M50 Open Cluster with a 70-90 mm Refractor: Common Mistakes to Avoid

Mistake 1: Searching when the cluster is too low

Why it happens. M50 is a winter-to-spring object for most observers, and its height above the horizon depends strongly on your latitude. The facts give its maximum altitude as 47° at 35° N, 32° at 50° N and 63° at 35° S. At 50° N, the cluster never climbs far, so you look through a thick layer of air and haze. Near the horizon the stars also blur and dim, which makes a faint cluster hard to separate from the background.

How to avoid it. Plan your sessions for February and March, when the cluster is well placed in the evening. Observe when it is near its highest point in the sky, which is the best time for a faint target from any latitude. If you are at 50° N, expect a weaker view than observers farther south, and choose a site with a clear view to the south before you start.

How to fix it. If the cluster is still low, do not keep searching the horizon area. Work on other targets until it climbs, then return to it. Observers at 35° S will find it higher, so the same dates give a clearer view there.

Mistake 2: Using too much magnification

Why it happens. The cluster is small, about 14′ across, so it is tempting to reach for a high-power eyepiece to make it bigger. High power also seems like a way to split the stars apart.

At high magnification, the exit pupil becomes very small. The image gets darker, and the faint stars that make up the cluster drop below what your eye can register. The result is a dim, thin patch where the cluster was easier to see at lower power.

How to avoid it. Start with a low-power eyepiece, typically in the range of 20x to 40x for a 70-90 mm refractor. At that power, the cluster fills a small part of a standard eyepiece field and the stars stay bright enough to count as a group. These figures are typical guidance, not measurements, and the exact power depends on your focal length and eyepieces.

How to fix it. If the cluster looks like a faint smudge, switch to a lower-power eyepiece before you change anything else. Step up in power only if the stars are clearly resolved and the image stays bright. Once you find a magnification that works, note it in your observing log so you can return to it quickly.

Mistake 3: Expecting a nebula or a glowing patch

Why it happens. Many deep-sky targets are gas clouds, and the word "cluster" can be misread as a diffuse object. Observers who have seen photographs of bright nebulae often expect something similar, then find only a small field of points.

M50 is a star cluster. A 70-90 mm refractor resolves its stars individually, so the view is one of points of light, not a glow. Under a suburban sky, though, the faintest stars merge into a haze at low power, and that haze can look like nebulosity if you are not expecting it.

How to avoid it. Expect a group of individual stars. Look for a clump of points that is denser than the stars around it, with a visible edge where the density drops off. Compare the cluster with an empty patch of sky a few degrees away. The contrast between the two is the real signal.

How to fix it. If you cannot tell the cluster from the background, let your eyes adjust for longer, then compare the field at two magnifications. If the haze remains and the stars never separate, the sky may be too bright for the cluster to show its full detail. Try again on a darker night or from a darker site. Do not record the object as a nebula.

Mistake 4: Star-hopping to a neighboring patch

Why it happens. Monoceros is not a rich constellation, and it contains several faint groupings and star fields that look alike at low power. A small error in your hop can take you to a different group of stars, and it is easy to mistake that group for the cluster.

Finder scopes also cause trouble. If the finder is not aligned, the star you center in it will not be the star you center in the main telescope.

How to avoid it. Start from Sirius, which is very bright and is in the neighboring constellation Canis Major. Move from Sirius north-east by roughly one fist-width at arm's length, which is a rough guide to about ten degrees. Check your position against a star chart before you move the scope, so that you know which faint stars to expect near the cluster.

Align your finder on a bright star far from the target before you begin. Then center the bright star in the finder, and check that it is centered in the main eyepiece too.

How to fix it. If the field looks wrong, go back to Sirius and repeat the hop more slowly. Note the stars around the cluster and compare them with your chart. When you have the right field, the cluster should stand out as a compact group, not as one faint star among many. If you are still unsure, try a slightly different magnification and sweep the field gently to see whether the group holds its shape.

Mistake 5: Ignoring focus, dew and temperature

Why it happens. A refractor's focus shifts as the tube cools or warms, so a sharp image at the start of the evening can soften an hour later. Humid suburban nights also deposit dew on the front lens, which scatters light and softens the image.

When the focus is off, the stars in the cluster become fuzzy and merge together. The cluster then looks like a smudge, even though the sky is clear.

How to avoid it. Allow the telescope to reach the outside temperature before you start, usually about 30 minutes. Focus on a bright star at the beginning of the session, then check focus again on a star near the cluster.

Use a dew shield on the objective. If dew forms, a dew heater or a warm cloth can clear it, and you should dry the lens before you continue. Do not wipe the lens with anything abrasive.

How to fix it. If the stars look soft, refocus slowly on a bright star and adjust in small steps. If the image stays soft even at good focus, check for dew on the lens first. Then check that the eyepiece is fully seated and that the tube is not still cooling.

Mistake 6: Rushing and looking straight at faint stars

Why it happens. Faint stars are easiest to see with the eye's rod cells, which are more sensitive in dim light but work best after the eye has dark-adapted. Rushing ruins this. A phone screen or a white torch can reset your eyes within seconds, and the cluster disappears again.

Looking straight at a faint group can also cause it to vanish. Your central vision is less sensitive to dim light, so stars can seem to fade when you look directly at them.

How to avoid it. Give your eyes about 20 to 30 minutes in the dark before you judge the view. Use a red light for notes and charts. Keep phones and screens dimmed or off while you observe.

Use averted vision. Look slightly to one side of the cluster so that its light falls on the more sensitive part of your retina. Shift your gaze in small steps and let the stars appear in the periphery.

How to fix it. If the cluster looks faint and you are impatient, pause and wait for your eyes to adjust before you change anything. Then try averted vision. If the stars still do not separate, look at a brighter target for a while and return to the cluster later.

For comparison, a larger reflector of about 130 mm will show more stars in the same cluster. A 70-90 mm refractor will show fewer, but the group still forms a clear pattern once you avoid these mistakes.