Tweedledee, Tweedledum Clusters - IC 4756 and NGC 6633
High in the summer Milky Way, two neighboring open clusters form one of the sky’s most charming binocular sights: IC 4756, known to amateurs as the Tweedledee Cluster, and NGC 6633, its partner, the Tweedledum Cluster. Both shine around magnitude 4.6, and both are large enough to stand out beautifully in binoculars or a wide-field telescope, sitting just three degrees apart on the border of Serpens and Ophiuchus.
The playful names come from Lewis Carroll’s Through the Looking-Glass, where the characters Tweedledee and Tweedledum are close companions, nearly alike yet not identical. Amateur astronomer and author Stephen James O’Meara popularized the nicknames, noting how these two clusters share the same binocular field, look like siblings in brightness, and yet reveal different personalities.
IC 4756 (Tweedledee) is the more sprawling and delicate of the pair, covering almost a full degree of sky. Its stars form loose chains and arcs, giving the impression of a gentle starry river flowing across the background glow of the Milky Way. NGC 6633 (Tweedledum), by contrast, is smaller and more compact, with brighter stars gathered in a richer, denser knot that makes it pop immediately to the eye. Seen together, they offer a delightful study in contrasts—two clusters that share the same stage but play different roles.
Tweedledee, Tweedledum Clusters
Rating: 🔵 Stagazer Favorite
Level: 🟢 Moderate
How:
When: Jun - Oct
🌟 A Unique Pairing Few open clusters are so close in brightness, size, and position that they can be viewed side by side in the same wide field. This makes IC 4756 and NGC 6633 not just good targets, but a matched set, like siblings shining together.
🌟 Two Characters, Side by Side IC 4756, the Tweedledee Cluster, sprawls across almost a full degree, its stars scattered loosely in gentle chains that look like a river of starlight. NGC 6633, the Tweedledum Cluster, by contrast, is smaller and denser, with brighter stars arranged in a tighter grouping. Observed together, they showcase the diversity of open clusters.
🌟 Binocular Splendor This pair shines best in binoculars or a wide-field telescope, where the contrast between sprawling IC 4756 and compact NGC 6633 can be enjoyed in a single glance. They are ideal for casual stargazing under a summer sky, and perfect for beginners to add to their observing list.
🌟 A Milky Way Showcase Both clusters sit against the rich background of the summer Milky Way. In dark skies, they appear not as isolated patches but as glittering highlights set within the galaxy’s broader star clouds.
Open cluster: a loose group of a few dozen to a few thousand young stars that formed together and are loosely bound by gravity.
Tweedledee, Tweedledum Clusters
Since the Tweedledum Cluster (NGC 6633) is the denser, more obvious cluster, of the two, it's generally best to find it first. It's in the constellation Ophiuchus, which is dim, but the Summer Triangle makes it easy to find.
Find the three stars that form the Summer Triangle, Vega (+0.02), Altair (+0.93) and Deneb (+1.33). Deneb is the brightest star in the constellation Cygnus, the Swan. It forms the Swan's tail. Find Albireo, the star that forms the tip of the Swan's beak.
Hop from Deneb to Albireo and then another long hop to relatively bright Cebalrai (+2.76) in Ophiuchus. To confirm you have the right star, find brighter Rasalhague (+2.08) just 8° north of Cebalrai.
Now for the bit more challenging part. Draw a mental line in the sky that stretches all the way from Cebalrai to Altair in the Summer Triangle. This is a long line, about 32° in length. NGC 6633 lies along this line one-third of the way from Cebalrai to Altair, or 11° -- about the width of your closed fist held at arm's length. Using binoculars, a telescope finder or a low-powered eyepiece, point to this location and look for the target. If you are using a telescope, this step can be made easier by first using binoculars to scan the sky starting at Cebalrai and moving toward Altair until you find NGC 6633. Once found, center and explore.
Hop east just 3° to the Tweedledee Cluster (IC 4756). Binoculars or a wide field telescopes makes this hop easy since both clusters can be viewed at the same time in the same view. Use higher power to explore each in detail.
Tweedledee, Tweedledum Clusters
✅ Use Wide Fields Binoculars (7×50 or 10×50) or a telescope with a 3°+ field of view capture both clusters together best.
