M78 (NGC 2068) Reflection Nebula: Best Timing with a 200-300 mm Dobsonian

Why timing matters for M78

A reflection nebula is dim because it only returns light from the stars that illuminate it. The glow is spread over about 4.5 arcminutes, so the difference between the nebula and the sky background is small. Anything that brightens the sky or lowers the object in the sky reduces that contrast.

For M78, plan around three things: the season, the altitude of the object when you observe, and the brightness of the Moon. Sky quality and equipment preparation affect the result too, but these three decide whether the nebula is worth the effort on a given night.

Best season and time of night

The best months to observe M78 are January and February. During these months Orion is well placed in the evening sky, and the object is high enough to observe for several hours after dark.

Using culmination as your window

An object is at its highest when it crosses the meridian, the imaginary north-south line through the sky. This point is called culmination. Observe as close to culmination as you can, because the nebula is then at its highest altitude above the horizon.

In early January, M78 culminates at roughly 10 p.m. local clock time. By early February, culmination comes around 9 p.m. The times move earlier by about four minutes each day, and daylight saving time shifts them by an hour. Use a planetarium program or a sky chart set to your location and date to confirm the exact time for your session.

Plan a window of one to two hours on either side of culmination. Outside that window, the object is lower, the air is thicker, and the view is weaker.

Watch the season's end

By late February, M78 sets earlier in the evening, and by March it is low in the west soon after dark. If you wait too long in the season, the object may be too low for a clear view before the sky has fully darkened. Start your session early in the evening when the object is still well above the horizon.

Altitude at your latitude

The maximum altitude of M78 depends on your latitude:

• At 35° N, the object reaches 55° above the horizon.
• At 50° N, it reaches only 40°.
• At 35° S, it reaches 55°, but it culminates to the north of the zenith rather than the south.

The higher the object, the less atmosphere your light passes through. Thinner air means less light lost to extinction and less distortion from turbulence. At 50° N, the lower maximum altitude makes the view noticeably weaker than at 35° N, even on a good night. At this latitude, schedule your observing during the closest possible approach to culmination, and avoid starting late when the object has already dropped toward the west.

Observers at 35° S see the object in the northern part of the sky. Face north for the highest position, and use the same January and February window.

Moon phase and twilight

Choose a dark Moon period

The Moon is the largest source of sky brightness you can control. A full Moon raises the sky background across the whole field and washes out the faint glow of a reflection nebula. Aim for the period around the new Moon, when the Moon is below the horizon for most of the night. A thin crescent that sets soon after sunset is also acceptable.

If the Moon is above the horizon, observe M78 only if the Moon is far from the nebula and low in the sky. Even then, the sky background will be brighter than on a moonless night, so expect less contrast.

Start after twilight ends

Begin observing after astronomical twilight ends, which is when the Sun is more than 18° below the horizon. Before this point, the sky is still too bright to show the faint glow. In January and February, the evening twilight can last well into the early evening at higher latitudes, so check the exact end time for your location.

Give your eyes 20 to 30 minutes of darkness before judging faint detail. Avoid looking at bright screens or lights during this time, because they reset your dark adaptation.

Sky conditions that matter most

From a suburban sky, where the naked-eye limit is about magnitude 5, M78 is a difficult target. Its magnitude of 8.0 means it is faint, and its low surface brightness means it is harder to see than a compact object of the same magnitude. Expect the nebula to be marginal from a suburban site, and aim for the best sky you can reach.

Transparency and haze

Transparency is more important than seeing for this target. A night with clear, dry air shows more of the faint glow than a night with steady but hazy air. Check the horizon for haze, humidity, and smoke, because all of them reduce contrast. Winter nights are often clearer and steadier than summer nights, which helps the object.

Light pollution

Point your telescope away from streetlights and bright buildings, and shield your eyepiece from direct light. A light dome near the horizon in the direction of your target lowers the contrast of a faint nebula, even if the sky overhead looks dark.

A broadband light-pollution filter can help modestly from a city or suburb. Narrowband filters, such as oxygen-III or hydrogen-beta, and ultra-high-contrast filters are designed for emission nebulae. M78 shines by reflection, so those filters do little to help. Choose a broadband filter if you want to test one, and check the view with and without it.

Dew and thermal equilibrium

Dew can form on the corrector or the eyepiece and reduce contrast quickly. Use a dew heater or a dew shield if you have one, and check the optics regularly. Allow your telescope to cool to the outside temperature for 30 to 60 minutes before observing, because a warm mirror creates air currents that blur the view.

Finding M78 with a 200-300 mm Dobsonian

Start with a low-power eyepiece that gives a wide field, such as a 25 to 32 mm eyepiece. This gives a field of about one degree, which is enough to see the stars around the target. Magnifications in the range of 40 to 60 times are typical guidance for this step.

1. Locate Alnitak, the eastern star of Orion's Belt. It is the brightest star at the eastern end of the three-star belt.
2. Move your telescope north-northeast from Alnitak. The nebula lies about two degrees north and a little more than one degree east of this star.
3. Look for a faint, hazy patch near a brighter star. The brighter star is the one illuminating the nebula, and the glow is centered on it.
4. Once you have the field, increase the magnification to between 60 and 80 times to improve contrast on the faint glow. Do not increase magnification further, because the nebula becomes harder to see as it spreads out.

The Great Orion Nebula lies southwest of Alnitak, below the belt. It is a much brighter object, so it is useful as a reference point in the same region of the sky. It is not a stepping stone to M78, though, because the two objects are separated by several degrees.

What you will see at the eyepiece

With a 200-300 mm Dobsonian from a dark, transparent site, M78 appears as a faint glow around its illuminating star. The glow is pale and gray. Do not expect color or defined structure, and do not expect dark lanes or filaments. Those details require larger instruments, longer exposures, or photographs.

Use averted vision to see the glow. Look slightly to one side of the nebula so that light falls on the more sensitive parts of your retina. Move your gaze slowly and let your eyes adjust for a few minutes. The glow may appear and disappear as you shift your view, which is normal for an object at this brightness.

If you cannot see the glow after a careful search, check your starhop, confirm the field, and then try again on a later night with a darker Moon or better transparency.