Ptolemy Cluster (M7) with an 8-inch SCT: A Star-Hop from Shaula

Where the cluster sits

Within Scorpius, the cluster lies to the east-northeast of Shaula, roughly 4 to 5 degrees away. The apparent diameter is 22 arcminutes, a little under three-quarters of the width of the Moon. At that size, it fills a small part of a low-power eyepiece field and is easy to lose if your hop is off by a fraction of a degree.

From a suburban sky with a naked-eye limit near magnitude 5, the cluster can be picked out by eye once you know where to look. Its combined magnitude of 3.3 is bright enough for binoculars to show it as a compact group, and the 8-inch SCT resolves the individual stars.

Star-hop route from Shaula

Step 1: Locate Antares

Find Antares, the orange-red star near the center of Scorpius, which marks the heart of the scorpion. It is bright and easy to see from a suburban sky. In July and August it is well placed in the evening, so start the hop soon after dark while the stars are still high enough to be clearly visible.

Step 2: Follow the tail to Shaula

From Antares, look east-southeast along the curve of the tail. The tail is a hooked chain of stars of similar brightness, and it spans roughly 16 degrees from Antares to its tip. That is about two or three wide binocular fields, so keep your sweeps short and count the bends in the curve as you go.

Shaula sits at the tip of the hook, at the stinger. Lesath lies just to its southwest, and the two form a close pair. Shaula is the brighter of the two. If you can see the pair as separate stars, you have identified the correct starting point.

Step 3: Make the long hop with binoculars

Center Shaula in a wide binocular field, typically 5 to 6 degrees across. Then pan toward the east-northeast by about one field width. The cluster should appear near the eastern edge of the field as a compact clump of stars. Slide the binoculars slowly until the clump is near the center. A 10x50 binocular is a common choice for this step, and the group is easy to spot against the surrounding stars.

Step 4: Refine with the 8-inch SCT

Put a low-power eyepiece in the 8-inch SCT and aim the optical finder at Shaula. Center Shaula in the telescope first. Then move the telescope east-northeast in steps no larger than the finder field, which is typically 6 to 8 degrees. The SCT's field at low power is much narrower than a finder field, so you will need several short moves. After each move, check your position against a star you can recognize, and keep track of how many moves you have made.

Confirming you have arrived

Check these points before you decide that you have found the cluster:

• Individual stars. At low power, the cluster should resolve into a loose group of separate stars, not a single fuzzy patch.
• Size and spread. The group should cover about 0.4 degrees, which is the 22-arcminute diameter, with a denser concentration toward the center and fewer stars at the edges.
• Contrast with the field. Compare the group with the stars just outside it. The Milky Way star density here is rich, so the contrast is less sharp than in a dark field. Judge by the concentration of stars rather than by the mere presence of stars.
• Repeat the hop. Return to Shaula and make the hop again in the same way. If you arrive at the same group both times, the position is confirmed.

If the group is not in view after several hops, return to Shaula and check that you identified it correctly. Confusing Shaula with a nearby star in the tail is the most common cause of a missed target.

Eyepiece choice and expected view

With an 8-inch SCT, the expected view is a group of individual stars. A typical 8-inch SCT has a focal length near 2000 mm, so magnification and field size depend on the eyepiece. These figures are typical ranges, not measurements:

• Low power, about 50 to 65x (a 32 mm to 40 mm eyepiece). The true field is roughly 0.8 to 1.0 degrees. The cluster sits in the middle of the field, and the group is easy to scan.
• Medium power, about 80 to 100x (a 20 mm to 25 mm eyepiece). The true field is about 0.5 to 0.6 degrees. The cluster fills most of the field, and the pattern of stars is clearer.
• High power, above 130x (a 15 mm or shorter eyepiece). The cluster fills the field and the stars become dimmer. Avoid high power when searching, because the field is too narrow to confirm your position.

Be realistic about the faint members. In a suburban sky, the brighter members of the group are the most reliable part of the view, and the faintest stars are hard to separate from the background. The SCT's central obstruction also reduces contrast slightly, so a cluster seen against a bright background may look less defined than it does on a dark night.

Visibility at 35° N, 50° N and 35° S

At 35° N, the cluster reaches a maximum altitude of 20 degrees. It stays low in the sky, so trees, buildings and haze matter. Observe in July or August, soon after dark, while the cluster is still above the southern horizon. Choose a site with a clear view to the south. The light from a low target crosses more air, so the cluster appears dimmer and softer than it does at higher altitude.

At 50° N, the cluster reaches a maximum altitude of only 5 degrees. It barely clears the horizon. Unless you have an unobstructed southern horizon and clear, low-haze conditions, the target is impractical with an 8-inch SCT. A target that sits higher in your sky will give a more reliable session.

At 35° S, the cluster reaches a maximum altitude of 90 degrees, so it passes almost overhead. Antares and Shaula both sit high in the sky, which makes the star-hop comfortable and reduces the amount of air the light passes through. July and August are winter months in the Southern Hemisphere, so the nights are long and dark, which gives you more time to make the hop.

Practical tips for a suburban sky

• Cool the telescope. An SCT's corrector plate and tube need time to match the outside air. Allow 30 to 60 minutes before you judge fine detail.
• Manage dew. The corrector plate can fog on a clear, humid night. A dew heater or a dew shield helps keep the optics clear.
• Adapt your eyes. Spend 10 to 15 minutes away from white lights before you start the hop. Use a red light to read your chart.
• Prepare the finder. Make sure the optical finder is aligned on a bright star before the session, and keep a printed or on-screen chart with the Shaula hop marked.