Star-Hop to Winnecke 4 (M40) with a 70-90 mm Refractor

What you are looking for

Winnecke 4 is a visual double. Its two stars are close enough that you will need to use a moderate magnification to separate them, and at low power they may look like a single faint point. The pair is not a target you can identify by its shape or by any extended glow. Your confirmation depends on the arrangement of the faint stars around it, which you will compare with a star chart at each stage.

Because the combined magnitude is 8.0, the pair is fainter than most of the stars you can see with the naked eye from a suburban site. Treat the pair as a test of your finder and your technique rather than as a showpiece.

Before you start

Use a star chart or planetarium application that shows stars to at least magnitude 8, and set it to your local time and location. Print or display a chart of the field around Megrez, since you will be comparing it with the sky at each step.

Choose a clear evening in April or May, when the target is best placed for observers in the Northern Hemisphere. Let your eyes adapt to the dark for at least 15 minutes, and keep a red-filtered torch for reading your chart. A white light will reduce your dark adaptation and make the faint stars harder to see.

Set up the refractor on a stable mount, and use the lowest-power eyepiece first. Check that the finder is aligned with the main tube against a bright star before you begin. A misaligned finder will send you to the wrong field.

Winnecke 4 is visible from northern latitudes only. At 35° N it reaches about 67° above the horizon at its highest, and at 50° N it climbs to about 82°, which places it nearly overhead. From 35° S it never rises, so observers in the Southern Hemisphere will need a different target.

Star-hop route from the Big Dipper

Step 1: Find the Big Dipper

Locate the Big Dipper, which is part of Ursa Major. It is easy to find from a suburban site because its seven brightest stars stand out even under light pollution. The four stars that form the bowl are the key to this route, and the three stars that curve away from the bowl form the handle.

Label the bowl stars in your head so you can keep track of them. The two stars on the side farthest from the handle are Dubhe and Merak. The two on the side nearest the handle are Megrez and Phecda. Megrez is the bowl corner that joins to the handle, so it is the star you want.

Step 2: Reach Megrez

Megrez is a third-magnitude star, bright enough to see clearly in your finder. Put it in the centre of your finder and confirm that you have the right star by checking that the handle begins from it and extends away toward the east. The handle's stars, Alioth and Mizar, lie in that direction.

Once you are on Megrez, keep the star in the finder and move on to the next step without changing your position on the sky.

Step 3: Move north from Megrez

From Megrez, move about one and a half degrees north. This is up and slightly toward the handle side of the Dipper, and it is roughly the distance from one edge of a finder field to the centre. If you do not have a finder with a known field, a clenched hand held at arm's length spans about 10°, so measure one and a half degrees as roughly one-seventh of that span.

The field you reach should contain several faint stars that are not in the bright Dipper pattern. Use your chart to match the arrangement. If you can identify two or three of the chart's faint stars, you are close.

Step 4: Switch to a low-power eyepiece

Change to the lowest-power eyepiece you have, which gives the widest field. Centre the field where your chart shows the pair, and look for the faint stars around it. The pair should sit near the centre of the field. Because the pair is tight, you may see only one point at this power. That is normal, and you should not assume you have missed the target.

Confirming you have arrived

There are three checks you can use before you move to a higher magnification.

First, compare the faint field stars around the centre with your chart. Match at least three of them in position and relative brightness. If you cannot match them, move back to Megrez and repeat the steps, paying close attention to the direction and distance.

Second, look for a single point that is slightly elongated or a little brighter than its neighbours. This suggests that the pair is nearby but not yet resolved. Increase the magnification gradually to see whether the point divides.

Third, check that the object does not look like an extended smudge. Winnecke 4 is a pair of stars, so if the centre of the field looks fuzzy and diffuse, you are on the wrong patch of sky or the focus is off. Adjust the focus carefully, and if the view remains fuzzy, return to Megrez and repeat the hop.

What the view looks like

Through a 70-90 mm refractor, the usable magnification range is about 30x to 60x for most observers. Start at the low end, where the field is wide and the stars are easier to find. Increase the power only if the air is steady and the star points stay sharp.

At the higher end of this range, the two stars may separate into a tight pair. Expect them to be small and close together, and expect the pair to appear faint. From a suburban sky, the field around the pair will show fewer stars than a dark-sky field, so the pair will stand out less clearly.

Do not expect a dramatic view. The pair is faint, and the separation is small. A steady, careful look and a good chart will serve you better than high magnification in poor air.

Seasonal and latitude notes

April and May are the best months to observe Winnecke 4, when the target is high in the evening sky for observers in the Northern Hemisphere. The Big Dipper is also well placed in those months, which helps you find the bowl quickly.

At 35° N, the target does not set, but it swings low toward the north at its lowest point. At 50° N, it passes nearly overhead, which makes the field easier to reach with a short finder path. Check the altitude at your own latitude on your chart before you start, so you know whether the field will be high enough for a clear view.