Orion Nebula (M42) Visual Observing: Common Mistakes With a 70-90 mm Refractor

Expecting a photograph

The most common mistake is judging the view against long-exposure images. Photographs gather light over many minutes or hours and show colors, filaments and dark lanes that the eye cannot collect in a single look. Comparing your eyepiece view with those pictures almost always leaves the eyepiece view looking poor.

Visually, the nebula is a faint, grey glow, sometimes with a slight green tint, wrapped around a group of stars. The stars are the brightest part of the view. The nebulosity is subtle and its brightness varies from place to place. Expect this:

• A tight knot of four bright stars near the center, known as the Trapezium.
• Faint, milky nebulosity around and between those stars.
• Fainter stars scattered through the glow.

Once you accept that this is the normal view, you can look for the structure in the nebula instead of waiting for it to look like a picture.

Observing it low in the sky

Altitude matters more for this target than many people expect. From 35° N the object reaches about 50° at its highest, from 50° N only about 35°, and from 35° S about 60°. The lower the object, the more atmosphere its light must cross. That air blurs the view, adds glow from distant lights and reduces contrast, which is the quality the nebula depends on most.

Check the altitude before you set up. If the object is still low, wait until it climbs higher in the sky. In January and February, when the object is best placed, the evening hours are usually the most practical time to observe. If you live at a northern latitude, expect a lower view than observers farther south will get, and do not judge the instrument by a view taken near the horizon.

Observers in the southern hemisphere have an advantage here. The object climbs much higher, so the air is steadier and the nebula is easier to see.

Also check the direction you are facing. Trees, roofs and buildings can block the lower part of the sky, which is where the object sits for many observers during its early evening window.

Using too much magnification

When a target looks faint, people often raise the magnification to make it bigger. With the Orion Nebula this does more harm than good. The nebula covers an apparent size of 90 by 60 arcminutes, so a high-power eyepiece spreads its light across a large area and makes the glow fade into the background sky.

Start with low power. For a 70-90 mm refractor, a range of roughly 20x to 40x usually shows the nebula best, using a wide-field eyepiece. At these powers the field is large enough to include the full bright region and the surrounding glow.

Higher powers do reveal the Trapezium stars more sharply. Use them briefly to check the stars, then return to a lower power to look at the nebula. If the glow vanishes as soon as you increase the magnification, that is the expected behavior for an extended object, not a fault in the telescope.

Framing the nebula off-center

A narrow field of view causes a second problem. If you center the Trapezium in a small field, the bright nebulosity at the edges falls outside the view and the object looks smaller than it is. Many people then conclude that the nebula is weak.

Use a wide-field eyepiece and center the view on the bright region, not only on the Trapezium. Then move the telescope slowly across the field. The outer parts of the nebula are easier to notice when they drift through the view than when they sit at the edge of a static field.

Dark adaptation and sky brightness

The nebula's glow is low in contrast against the sky. A bright phone screen, porch light or streetlight can stop your eyes from adapting, and the faint nebulosity disappears in the glare. Dark adaptation usually takes about 20 to 30 minutes, so do not expect a good view in the first few minutes after you step outside or look at a bright screen.

Take these steps before you observe:

• Turn off or shield nearby lights, and turn down the screen on your phone.
• Use a red torch for reading charts and adjusting equipment. Red light affects dark adaptation less than white light does.
• Choose the darkest spot you can reach, away from the direct glow of street lamps.
• Keep your eye on the eyepiece for a few minutes before you judge the view.

Suburban glow over the horizon also reduces contrast. You can shield a nearby light with a board or a cloth, but you cannot remove the sky glow. Accept that the nebula will be fainter than it would be from a dark rural site and plan your observing time around that.

Staring directly at the nebula

Looking straight at a faint object places its light on the central part of the retina, which is less sensitive in dim conditions. Many observers miss the nebula because they stare at the center of the view.

Use averted vision instead. Look slightly to one side of the nebula, a little away from the center of the eyepiece. The faint glow often appears more clearly in the edge of your sight. Move your gaze in small steps and watch for the grey nebulosity to appear and disappear. Practice this technique for a few minutes and you will usually see more structure.

Filter and focus mistakes

Using the nebula filter the wrong way

A nebula filter is one of the most useful accessories for this target, and the expected view from a 70-90 mm refractor is noticeably better with one. The filter passes the specific wavelengths that glowing gas emits and blocks much of the other light, so the nebula stands out against the background. The stars get dimmer, which can make the view look different at first.

Several mistakes come up with filters:

• Expecting a dramatic change in a bright sky. A filter improves contrast, but it works best under dark skies and after full dark adaptation.
• Putting the filter in and out repeatedly. Each change resets your adaptation. Set the filter in place before you start, and compare the views by changing eyepieces rather than removing the filter.
• Using a high magnification with the filter. The same rule applies as without it: low power works better for the extended glow.

Compare the nebula with and without the filter at the same magnification. If you see no difference at all, check that the filter is screwed in securely and that it is designed for emission nebulae.

Focusing on the stars alone

The Trapezium is bright and sharp, so it is tempting to focus on it and stop there. The focus that makes the stars look best is not always the focus that makes the nebula look best. Focus carefully on the bright stars, then check the nebula. Make small adjustments and compare the glow at each step.

A poorly focused view looks soft in both the stars and the nebula. Move the focuser slowly through the point of best focus. Do not rush it. Small, careful adjustments give a clearer result than large turns.

Setting up too quickly

A refractor or a reflector brought straight from a warm room will shimmer, because the tube and optics are warmer than the outside air. The shimmer makes the nebula look mottled and unsettled. Set up the instrument outside early, and let it reach the outside temperature before you judge the view. For a larger tube this can take 30 to 60 minutes.

Keep the optics covered until you start observing. Dew can form on the front lens, which causes blur and glare. A dew shield or a warm cover helps in humid conditions.

Identifying the target

Some observers spend a long time searching the wrong patch of sky. Start with Orion's Belt, the short line of three similar stars near the center of the constellation. Orion's Sword hangs below the Belt, and the middle star of the Sword is the nebula region. In a pair of binoculars or a finder scope, the middle of the Sword looks fuzzy rather than point-like.

Use a low-power finder or binoculars to confirm the fuzzy patch before you move to the telescope. Then point the telescope at the same patch and check the Trapezium first. If the bright stars are visible and the faint glow surrounds them, you have found the nebula.

Do not confuse the glow with a faint star or with a reflection in the eyepiece. Move your eye slightly off-axis and check whether the glow stays in place.