M107 and M70 Through a 114-150 mm Reflector: Comparing Two Globular Clusters

Why Compare These Two Clusters

Globular clusters are dense balls of old stars that orbit the galactic center. Through a telescope they range from easy, resolved swarms to faint, unresolved glows. M107 and M70 sit near the low end and the middle of that range, so they show the difference clearly.

M107 is the fainter of the two and looks like a soft, evenly lit patch. M70 is generally the brighter and more condensed, with a denser central region. Viewing them one after the other trains your eye to notice differences in surface brightness, which is the skill most needed for faint deep-sky work.

What M107 Looks Like

Through a 114-150 mm reflector from a suburban site, M107 appears as a round, softly glowing ball. Its edges fade gradually into the background, and the glow is faintly brighter toward the middle. The cluster's magnitude of 8.9 places it well below the naked-eye limit, which is about magnitude 5 under suburban skies.

Expect the following:

• A round, grey-white haze roughly the size given in the facts, about 7.8′ across.
• A center that is brighter than the edges but not sharply defined.
• Little or no visible resolution into individual stars at low power.
• Occasional glimpses of faint specks at the edge when the sky is steady and transparent, usually at higher power and with averted vision.

Do not expect the cluster to show a bright, starry core or a dramatic spray of stars. Under suburban light pollution, M107 is best described as a clear, round smudge that gets easier to see as your eyes adapt.

What M70 Looks Like

M70 is usually the more obvious of the pair. At the same magnification, it tends to look brighter and more compact, with a denser center. Its outer region is more diffuse than its core, so the view has a stronger sense of concentration.

Compare the two in the same eyepiece:

• M70 presents a brighter, more defined central glow.
• M107 presents a fainter, more evenly spread haze.
• Both are round, so shape alone does not separate them. Brightness and central concentration do.

With good transparency and steady air, the brightest parts of M70's core may show a granular texture. Treat this as a bonus, not something to count on from a suburban site.

Finding Each Cluster

Both clusters are found by star hopping, using bright, easily identified stars as waypoints. Start with a low-power finder or a wide eyepiece, then move to higher power once the cluster is in view.

Finding M107

M107 lies in Ophiuchus, north of Antares, the red star in Scorpius. Antares is one of the brightest stars in the southern summer sky and is easy to locate. From Antares, move roughly 13 degrees north. The area is sparsely populated with bright stars, so use a finder chart or planetarium software to confirm that you are looking at the right field.

Use a wide-field eyepiece, roughly 40x to 60x, to locate the cluster. Once you have it, increase magnification gradually to see the cluster's shape more clearly.

Finding M70

M70 lies in Sagittarius, to the south and east of M107. A useful waypoint is Kaus Australis, a bright star in the Teapot pattern of Sagittarius. From there, the cluster sits within the same rich star fields that surround the Teapot. Because Sagittarius is dense with stars, a wide-field eyepiece and a star chart are especially helpful here.

Move from one target to the other in a single session. Note the star patterns along the way. The change in star density will help you judge how far you have moved.

Timing and Altitude by Hemisphere

M107 is best placed in July. Its maximum altitude depends strongly on your latitude, so the table in this guide shows the figures to plan around. The following notes apply to typical observing conditions.

Observers at 35° N

From 35° N, M107 reaches a maximum altitude of 42 degrees. This is a comfortable height above the horizon. The cluster is well placed for an early evening session in July, and it clears the worst of the low-altitude haze and turbulence.

M70 is also a summer target in this part of the sky. Check its altitude for your date and time with planetarium software, and aim to observe both clusters while they are reasonably high.

Observers at 50° N

From 50° N, M107 reaches a maximum altitude of only 27 degrees. At this height, the cluster is viewed through more atmosphere, which softens detail and reduces contrast. Observe it as early as possible after it clears any local obstructions, and avoid trying to push magnification high.

For this latitude, a larger aperture gives a more comfortable view of a faint, low target. If your telescope is at the smaller end of the 114-150 mm range, expect the cluster to be harder to see and plan for a longer session.

Observers at 35° S

From 35° S, M107 reaches a maximum altitude of 68 degrees. This is an excellent height, and the cluster is easy to locate and observe for a long period. It is one of the easiest ways to see this cluster, as the air is steadier and the contrast is better.

From the southern hemisphere, M70 also lies in a well-placed part of the sky during the same season. Use this to your advantage: the two clusters can be compared in a single long session.

Technique for a Suburban Sky

Light pollution is the main limit on what you can see. Use the following techniques to get the most from your telescope.

Choose the Right Magnification

Use a low to moderate magnification, roughly 60x to 100x, to see the overall shape and brightness of both clusters. Higher magnification, roughly 120x to 150x, can help you judge whether the outer edges show any star-like texture. Avoid going much higher, because the cluster becomes fainter as magnification increases and the image may look dim.

Use Averted Vision

Look slightly to one side of the cluster rather than directly at it. The edges of your retina are more sensitive to faint light, so the cluster may appear brighter and more defined when you look just off center. Move your eye slowly and pause for a few seconds at each position.

Dark-Adapt Your Eyes

Allow your eyes 20 to 30 minutes in darkness before you start. Avoid bright white lights, and use a dim red light when reading charts. Once your eyes have adapted, you will notice faint detail that was not visible at first.

Check Transparency

Clear nights vary a great deal. On nights with haze or high humidity, both clusters will look fainter and less defined. On steady, transparent nights, the cluster edges are easier to judge and the brighter parts of M70 may show more structure. If conditions are poor, observe the brighter M70 first and return to M107 on a better night.

What Each Cluster Teaches

M107 and M70 teach different lessons, and both are useful for building observing skill.

M107 teaches you to work with faint, low-contrast targets. Its soft, even glow requires patience, averted vision and good dark adaptation. Observing it regularly builds the ability to detect faint extended objects, which applies to many other deep-sky targets.

M70 teaches you to notice central concentration and brightness gradients. Comparing its dense core with the more open appearance of M107 helps you understand how different globular clusters can look through the same instrument. It also gives a useful reference point when judging whether a faint object is brighter in the middle or more uniform across its face.

Observing both in the same session turns the comparison into a lesson in judgment. Note what you see, record the magnification and sky conditions, and repeat the comparison on another clear night to confirm your impressions.