larger value means a darker sky, the opposite direction from the Bortle scale. The typical correspondence is as follows.

Bortle class	SQM (mag/arcsec²)	Sky characteristics
1	about 21.7–22.0	Pristine dark sky; zodiacal light and airglow clearly visible
2–3	about 21.3–21.7	Rural dark sky; rich Milky Way structure
4	about 20.4–21.3	Rural–suburban transition; Milky Way visible but the low sky is bright
5–6	about 19.1–20.4	Suburban; Milky Way faint or only discernible near the zenith
7–8	about 18.0–19.1	City edge to urban; only bright stars visible
9	below about 18.0	Inner city; the whole sky is bright

Note that SQM readings are affected by the Moon phase, zodiacal light, and airglow; they should be taken toward the zenith on a moonless night away from the Milky Way plane, with conditions recorded for comparison.

Light Pollution Maps

Light pollution maps (such as lightpollutionmap.info) overlay satellite-observed artificial nighttime brightness on a terrain base map, usually presented with color coding: deep blue to black corresponds to Bortle 1–3, green to 4–5, yellow-orange to 6–7, and red-white to 8–9. Most maps let you click any coordinate to read off that point’s Bortle class and modeled SQM value, making them the most efficient tool for quickly screening candidate sites before a trip.

A rule-of-thumb correspondence

Being able to see a clear Milky Way band roughly corresponds to Bortle class 4 or darker; if you cannot even count all the bright stars of the Big Dipper, the sky is basically class 7 or higher—an urban sky. Maps give modeled values, which often deviate in the field due to local light sources or atmospheric conditions, so it is more reliable to verify with an SQM or your eyes once you arrive.

Atmospheric Seeing

Atmospheric seeing refers to the shimmer and blur of star images caused by atmospheric turbulence, and is the main factor limiting the resolution of planetary, lunar, and high-magnification visual observing.

Cause: as light passes through atmospheric layers whose refractive index varies with temperature and wind speed, it undergoes random deflection, broadening a point source into a diffuse blob that jitters constantly. Strong wind shear brought by the high-altitude jet stream, convection from the heated ground, and heat sources in the telescope’s near field (the ground, walls, and air inside the tube) all degrade seeing.

Measurement: it is usually expressed as the full width at half maximum (FWHM) of the star image at a wavelength of 500 nm, in arcseconds (″). Another common quantity is the Fried parameter (r₀), the equivalent aperture at which “the diffraction-limited resolution exactly equals the seeing-limited resolution”; a larger r₀ means better seeing. The relationship between the two is approximately:

ε ≈ 0.98 · λ / r₀

where ε is the seeing angle (in radians), λ is the wavelength, and r₀ is the Fried parameter.

Seeing FWHM	r₀ (at 500 nm)	Rating
about 0.4″	about 25 cm or more	An excellent night at a top-tier site
around 1.0″	about 10–15 cm	Good seeing at an average site
2″–4″	about 3–6 cm	Average to poor; planetary detail is limited

Because resolution improves with wavelength, the seeing limitation is relatively relaxed at longer wavelengths, which is one reason large-aperture telescopes more easily approach the diffraction limit in the infrared. Seeing quality is unrelated to sky transparency: a cloudless high-pressure clear night may have very poor seeing due to a high-altitude jet stream, while a night with dense thin cloud may instead have steady star images.

Weather and Forecasts

Before observing, you need to combine multiple data sources to judge whether the night is usable. The following types of forecast are tailored to astronomical use.

• Clear Sky Chart (cleardarksky.com): hourly charts based on a Canadian meteorological model, specifically forecasting cloud cover, transparency, and seeing—three things that ordinary civil and aviation forecasts do not provide. The chart uses colored blocks to show conditions for each time slot over roughly the next 48 hours, with each site being a point forecast for about a 9-mile radius. Coverage is mainly North America.
• Clear Outside (clearoutside.com): a global astronomy forecast that gives, by time slot, total cloud cover and high-, mid-, and low-layer cloud, precipitable water, humidity, wind, dew point, seeing, and visibility above sea level, and marks twilight and Moon phase information.
• Satellite imagery and numerical weather prediction: used to judge the movement trend of cloud systems, confirmed again just before departure.
• Jet stream forecast: a wind-speed chart at the high-altitude 200–300 hPa level. Seeing is often poor beneath the core of the jet stream, and planetary observers deliberately avoid nights when the jet passes overhead.

