Trifid Nebula (M20) Star-Hop Guide for Visual Observers

Before you start

Choose an evening in July or August if you can. The target is best placed then, and the Sagittarius region is high in the southern evening sky. Outside those months it rises later and sets sooner, so check the timing for your date and location with a planetarium program or a printed star chart.

Plan for two instruments. A pair of 10x50 binoculars shows the star fields you need for the first part of the hop. A 70 to 90 mm refractor with a low-power eyepiece shows the nebula itself. A nebula filter improves the view, so bring one if you have it.

Allow 20 to 30 minutes for your eyes to dark-adapt before you start the hop. Keep a red-light torch for reading your chart, and avoid white light, which destroys night vision.

Choose a site with a clear view of the southern horizon. At northern latitudes the target stays low, and a nearby roofline, tree or street light can block it entirely.

Star-hop route from Kaus Borealis

Use the teapot, the familiar pattern of Sagittarius, as your starting point. Its stars are bright enough to see from a suburban sky, and Kaus Borealis is a third-magnitude star in its northern part. The route below takes you from that star to the Trifid in four moves.

1. Find the teapot and Kaus Borealis. Locate the teapot low in the south during evening. Identify Kaus Borealis, which is the star you will use as the anchor for the hop.

1. Center Kaus Borealis in the binoculars. A 10x50 binocular shows a field of roughly 5 to 6 degrees. Put the star near the middle of the field, then hold the binoculars steady for a few seconds to confirm the position.

1. Move about 6 degrees west, angling slightly north. The Trifid lies about 6 degrees west of Kaus Borealis and a little north of that line. In practice this is about one binocular field to the west. Sweep slowly, because the field will contain many faint stars and the target is not bright enough to stand out on its own.

1. Find the Lagoon Nebula, then move north. Look for a hazy patch in the binocular field. On a steady night in suburban skies, the Lagoon Nebula can appear as a soft glow. Once you have it, use your telescope's finder or a low-power eyepiece to center it. The Trifid lies about 1.5 degrees north of the Lagoon Nebula, which is roughly a quarter of a 6-degree finder field. Move north by that amount to reach the Trifid.

If the Lagoon does not show in the binoculars, do not try to force the hop. Instead, return to Kaus Borealis, check that you are centered on it, and repeat the westward move slowly. Mistaking a faint star cluster for the nebula is common at this stage, so do not stop at the first hazy patch you see.

Confirming you have arrived

A hop is only useful if you can confirm the target. Use these checks at the telescope.

• Look for a faint glow about 28 arcminutes across. This is just under half a degree. At 25 to 40 times magnification, the glow fills only a part of the field, so do not expect it to fill the eyepiece.
• Check for a small group of stars near the center. The nebula contains a loose group of stars. In a suburban sky, these stars are often easier to see than the surrounding glow, so use them as a second confirmation.
• Use averted vision. Look slightly to one side of the nebula rather than directly at it. The rods of the eye, which are most sensitive to faint light, lie off-center, so a faint glow becomes easier to detect this way.
• Compare with and without the filter. Put in the nebula filter, then remove it. If the glow brightens and gains contrast with the filter in place, you have most likely found the nebula rather than a background haze.
• Tap the telescope. A real nebula stays in place when the tube moves slightly. A reflection or a flaw in the eyepiece will move or flicker.

If you cannot satisfy at least two of these checks, you have probably not reached the Trifid. Return to the star-hop steps and try again.

What to expect at the eyepiece

The Trifid is a faint glow in a suburban sky. In an 70 to 90 mm refractor, it appears as a soft, grey patch rather than a clear structure. Most observers will not see colour.

A nebula filter improves the view. It darkens the background sky while passing the light from the nebula, so the glow stands out more clearly. The filter also helps you judge whether a faint patch is a nebula or simply a cluster of stars.

Dark lanes of dust may be visible through the glow when conditions are steady and your eyes are dark-adapted. Their presence depends heavily on the sky and the instrument, so treat any lane detail as a bonus rather than the goal. The three-lobed shape that gives the nebula its name is difficult to see from a suburban sky, and a larger aperture or darker site helps.

Viewing from different latitudes

The target's altitude changes considerably with your latitude. The table above gives the maximum altitude, which is the highest point the target reaches in the sky. The higher the altitude, the thinner the atmosphere between you and the nebula, and the easier the view.

At 35° N, the target reaches a maximum altitude of 32 degrees. It is low but observable. Start the hop as soon as the teapot is clear of the trees or rooftops to the south. Use low magnification and a clear southern horizon, and expect the view to be less steady than at higher altitudes.

At 50° N, the target reaches only 17 degrees. It is very low, and atmospheric haze and light pollution near the horizon make it difficult. Attempt the hop only on nights with a flat, unobstructed southern horizon and good transparency. A dark site with a clear horizon gives the best chance of success.

At 35° S, the target reaches a maximum altitude of 78 degrees, close to the zenith. The Trifid is high in the sky, and the route from Kaus Borealis is comfortable to follow without a long neck angle. The view is generally steadier, and the nebula is easier to confirm with averted vision and a filter.