Altitude, Airmass and Timing a Target for Better Views with Any Telescope

Why altitude decides the quality of your view

Light from a target travels through Earth's atmosphere before it reaches you. The lower the target, the longer that path, and the more air, dust, water vapor and haze the light must cross. Astronomers call the amount of air along the line of sight the airmass. At the zenith, directly overhead, the airmass is about 1. At 30 degrees above the horizon it is about 2, and at 12 degrees it is about 5. Those figures are typical values, not measurements of your site, but they show how quickly the air thickens near the horizon.

What airmass means in practice

A longer path through the air scatters and absorbs more light. Stars and deep-sky objects appear dimmer, and the atmosphere blurs them more because air moves in layers with different temperatures and speeds. This blurring is called seeing. Near the horizon, you also see more of the glow from towns and roads, because that glow is reflected and scattered by the same thick air.

For most visual observing, you should aim to keep your target above about 30 degrees. Above that height, the view usually improves noticeably. Below about 20 degrees, expect a significant drop in contrast and sharpness, even in a dark sky. If you are imaging, the penalty is larger, because atmospheric color shifts and extra turbulence affect long exposures more strongly.

Problem 1: The target looks soft, shimmering or colored

How to recognize it. The target is low in the sky, and the image seems to boil, wobble or lose its fine detail. Stars may flare in different colors as you look at them, and planets may show color fringes even in a well-made instrument. You may also notice that the background sky looks brighter in one direction.

Why it happens. At low altitude, light crosses more turbulent air and more layers of different temperature. Each layer bends the light slightly, which smears the image. Colored fringes appear because the atmosphere bends blue light more than red light. Nearby buildings and sources of light add to the problem.

How to fix it.

• Move the target higher before you judge its detail. If the mount or your schedule allows, wait until it rises above 30 degrees.
• Use a lower magnification. A smaller image is less affected by seeing, and the overall view is steadier.
• Choose a different target in the same region of the sky that is higher at the time you observe.
• If you are imaging, shorten individual exposures and keep more frames. Discard frames that are obviously blurred, and stack the rest.
• Avoid comparing a low target with a high one on the same night. Judge the instrument and the sky by the view you get when the target is high.

Problem 2: The target never gets high enough

How to recognize it. The target rises, but it only reaches a low altitude at its best point in the sky. Some targets in your region may never climb above 30 degrees from your location. This is a property of the target's position and your latitude, not a fault in your equipment.

Why it happens. Every target has a fixed declination, which is its angular distance north or south of the celestial equator. From your latitude, the highest altitude a target reaches is approximately 90 degrees minus the difference between your latitude and the target's declination. For example, a target with a declination of 10 degrees north, seen from a latitude of 50 degrees north, reaches a maximum altitude of about 50 degrees. A target with a declination of 40 degrees south, seen from the same latitude, reaches only about 0 degrees, so it does not rise at all from there.

How to fix it.

• Calculate the maximum altitude before you choose a target. Subtract the absolute difference between your latitude and the target's declination from 90 degrees.
• If the maximum altitude is below 30 degrees, pick a different target, or wait for a season or year when the target is in a more favorable part of the sky for your location. This does not change the target's properties, only the altitude at which you see it.
• If you travel, plan observing trips to a latitude where your chosen targets reach higher altitudes.
• Use planetarium or sky-chart software that shows the altitude of a target at a given time and place. Check it before the session rather than estimating by eye.

Problem 3: You arrive when the target is already past its best

How to recognize it. The target was high when you began planning, but by the time you set up, it is low or setting. You may spend much of the night on targets that are rising, when the rising ones are not yet high enough to observe well.

Why it happens. A target reaches its highest altitude when it crosses the observer's meridian, the imaginary line running north to south through the zenith. Astronomers call this transit. The target's altitude is highest at transit and falls off on both sides. The time of transit depends on the target's right ascension and on the local sidereal time, which runs slightly faster than solar time. Each night, a target transits about four minutes earlier by clock time than the night before.

How to fix it.

• Find the transit time for your target before the session. Most planetarium software lists it directly. Plan to observe within about two hours of transit when the altitude is at its best.
• Check the time of your session against that window. If transit is at 23:00 local time, aim to observe between about 21:00 and 01:00.
• Begin with targets near transit, then move to targets that are rising or setting later, when their altitude makes them less suitable for detailed work.
• Keep a short written plan. List each target, its transit time and the time you intend to observe it, so you can adjust your order if the sky clouds over or you run late.

Problem 4: The target is high, but the view still looks poor

How to recognize it. The altitude is good, but the image is dim, low in contrast or affected by haze, or it looks different from what you expected. Stars may look fainter than they should, or the background may appear brighter than you anticipated.

Why it happens. Altitude is one factor among several. Local light pollution, dew, temperature changes, a poorly acclimated instrument, and obstructions near the horizon can all reduce the view, even when the target is high. A target at 60 degrees over a bright suburban sky can look less contrasted than a fainter target at the same altitude from a dark site.

How to fix it.

• Check your local horizon. Trees, buildings and streetlights can block parts of the sky that your target passes through during the session. Choose a target that stays above those obstructions.
• Wait for the instrument to reach the outdoor temperature. Large mirrors and thick lenses can take an hour or more to settle. Keeping the telescope covered outdoors before you start helps.
• Prevent dew with a dew heater or shield, or by removing condensation with a dry cloth. Dew scatters light and reduces contrast in the eyepiece and on the lens.
• Observe after astronomical twilight has ended. Starting too early leaves the sky bright, and the target looks flat.
• Set realistic expectations. From a suburban sky, many deep-sky objects appear as faint patches or only their brightest parts. A high position improves the view, but it does not make faint detail visible.

Problem 5: The target drifts or the mount struggles near the meridian

How to recognize it. This applies mainly to equatorial mounts. As the target approaches the meridian, the tracking may seem to fail, or the telescope may need to be reoriented. The target drifts from the centre of the field, and you may find the eyepiece on an awkward side of the mount.

Why it happens. On a German equatorial mount, the telescope tube passes over the top of the mount when a target crosses the meridian. Most mounts then perform a meridian flip, which moves the tube to the opposite side so it can continue tracking. During the flip, the target is briefly out of view, and some setups lose a few seconds or minutes of alignment. Some mounts also limit how far the telescope can point past the meridian.

How to fix it.

• Find out when your mount will flip, and plan to do the flip at a time when you are not in the middle of a critical observation or imaging sequence.
• After a flip, recenter the target and check focus and alignment before you continue.
• If your mount does not flip automatically, decide whether to observe on the other side of the meridian or wait for the target to move past it.
• For visual work, a fork-mounted or altazimuth mount usually avoids this issue, though tracking near the zenith can still be awkward on some designs.

Building a session around altitude

A good plan orders targets by altitude, not by how interesting they sound. The steps below give a simple method you can repeat before any session.

1. Note your latitude and the declination of each target you want to see.
2. Calculate the maximum altitude of each target from your location, and discard any that stay below about 30 degrees.
3. Find the transit time for each remaining target, and write it next to its name.
4. Schedule each target to fall within about two hours of its transit, starting with the targets that transit earliest after dusk.
5. Check the local horizon and remove any target whose path crosses known obstructions.
6. Plan a mount flip or other change of orientation at a point where it will not interrupt your best observations.
7. On the night, begin with the instrument cooled and free of dew, and watch the altitude of each target as the session progresses.

This method takes about twenty minutes to set up, and it prevents most of the problems described above. Once you have used it a few times, you will find it quick to repeat for any target.