When judging transparency, watch precipitable water and relative humidity: more water vapor means stronger extinction and scattering, lower contrast on faint objects, and too high a humidity carries a risk of dew.

Cross-check data sources

No single forecast model is accurate in every situation; you should cross-compare at least two or three sources and confirm once more just before departure. Cloud cover, seeing, and transparency often cannot all be ideal at the same time, and you need to decide whether the trip is worthwhile based on the night’s main goal.

Planning Tools and Visibility Checks

Planetarium and planning software is used to “rehearse” the entire night before setting out—checking the sky at any moment, an object’s altitude above the horizon, its transit time, and its rise and set times.

Software	Platform	Main use
Stellarium	Desktop / web / mobile	Simulate the sky at any moment; check object altitude, transit, moonrise and moonset
SkySafari	Phone / tablet	Real-time field reference, with Go-To planning paired with an equatorial mount
Telescopius	Web	Object library and visibility curves, field-of-view simulation, observing list management
Cartes du Ciel	Desktop	Free open-source planetarium with detailed catalogs and equipment control

The core output of these tools is the curve of an object’s altitude versus time and its transit time. Whether a given celestial object can rise at a given latitude, how high it can reach, and which months of the year are suitable for observing it depend on its declination and the observing site’s latitude; see /astronomy/observing/hemisphere-visibility/ for details. At transit an object is at its highest altitude, takes the shortest path through the atmosphere, and suffers the least extinction, making it the best time to observe or image—a principle directly related to the diurnal motion discussed in /astronomy/foundations/celestial-coordinates/ and /astronomy/foundations/apparent-motion/.

Moon Phase and Twilight

The usable observing time is determined jointly by the Moon phase and how far the Sun is below the horizon.

Moon Phase

Moonlight is natural “light pollution.” The week or so around new Moon is the golden window for deep-sky observing and imaging; the bright lunar surface and the planets, by contrast, are unaffected by moonlight and can be scheduled near full Moon when the target is near transit and high in the sky. When planning, also check moonrise and moonset times: even on a bright-Moon night, the interval from moonset to astronomical dawn remains usable for deep-sky work.

Twilight

Twilight is divided into three types based on how far the Sun’s geometric center is below the horizon (the degrees below refer to the angle below the horizon).

Type	Sun altitude range	Observing significance
Civil twilight	0° to −6°	Natural light is still sufficient for outdoor activity; bright planets such as Venus are visible
Nautical twilight	−6° to −12°	The horizon can still be discerned on a moonless night; bright stars can be used for navigation
Astronomical twilight	−12° to −18°	Once the Sun is below −18° the sky is fully dark, and point sources such as stars can be observed

True “astronomical night” begins after the Sun drops to 18° below the horizon and ends before the Sun rises to −18° the next day. The length of this period varies with season and latitude, and at high latitudes in summer it may disappear entirely (white nights); it directly determines the number of hours actually usable for deep-sky observing that night. The three types of twilight at dawn and dusk are fully symmetric, likewise defined by the altitude of the Sun’s geometric center. The “blue hour” often mentioned in photography roughly corresponds to the low-light stage between civil and nautical twilight.

Why deep-sky work waits for astronomical night

During astronomical twilight, residual scattered sunlight raises the night-sky background and depresses the contrast of faint objects. Deep-sky imaging should schedule the main exposures within astronomical night; the astronomical twilight period can be used for focusing, polar alignment, test framing, and other preparation, so that the time before full darkness is not wasted.

Target Lists and the Meridian Flip

Gather the Moon phase, weather, twilight, and visibility information into a single, time-ordered target list.

Ordering principle: arrange objects by the time they cross the meridian (transit) rather than by interest, trying to ensure each object is observed or imaged near transit when it is high in the sky. Handle objects already sinking toward the west first, and leave objects rising in the east for the latter half of the night, to reduce the effects of low-altitude atmospheric extinction and turbulence.

