M19 Globular Cluster in Ophiuchus: Sketching the View Through a Refractor

What the cluster looks like at the eyepiece

Shape and size

At low power, M19 appears as a round, softly edged patch. Its apparent diameter is 7.5 arcminutes, which is small compared with the field of most eyepieces. At 30x to 40x, the cluster fills roughly a tenth of a typical wide field, so you can center it easily and still see the stars around it.

When you use higher magnification, the outline becomes harder to define. The edge fades into the background sky, and the patch can look more compact. Draw the outline at the magnification where it looks most distinct, and note any change at higher power in your sketch.

Brightness and core

M19 has a visual magnitude of 5.6. That is just below the naked-eye limit of a suburban sky, which is about magnitude 5. Expect the cluster to be visible in a telescope or binoculars but not to the unaided eye from most suburban locations.

The brightness is concentrated toward the center. The core is the most obvious part of the cluster, and the outer regions fade gradually. In your sketch, show this by using the darkest pencil strokes at the center and blending outward to lighter tones. The outer edge should be drawn lightly, because it blends into the sky background.

Detail and stars

With a 70-90 mm refractor, you will not resolve individual stars in M19. The cluster appears as a smooth glow with no obvious granular texture. You may notice a faint mottled quality near the core, but this is hard to confirm when the sky is bright.

Larger apertures begin to show the cluster's granular structure at the edge, and the core becomes more textured. Do not expect a dramatic change with a 70-90 mm instrument. If you are sketching with this equipment, record the smooth glow and the concentrated core, and leave the outer region free of star-like points unless you can see them clearly.

Color

Globular clusters look white or grayish-white in small telescopes. M19 is no exception. Any hint of warmth in the glow is most likely the effect of your eye's color response at low light. In your sketch, use plain graphite or a neutral gray and do not add color.

Choosing magnification

For a 70-90 mm refractor, typical focal lengths are in the range of 400 mm to 700 mm. An eyepiece with a focal length of 20 mm to 30 mm gives magnifications in the range of about 15x to 35x with these focal lengths. Use these as starting points, not fixed values, because the exact result depends on your instrument.

Lower powers, roughly 15x to 25x, show the full extent of the cluster and the surrounding field. Use these to set the outline of your sketch. Medium powers, roughly 30x to 50x, usually give the best balance between brightness and detail for the core. Higher powers make the cluster dimmer and harder to read, so use them only to check the core structure.

Try several eyepieces during a session and sketch the cluster at two or three magnifications. Record each magnification on the drawing. A comparison between a low-power and a medium-power sketch is more useful than a single, highly detailed drawing.

Locating M19 and timing your session

M19 lies in Ophiuchus, the constellation that sits between Scorpius to the west and Hercules to the north. Use Scorpius and the bright star Rasalhague as guides. Star-hop from the bright stars of Ophiuchus toward the cluster, and confirm your position with a finder chart before you start the drawing.

The best months to observe M19 are July and August. During these months, the cluster is well placed in the evening sky. Your altitude above the horizon depends strongly on your latitude, and that changes how the view looks.

The maximum altitude of M19 is:

• 35° N: 29°. The cluster is low, so light from the horizon and the atmosphere can soften the view. Wait until it is as high as possible and sketch when the air looks steady.
• 50° N: 14°. The cluster stays very low. Trees, buildings, and the haze near the horizon may block it completely from some sites. Choose a location with an unobstructed southern horizon.
• 35° S: 81°. The cluster passes close to the zenith from this latitude. The view is usually steadier and more contrasted, and it is the easiest of the three locations for sketching.

From the Southern Hemisphere, you will see the cluster high in the sky during the same months. Note your latitude and the altitude on your sketch so that readers can compare your view with the conditions described here.

Preparing a sketch

Materials and setup

Use a sketchbook with smooth, off-white paper. Draw a circle on each page to represent the field of view, using a template or the eyepiece's apparent field. A circle keeps the cluster in proportion and makes the drawing easier to compare with later sketches.

Use a few graphite pencils of different hardness, a blending stump, and a soft eraser. A pencil with a soft lead works well for the core, and a harder pencil suits the faint outer edge. Have a small red-filtered light or a dim red light that does not disturb your dark adaptation.

Allow 15 to 20 minutes for your eyes to adapt to the dark before you start. Avoid looking at bright lights during this time. Keep the eyepiece and the drawing board arranged so that you can switch between them without losing your place.

Drawing steps

1. Center the cluster at low power and draw the outline of the glow inside your circle. Mark the position of any field stars that help you locate the cluster later.
2. Switch to a medium-power eyepiece and check the core. Note where the brightest part sits within the cluster.
3. Shade the core with a soft pencil, using light pressure and blending outward. Keep the shading smooth so that it does not look like individual stars.
4. Use a harder pencil for the faint outer edge. Draw it lightly, and stop where the glow merges with the background.
5. Write the date, time, instrument, eyepiece focal length, magnification, your latitude, the altitude of the cluster, and the sky conditions beside the drawing.

Do not add stars that you cannot see. Unseen stars make a sketch look more detailed than the eyepiece view and reduce its usefulness as a record.

How conditions change the view

The sky changes from night to night and from hour to hour. Three conditions matter most for M19.

Transparency. A clear, dry sky makes the cluster easier to see and gives a more defined edge. Haze, thin cloud, or humidity reduces contrast, and the cluster can disappear entirely. Under suburban light pollution, the sky background is brighter, so transparency has a larger effect than it would under a dark sky.

Seeing. Steady air keeps the core sharp at medium power. Turbulent air makes the core look swollen or wavering. If the image shakes, use a lower magnification and draw the overall outline rather than the core details.

Altitude. As noted above, the cluster appears dimmer and less defined when it is close to the horizon, because you are looking through more atmosphere. Sketch when the cluster is as high as possible during your session, and record the altitude so the drawing can be interpreted correctly.

Moonlight also reduces contrast. A bright moon in or near the same part of the sky can wash out the outer edge of the cluster. Check the moon's phase and position before you plan a sketching session.