M55 Star-Hop Guide for a 200-300 mm Dobsonian

What you need

Bring a 200-300 mm Dobsonian with its finder aligned, a low-power eyepiece, a medium-power eyepiece and a star chart or planisphere that shows stars to about magnitude 7 or 8 in this part of Sagittarius. A 10x50 binocular is useful for the middle stretch of the hop, because its wide field makes it easier to hold the path between stars.

Use a 25 mm eyepiece as your finding eyepiece. In a 200 mm f/5 instrument it gives about 40x, and in a 300 mm f/5 instrument about 60x. Eyepieces with a 50-degree apparent field give a true field of roughly 1.3 degrees at that power, which is wide enough to hold the whole route in a few fields. Switch to a 10 mm or 14 mm eyepiece only after you have found the glow, since higher power spreads a faint cluster thin.

Allow 20 to 30 minutes for your eyes to dark-adapt. Use a red light for reading charts, and keep white light off the finder and the eyepiece.

Starting point: the Teapot and Ascella

The Teapot is an asterism in Sagittarius with the rough outline of a teapot, with a lid, a body and a spout. Ascella, also called Zeta Sagittarii, is one of the stars that outline the lid. It is bright enough to see in a suburban sky without optics, and it is the anchor for the whole hop.

Nunki, also called Sigma Sagittarii, is another bright star in the lid region. Use it to confirm you have the right part of the Teapot before you start moving east. If Ascella and Nunki sit where your chart shows them, the rest of the route will line up.

Star hop: step by step

Work through these steps in order, checking each landmark before you move on.

1. Center Ascella in the 25 mm eyepiece. Confirm the position against Nunki and the other Teapot stars on your chart.
2. Move about 2.7 degrees up and to the left, toward Phi Sagittarii. On a chart with north up and east to the left, Phi lies northeast of Ascella. Phi is a third-magnitude star, so it is easy to pick out. Center it.
3. From Phi Sagittarii, move about 7.5 degrees east-southeast. This is the longest stretch. Switch to the 10x50 binocular for this part, since its field of about 5 to 6 degrees lets you cover most of the distance in one or two sweeps. Look for a loose field with few bright stars and check the stars against your chart as you pass them.
4. Return to the telescope and the 25 mm eyepiece once you are near the end of the stretch. The cluster lies in a sparse field, so look for the glow against a background of faint, scattered stars.
5. Center the glow. At 40x it occupies roughly a sixth of the field width, so it should be easy to see in the middle of the view once you are close.

If you do not see the glow, sweep slightly around the expected position. A small error in the direction of the last hop can move the cluster just outside the field. Re-check the chart and try again, rather than moving far in any one direction.

Confirming you have arrived

Several checks help you separate M55 from a random faint patch of sky.

First, look for a round, evenly lit glow with a brighter center. A single faint star looks like a point, while the cluster looks like a soft patch of light. Keep the glow centered and check that it does not change shape as you move the eyepiece slightly.

Second, go to a higher magnification. Switch to a 10 mm or 14 mm eyepiece and look at the edges of the glow. In a 200-300 mm instrument you should see individual stars at the edges, even though the center remains a blend. If you see only a smooth glow at high power, the view is still consistent with M55, since the core is too crowded to resolve from a suburban sky.

Third, use averted vision. Look slightly to one side of the glow. Faint, extended objects often appear more clearly when you do not look straight at them, because the outer part of the eye is more sensitive in dim light.

Fourth, compare the glow's position with your chart. The cluster should sit where the chart places it relative to the faint stars you have passed. If the position is off by more than a field, your last hop probably landed on a different patch.

What to expect from a suburban sky

Suburban light pollution lifts the background brightness of the sky and reduces contrast. M55 is a low-surface-brightness object, so it will look fainter and less defined than photographs suggest. The glow is soft, and resolving its edges into stars needs steady air and well-adapted eyes.

The region also has many faint stars, so the field can look busy. Use a chart with stars to at least magnitude 7 or 8, and check the stars you pass rather than relying on memory. A star that looks like a cluster member at low power may turn out to be a field star at higher power.

Be prepared for a view that is modest. In a suburban sky the cluster is visible but not dramatic, and the individual stars at the edges may be hard to hold for long. Darker sites give a clearer view, but the steps in this guide still work from a suburban location.

Observing from different latitudes

The cluster's altitude depends on your latitude, so the difficulty of the hop changes with where you observe.

At 35° N

At this latitude the cluster reaches a maximum altitude of 24 degrees. It is low in the southern sky and sits behind more atmosphere than it would from a southern site, which softens the view. Choose a site with a clear southern horizon, free of trees, rooftops and hills. Set up early in the evening in September, when the cluster is near its highest point, and allow the telescope to cool before you begin.

At 50° N

At this latitude the cluster reaches a maximum altitude of only 9 degrees, close to the horizon. Trees, buildings and haze often block it, and the light passes through a long path of air. Observing is only practical on a clear, steady night with an unobstructed southern horizon. Expect a dim and washed-out view, and treat the cluster as a marginal target from this latitude.

At 35° S

At this latitude the cluster reaches a maximum altitude of 86 degrees, nearly overhead. It is easy to reach with a Dobsonian, but the eyepiece will sit at a steep angle. Use a reclining chair or a stool, and keep the telescope on a base that rotates smoothly so you can follow the field without straining your neck. The star hop uses the same stars, but the orientation of the field will differ from northern charts. Check each landmark against a chart drawn for the southern sky rather than relying on the northern orientation.

Timing your session

The best months for M55 are August and September. In September the cluster is well placed in the early evening, and in August it is better placed later in the night. Begin the hop once the sky is fully dark, which in these months means after astronomical dusk. Start with the Teapot and Ascella, then work through the steps while your eyes are adapted.