✅ Use Low Magnification Higher magnifications break them apart and diminish their charm. Keep the view wide and sweeping.
✅ Compare and Contrast Spend time moving back and forth between the two clusters—notice how one is airy and sprawling while the other is concentrated and rich.
✅ Dark Skies Enhance In Bortle 1–3 skies, faint members in IC 4756 emerge, while NGC 6633 sparkles with brighter stars.
Tweedledee, Tweedledum Clusters
❌ Naked Eye
To the naked eye under very dark skies, each cluster may appear as a faint glow, but they blend into the Milky Way’s richness.
✅ Binoculars
Through binoculars, both clusters are splendid. IC 4756 appears as a loose, misty patch filled with faint stars, while NGC 6633 resolves into a rich grouping of brighter points. The contrast is striking.
✅ Small Telescope
In a small telescope with a wide-field eyepiece, the differences grow more obvious: IC 4756 spreads beyond the field, airy and delicate, while NGC 6633 is contained and bright, almost jewel-like.
✅ Medium Telescope
Medium telescopes reveal fainter stars in both clusters, but the essential contrast—one broad, the other compact—remains. These are clusters best enjoyed at low power, not under high magnification.
⚠️ Large Telescope
A large telescope won’t improve the overall framing—both IC 4756 and NGC 6633 are best appreciated wide-field in binoculars or small scopes—but it will enrich the view with fainter members, more color, tighter doubles, and a sense of depth against the Milky Way.
Tweedledee, Tweedledum Clusters
In dark skies, IC 4756 and NGC 6633 form a beautiful contrasting duo—one broad and diffuse, the other tighter and denser—set against the Milky Way. As light pollution increases, IC 4756 quickly diminishes while NGC 6633 holds on longer, but the charm of the paired view is best preserved in Bortle 1–4 skies.
🟣 Bortle 1–2. In pristine skies, both clusters stand out distinctly from the rich Milky Way background. IC 4756 looks sprawling, loose, and full of faint stars, while NGC 6633 appears more compact and sharply defined. Their close pairing creates a striking binocular-field jewel in the summer sky.
🔵 Bortle 3–4. The Milky Way remains strong, but the clusters begin to blend slightly into the background. IC 4756 loses a bit of its faintest sparkle, though its broad haze of stars is still attractive. NGC 6633 holds its form well, showing as the denser of the two.
🟡 Bortle 5–6. Light pollution weakens the Milky Way backdrop. IC 4756 fades into a sparse patch of scattered stars, no longer obviously a cluster. NGC 6633 remains noticeable, though its richness and contrast are reduced, and the pairing is less dramatic.
🟠 Bortle 7+. The Milky Way is gone. IC 4756 nearly disappears as a recognizable cluster, becoming just a few stars with little cohesion. NGC 6633 is still visible, but appears as a modest, loose grouping rather than a distinct open cluster, and the sense of a “twin cluster” pair is mostly lost.
Tweedledee, Tweedledum Clusters
DSOs within 48° or 2 outstretched hands at arm's length
Tweedledee, Tweedledum Clusters - IC 4756 and NGC 6633
Find the three stars that form the Summer Triangle, Vega (+0.02), Altair (+0.93) and Deneb (+1.33). Deneb is the brightest star in the constellation Cygnus, the Swan. It forms the Swan's tail. Find Albireo, the star that forms the tip of the Swan's beak.
Hop from Deneb to Albireo and then another long hop to relatively bright Cebalrai (+2.76) in Ophiuchus. To confirm you have the right star, find brighter Rasalhague (+2.08) just 8° north of Cebalrai.
Now for the bit more challenging part. Draw a mental line in the sky that stretches all the way from Cebalrai to Altair in the Summer Triangle. This is a long line, about 32° in length. NGC 6633 lies along this line one-third of the way from Cebalrai to Altair, or 11° -- about the width of your closed fist held at arm's length. Using binoculars, a telescope finder or a low-powered eyepiece, point to this location and look for the target. If you are using a telescope, this step can be made easier by first using binoculars to scan the sky starting at Cebalrai and moving toward Altair until you find NGC 6633. Once found, center and explore.
