Finding M13 with a 114-150 mm Reflector: Keystone Star-Hop

Before You Start

Timing and altitude

July and August are the best months, when Hercules stands high in the evening. From 35° N the cluster can pass almost overhead at 89°, and from 50° N it reaches 76°. Both are high enough that you look through little of the thick, hazy air near the horizon.

From 35° S the cluster only reaches 19° above the horizon. It can be found there, but it stays low, and haze and light-pollution glow near the horizon will wash it out. Southern observers should wait until Hercules is as high as it will get, and should expect a dimmer, softer view than northern observers get.

Setting up the telescope

Let a reflector cool outdoors for 20 to 30 minutes so the mirror matches the air temperature. Check the collimation if you have not done so recently; a globular cluster shows poor alignment as a lopsided, smeared glow.

Align the finder with a distant object in daylight, or on a bright star before you begin. A finder that points a degree off turns this hop into a search.

Eyepieces to have ready

• A low-power eyepiece giving a wide field, roughly 1.5° to 2° across, for the hop itself.
• A medium eyepiece giving about 100x to 150x for studying the cluster.
• Optionally, a higher-power eyepiece for the cluster's edge stars if the air is steady.

Directions in the eyepiece

A telescope may show an image flipped or mirrored, depending on the optics. Before you start, nudge the telescope toward Polaris or a known bright star and note which way the stars drift. Use that to match the charts and descriptions below to your own view.

Finding the Keystone

Hercules has no bright stars, so use its neighbours to get started. In summer evenings, find these two bright guides:

• Vega in Lyra, a blue-white star that is among the brightest in the summer sky. It is easy to see even through suburban glare.
• Arcturus in Boötes, an orange star that stands well to the west of Hercules at that time of night.

Hercules sits roughly between them. Look for a lopsided square of four moderately bright stars about halfway along the line joining Vega to the Arcturus region, slightly nearer Vega. This is the Keystone. The stars are around third and fourth magnitude, so they stay visible from a suburban sky with a limit near magnitude 5.

The four Keystone stars are:

• Zeta Herculis at the lower preceding corner, the brightest of the four on the western side.
• Eta Herculis at the upper preceding corner.
• Epsilon Herculis at the lower following corner.
• Pi Herculis at the upper following corner.

The cluster lies on the western side of the Keystone, along its upper edge.

The Star-Hop

Step 1: Pick the right side of the Keystone

Look at the edge of the Keystone that joins Eta Herculis and Zeta Herculis. This is the western edge, the side facing Arcturus. The cluster lies along it, not in the middle of the quadrilateral.

Step 2: Start at Eta Herculis

Put Eta Herculis in the middle of your finder or low-power eyepiece. It is the brightest and most obvious starting point for the hop, and it is easy to recognise as the upper western corner of the pattern.

Step 3: Move toward Zeta Herculis

Slide the telescope along the Eta-Zeta line. The two stars are roughly 7° apart, which is about one and a half fields of a typical 5° finder. You do not have to go the whole distance.

The cluster sits about one third of the way from Eta Herculis to Zeta Herculis, a little over 2° from Eta. It is not on the line itself but slightly toward the Keystone's inside.

Step 4: Look for the faint smudge in the finder

When you are about a third of the way along the edge, look in the finder for a small, hazy spot. In a suburban sky it may be only a pale glow, sometimes visible only with averted vision, when you look slightly to one side of it. In a 50 mm finder the glow looks like an out-of-focus star.

If you see nothing, do not move on. Check the finder with the eyepiece, and nudge the telescope by half a degree in each direction.

Step 5: Confirm in the main eyepiece

Switch to your wide-field eyepiece and sweep back and forth across the area. The cluster should appear as a round, soft patch, noticeably brighter in the centre. It should be the only fuzzy object in the field, apart from a very faint small smudge a short distance away that is a separate, much fainter galaxy.

A backup route using the finder field

If the line between Eta and Zeta does not work for you, use fields instead:

1. Centre Eta Herculis.
2. Move the telescope about half a finder field toward Zeta Herculis.
3. Move again by about a third of a field.
4. Check for a haze in the main eyepiece.

Repeat the sequence if the first pass fails. Small, repeated steps work better than one long jump.

Confirming You Have Arrived

Several features mark the right spot:

• Position. The object lies along the Eta-Zeta edge, closer to Eta, and well outside the Keystone's interior.
• Shape. It is round and appears roughly circular, not elongated like a galaxy.
• Brightness profile. The glow brightens gradually toward the centre, without a sharp nucleus.
• Stars nearby. A few faint field stars sit around the cluster, and one or two may touch its outer glow.
• Behaviour at higher power. At around 120x the glow grows larger and begins to look grainy, rather than turning into a single point.

If the object looks like a star at any power, you have stopped on a field star. If it is a small, elongated smear with a bright core, you may have found a nearby galaxy instead, and should return to the Eta-Zeta line.

What You Will See

The cluster has a magnitude of 5.8 and spans 17′, a little over half the width of the full Moon. That is bright and large for a small reflector, and the expected view in a 114-150 mm instrument is easy: a round, softly glowing ball.

At low power

At 40x to 60x the cluster is a round, hazy ball with a brighter middle and a fading edge. Stars are not resolved at this stage, and in a suburban sky the glow can look like a faint, evenly lit patch. The surrounding sky is dark enough that it stands out clearly.

At medium and high power

At 100x to 150x the outer parts start to look granular. With steady air and a well-collimated mirror, a few of the brightest stars on the cluster's edge may begin to appear as tiny points on top of the glow. With a 114 mm mirror expect only a few at best; with 150 mm you may see more, but the core remains an unresolved glow.

Do not expect the dense, fully resolved star field seen in photographs or through larger apertures. From a suburban sky, the view is a soft ball with a mottled edge.

Helping the view

• Use averted vision, looking slightly to one side, to bring out the cluster's outer halo.
• Give your eyes about 10 minutes away from bright lights and screens.
• Tap the tube gently or wait for it to settle if the image trembles.
• Keep the eyepiece clean and free of dew.

Troubleshooting a Failed Hop

The finder shows nothing

A suburban sky can hide the glow in a small finder. Skip the finder step and rely on the eyepiece. Place Eta Herculis in the low-power eyepiece, then move along the Eta-Zeta line in small steps.

Too many stars in the field

If the field looks crowded, you may be looking at the Milky Way's background rather than at the cluster. Return to Eta Herculis and recheck the direction.

The image looks flat or smeared

Check collimation, and let the mirror reach air temperature. A warm mirror or a misaligned one reduces contrast on a faint object.

You are in the southern hemisphere

The cluster is only 19° above the horizon from 35° S, so the Keystone pattern also sits low and may appear tilted. Use the same Eta-Zeta edge, but allow more time and choose a night with transparent air.