M21 (NGC 6531) Star-Hop Guide for Binoculars and Small Telescopes

Equipment and sky conditions

A 10x50 binocular, or a 70 to 90 mm refractor with a low-power eyepiece, shows M21 as a group of individual stars. A wider field makes the star-hop easier, because you can see the bright guide stars and the cluster in the same view. Use a finder chart or planisphere that shows stars to about magnitude 6 so you can match what you see with the chart.

Allow about 15 to 20 minutes for your eyes to dark-adapt before you start. Keep any white light away from your eyes, and use a red torch if you need to read a chart. A suburban sky, with a naked-eye limit near magnitude 5, will hide many of the fainter stars in the cluster. If your view is washed out, move to a darker site or wait until the sky is fully dark.

Start at low magnification, between about 20x and 40x. This gives a field wide enough to hold the cluster with its surrounding stars. Higher powers make the cluster smaller and fainter, and they make it harder to hold steady.

Star-hop from Kaus Borealis

Kaus Borealis (Lambda Sagittarii) is one of the brighter stars in the teapot-shaped pattern of Sagittarius. It lies on the northern side of that pattern, above Kaus Media (Delta Sagittarii) and Kaus Australis (Epsilon Sagittarii), so it is a good place to begin.

1. Find the teapot. In July and August evenings, look low in the south for the teapot-shaped pattern of Sagittarius. From the Southern Hemisphere, the teapot stands high. Locate Kaus Borealis on the northern side of the pattern.

1. Set your field. Center Kaus Borealis in your binocular or finder field. Check that you can see the stars around it, which will help you confirm your position.

1. Move about 6 degrees west. Sweep west from Kaus Borealis until you have covered about 6 degrees. In a 10x50 binocular, this is a little more than one field width, so do this in two or three short sweeps and check the stars you pass against your chart.

1. Move about 3 degrees north. Shift upward by about 3 degrees. M21 lies in a field with a fairly sparse background, so you should notice that the stars become less crowded as you approach it.

1. Confirm the cluster. At this point, M21 should be near the center of your field. Increase magnification only after you have seen the cluster at low power.

If you can see Mu Sagittarii, use it as a shorter route. Mu Sagittarii lies about 4 degrees north-west of Kaus Borealis. From Mu Sagittarii, M21 is about 4 degrees west and about 1.5 degrees south. This is a single, short hop that can be quicker than the route from Kaus Borealis, especially when the teapot is low or partly blocked by a building or tree.

Confirming you have arrived

Several bright landmarks in the same region can help you confirm your position before you look for the cluster.

• Trifid Nebula. This nebula lies less than 1 degree to the south-west of M21. It appears as a faint, hazy patch, and in a small telescope it has a dark lane through it. Do not mistake it for the cluster: the Trifid is a nebula, not a group of stars.
• Lagoon Nebula. This nebula lies about 2 degrees south of M21. It is a larger, brighter patch in binoculars, and it is a useful marker for checking the southern side of your field.

Once you think you have found M21, look for a loose group of faint stars with no obvious nebulosity around them. The cluster will look like a small scatter of stars, typically a few dozen, that are noticeably denser than the stars around them. Its overall width is about one-fifth of the Moon's apparent width, so it is small in the field.

To check your identification, move the field slightly and watch the group. If it stays in the same place relative to the bright stars, you have found it. If it drifts with the stars around it, you are looking at a chance alignment, so return to the guide star and repeat the hop.

It helps to note the position of M21 relative to a nearby bright star in your field, such as the stars you used as guide stars. Write down the field, the eyepiece, and the orientation. That way, you can return to the cluster quickly on later nights.

Latitude and altitude notes

M21 is visible from both hemispheres, but its altitude depends strongly on your latitude. The maximum altitudes are 33 degrees at 35 degrees north, 18 degrees at 50 degrees north, and 77 degrees at 35 degrees south.

At 35 degrees north, M21 reaches a maximum of 33 degrees. The cluster is low in the southern sky, and you will need a clear southern horizon. Start your hop early in the evening, while the teapot is still well placed. The cluster is easier to see when it is higher, so avoid looking through a long column of air near the horizon.

At 50 degrees north, M21 reaches a maximum of only 18 degrees. At this altitude, the light passes through a thick layer of atmosphere, and haze and light pollution near the horizon can hide faint stars. Choose a site with an open southern horizon, and go out as soon as the sky is fully dark. The cluster may be visible only for a short time in July and August.

At 35 degrees south, M21 reaches a maximum of 77 degrees. The cluster is high in the sky, which makes the star-hop easier and the view steadier. From the Southern Hemisphere, the teapot is well placed, and Kaus Borealis is easy to find.

In both hemispheres, July and August are the best months for observing M21. In those months, Sagittarius is well placed in the evening. By October and later, the cluster sets earlier in the evening, and you will have less time to observe it before it is low in the west.

What you will see

At 70 to 90 mm, M21 appears as a group of individual stars. You may see a few dozen faint points, most of them of similar brightness, against a background of fainter field stars. The group is small, and it will look better at low power than at high power. At 20x to 40x in a 70 to 90 mm refractor, the group fills a fair part of the field and the stars are clearly separated from one another.

Do not expect the cluster to look like a photograph. Long exposures show colours and dense star fields that the eye does not see at the eyepiece. In a suburban sky, the cluster will look faint and modest, and you will need to be patient while your eyes adjust. Averted vision, where you look slightly to one side of the cluster, often makes faint stars easier to detect.