Meridian flip: the meridian is the great circle on the celestial sphere connecting due north, the zenith, and due south, and an object is at its highest altitude as it crosses the meridian. When tracking with a German Equatorial Mount, if you continue tracking westward after the target crosses the meridian, the tube will eventually collide with the tripod or base. You therefore need to perform a “meridian flip”—swapping the tube and counterweight to the opposite side around the right-ascension axis, so the telescope continues tracking after switching from one side of the meridian to the other. The flip interrupts the current exposure and temporarily loses the framing and guiding, so long-exposure imaging needs to reserve time for the flip during planning and re-register the framing afterward using plate solving. Fork mounts and some designs can avoid this problem.

A target list entry usually contains the following fields.

Target	Transit time	Maximum altitude	Suggested focal length / field	Notes
M42 Orion Nebula	21:40	55°	Medium focal length	Shoot first; sinks west after midnight
M51 Whirlpool Galaxy	00:30	70°	Long focal length	Crosses meridian in the latter half of the night; watch for the flip
Galactic center	02:10	30°	Wide angle	Best before dawn; large low-altitude extinction

Record each target’s coordinates, required field of view or focal length, and suggested exposure, and reserve buffer time for focusing, polar alignment, meridian flips, and switching targets.

Planning a Night

1. Set the window: check the Moon phase and moonrise/moonset to determine the usable interval that is moonless or after moonset; choose a suitable Moon phase based on target type (deep-sky / planetary and lunar).

2. Check the weather: combine the Clear Sky Chart, Clear Outside, satellite imagery, and jet stream forecasts, compare cloud cover, transparency, and seeing, and confirm once more just before departure.

3. Compute astronomical night: determine the interval from the end of astronomical twilight to the start of astronomical dawn—this is the effective duration usable for deep-sky work.

4. Order by transit: use Stellarium or Telescopius to list each target’s transit time and altitude curve, order them by time window, and mark targets that will undergo a meridian flip.

5. Build the list: for each target write down coordinates, field of view, exposure, and notes, and prepare a backup target and a backup site.

Check visibility first

Whether a target can rise at your latitude and how high it gets decides whether it is worth putting on the list. Check hemisphere visibility

Then check conditions

How to read the on-site atmosphere and sky state. Observing conditions

Reserve a backup plan

Weather is the most variable factor. Always prepare a backup target on the other side of the sky and a backup site, so you can switch immediately when the main target is blocked by cloud and avoid wasting the whole night.

Logging and Review

After observing ends, recording the night’s actual conditions and results is the basis for improving future planning. Recommended records: date and location, SQM reading or visual Bortle estimate, subjective ratings of seeing and transparency, Moon phase, the targets observed or imaged and their parameters, and equipment problems and their solutions. Imagers should additionally record the actual number of exposure frames for each target, gain, filters, and discard ratio. Records accumulated over the long term help identify a site’s usability patterns across different seasons and calibrate the local accuracy of each forecast source.

Under a Bortle class 2–3 dark sky, the Milky Way's dust lanes and star-cloud structure are clearly discernible; such a sky is a prerequisite for wide-field Milky Way and faint nebula imaging. 图源 Bruno Gilli/ESO · CC BY 4.0

After completing the workflow above, an observing session will come closer to running as planned rather than relying entirely on on-the-spot judgment. For on-site equipment setup, reading the atmosphere, and visual technique, continue with /astronomy/observing/conditions/; for the basics of celestial coordinates, magnitude, and visibility, see /astronomy/foundations/magnitude/ and /reference/glossary/.

References

• Twilight — Wikipedia: the Sun-altitude definitions and observing significance of civil, nautical, and astronomical twilight.
• Astronomical seeing — Wikipedia: the causes of atmospheric seeing, its FWHM and Fried-parameter measures, and typical values.
• Clear Sky Chart — Wikipedia: the principles of hourly cloud-cover, transparency, and seeing forecasting for astronomical use.
• Bortle Scale & Sky Quality Meter (NightSkyPix / AstroBackyard): the correspondence between Bortle classes 1–9 and SQM (mag/arcsec²).
• Light Pollution Map (lightpollutionmap.info): a global light pollution and Bortle-class lookup tool based on satellite data.
• Automated Meridian Flip — N.I.N.A. documentation: the principle of the German equatorial mount meridian flip and its handling in the imaging workflow.