M106 Star-Hop Guide: Finding NGC 4258 with a 200-300 mm Dobsonian

Before you start

Use a printed star atlas or a planetarium application that shows stars to about magnitude 9, and set the chart to match your local time and date. Arrive at the eyepiece with the field of view you plan to use already chosen, because changing eyepieces mid-hop wastes dark adaptation time.

Equipment and conditions

A 200-300 mm Dobsonian is well suited to this object. Use a low-power eyepiece, roughly 25-40x, for the hop itself. The wide field makes it easier to see the faint stars that mark your path. Switch to roughly 80-120x once you think you have the galaxy, so the oval shape separates from the background.

A 10x50 binocular is a useful finder for the first stages of either route. Its field of roughly 5 to 6 degrees shows the bright stars you need and some of the faint ones in between.

Allow at least 10 to 15 minutes for your eyes to adapt to the dark. Use a red light for reading the chart, because white light resets your night vision.

Sky conditions

From a suburban sky, with a naked-eye limit around magnitude 5, M106 is at the edge of what is comfortable. The galaxy is magnitude 9.3, so you will need steady attention and some patience. Haze, a bright horizon glow, or a moon close to the target will make the oval much harder to see. A clear, moonless night is the best time to try.

Route 1: From Alioth, the Big Dipper's handle

This route is the most direct from the Big Dipper, the most easily recognized pattern in the northern sky. It requires a single long hop south.

Step 1: Find the Big Dipper

Look for the seven stars of the Big Dipper, part of Ursa Major. The four stars forming the bowl are at the top, and three stars curve out from the bowl to form the handle. In spring evenings the pattern is high in the northern sky for most observers in the Northern Hemisphere.

Step 2: Locate Alioth

Alioth is the middle star of the handle, with an apparent magnitude of about 1.8. It is one of the easiest stars in the pattern to pick out. Its neighbor, Megrez, sits where the handle meets the bowl, and Alioth lies just beyond it along the handle.

Step 3: Hop south about 11 degrees

From Alioth, move about 11 degrees almost straight south. Use your hand at arm's length to estimate: a closed fist covers roughly 10 degrees, so this is about one fist plus a little more. The stars in this region are sparse and fainter than those of the Dipper, so keep your finder or binocular field centered on the bearing rather than on individual stars.

The galaxy lies nearly on this line. If you have located the correct patch of sky, you will see a faint, blank-looking area where the field of stars thins. Move the telescope slowly across it at low power.

Step 4: Confirm the bearing with Cor Caroli

Cor Caroli, the leading star of Canes Venatici, is about 18 degrees south of Alioth, a little further south than the galaxy. Locating it helps you check that you are on the correct bearing. It is a bright star at magnitude 2.9, and it will be easy to see in the same finder field as Alioth if you are patient with the hop.

Route 2: From Cor Caroli, through Chara

This route is useful when Cor Caroli is easier to see than the Big Dipper, or when you want a second approach to check your position. It uses two intermediate stars and two short hops.

Step 1: Find Cor Caroli

Cor Caroli is Alpha Canum Venaticorum, at magnitude 2.9. Use Alioth as your starting point, since it lies about 18 degrees north of Cor Caroli, and sweep south until you find it. The star is bright enough to stand out from the surrounding faint field.

Step 2: Hop to Chara

Chara is Beta Canum Venaticorum, at magnitude about 4.3. It lies roughly 5 degrees from Cor Caroli, to the west and slightly north. Move in that direction until you see a star of about the brightness of the stars around the Big Dipper's bowl. In a suburban sky, Chara is visible to the naked eye, but it will be one of the fainter stars you see.

Step 3: Hop to M106

From Chara, M106 lies about 6 to 7 degrees further away, toward the north-northwest. Move your finder roughly 6 degrees north and a little west. This is the final hop, and it is the one most likely to be affected by field-star confusion, so take your time and check the field carefully on your chart.

Confirming you have arrived

Finding the galaxy is only half of the task. Confirm that you have arrived before you spend time trying to see detail.

Check the field against your chart

Compare the faint stars in your eyepiece with the chart at the same magnification. Look for the pattern of stars around the galaxy, and check that you are not confusing a field star for the object. If the chart shows several faint stars in a tight group and you see a similar group, you are close.

Look for the oval glow

At the magnification suggested for confirmation, M106 looks like a small, elongated glow. Its apparent size is about 17 by 7.2 arcminutes, which is roughly a quarter of the width of a 1-degree field at a typical low power. It is brighter toward the centre and fades gradually toward its edges. At high power, the oval shape will be clearer, but the galaxy will be dimmer, because the light is spread over a larger area.

Use averted vision. Look slightly to the side of the galaxy, so the light falls on the more sensitive part of your retina. The glow will often appear and disappear as you move your gaze, so hold your view steady for several seconds before deciding.

Check for a stable target

A star that appears fuzzy in poor seeing can be mistaken for a galaxy. A true galaxy will keep the same elongated shape as you move your eye and as you change eyepieces. If the glow looks like a point of light at higher power, or if its shape changes with the seeing, you are probably looking at a field star or at atmospheric blur.

What you will see

From a suburban sky with a 200-300 mm Dobsonian, the expected view is an oval glow. The core is the brightest part. The outer areas are faint, and you may not see the edges clearly without a dark sky.

The spiral arms of M106 are not visible from a suburban site with visual observation. Long-exposure photographs reveal them, and dark-sky observers with large telescopes sometimes see hints of structure. Do not expect that level of detail. A visual view from a suburban sky is a soft, faint oval, and that is the appropriate expectation.

Use a slightly larger aperture, a darker site, or a longer session if you want to see more. Each step will improve the view, but none of them will make it bright.

Visibility by latitude

The best time to observe M106 is in April and May, when it is well placed in the evening sky. The altitude above the horizon depends strongly on your latitude.

From 35 degrees north, the galaxy reaches a maximum altitude of 78 degrees, so it is high in the sky and easy to position. From 50 degrees north, the maximum altitude is 87 degrees, which places it almost directly overhead. Observers at these latitudes have a clear, unobstructed view and fewer problems with atmospheric distortion.

From 35 degrees south, M106 reaches a maximum altitude of only 8 degrees. It is barely above the horizon, and the air at that low altitude distorts the image badly. Observers in the Southern Hemisphere can still try, but they should expect a faint, unsteady view, and they should observe only when the galaxy is at its highest. Seeing the oval glow from that position is unlikely with a suburban sky and a 200-300 mm telescope.