The bright star Alnitak (Zeta Orionis, the easternmost star in Orion's belt) is at the center of one of the most remarkable and picturesque regions in the sky. The radiation of this hot star excites the gas in the surrounding nebulae and causes it to emit light. The nebula strip running south from Alnitak is the designated IC 434, into which the famous dark nebula called the Horsehead Nebula (Barnard 33) protrudes from the eastern side. The dark nebula's shape resembles the head and neck of a horse, hence its name. Dark nebulae are clouds of dust in space that obscure stars and other objects behind them.
The other nebula on to the east of Alnitak is called the Flame Nebula (NGC 2024). It is overlayed by several dark lanes, the main one of these splits NGC 2024 into two parts. This nebula certainly has a flame-like appearance. Its western part is also overlayed by blue glare from the star Alnitak. The unusual color of this nebula has been noticed, in color photographs it usually appears yellow or a sort of orange almost unique to this object.
To the upper left of the Horsehead is the blue reflection nebula NGC 2023. Other reflection nebulae in this photo are IC 431, IC 432 and IC 435, all of them located around bright stars.
Along with the Great Orion Nebula M42/43 and its surrounding nebulae, this region is part of a large nebulous complex where stars are forming out of the dust and gas. Located at a distance of about 1,500 light years, this complex is the closest star forming region to our own solar system.
The Horsehead Nebula in Orion is a well-known photo target, pictures of it are found in many astronomy textbooks and surely every astrophotographer is proud to present his own version of the famous dark nebula. Indeed these photos look spectacular: The orange-red Flame Nebula NGC 2024, the deeply red nebulous stripe IC 434 with the high-contrast dark protrusion of the Horsehead Nebula B33 and the brilliant blue-white star Alnitak (Zeta Ori) make for a nice composure.
Visual observers, drawn by these colorful impressions, are of course tempted to try their luck with B33. Often they get disappointed because the Horsehead is a tough challenge for not-so experienced skywatchers. Seeing the Horsehead is possible in a premium 6-inch telescope, but it takes a perfectly dark sky and much skill in using averted vision. A Hβ filter is very helpful.
But first, B33 must be located. That is not a difficult task, as the three belt stars of Orion are conspicuous to the naked eye, Alnitak is the easternmost of them. Immediately to the east of Alnitak the Flame Nebula NGC 2024 is located, which is quite bright and would be easily observable but for the extremely bright star Alnitak, which outshines the Flame Nebula and tends to ruin an observer's dark adaption. Positioning Alnitak outside the field stop should help. As the flame nebula is not small, using low magnification makes sense. At least the dark central lane which bisects the nebula should be readily observable, even without filter; if not, forget about observing the Horsehead - the Flame Nebula is a good indicator for that.
The next step is finding IC 434, which is a stripe of nebulosity running in north-southern direction. It is a lengthy stripe, therefore low magnification is again adequate, an exit pupil of 4.5 to 5 millimeters should be ok. More is usually not practical because the sky is seldom that dark...
A Hβ filter is extremely helpful for detecting IC 434 because the nebula's contrast against the sky background increases a lot. The observability of IC 434 is another indicator for detecting the Horsehead: If it just very faint in the Hβ filter, B33 will be hard if not impossible.
Ok, now for the real challenge: Hunting for B33. First, take a look at a detailed sky chart or check a photograph for the position of the dark nebula in correlation to the surrounding stars. Starting at Alnitak and going south, there are two relatively bright stars, the first one fainter, the second one brighter, of 7th magnitude. These two mark the higher-contrast eastern edge of IC 434. East of the second star there is another star surrounded by not-so-faint nebulosity designated NGC 2023. Drawing an imaginary line from NGCC 2023 to the 7th magnitude star, and extending it across IC 434, you will find another two relatively bright stars (the northern one brighter, the southern one fainter) not quite aligned with the eastern edge of IC 434. Exactly there, at the eastern edge of IC 434, B33 is located. To see it, use averted vision and keep the eye steady by fixing one of the stars. If the conditions are excellent and you get a little experience in observing B33, you can even detect the Horsehead shape so well-known from photographs: The neck, the straight edge above the head, and a little bit of the snout, which dissolves into the nebulous background.
If the sky is extremely dark, it is possible for a skilled observer to detect B33 without Hβ filter, but more than a break in the nebulous band IC 434 can usually not be seen.
The fact that seeing the Horsehead in a 6" scope is actually not extremely difficult for an experienced observer makes detection reports with smaller apertures under perfectly dark skies plausible. There are reports (by Jay Reynolds Freeman) about a detection of B33 in his youth with a 4.5" Newtonian, without any filter. But that takes an extremely dark sky and his excellent eyes.
Hunting for B33 is easier in an 8" telescope, where seeing the Horsehead in an Hβ filter is not extremely tough for an experiences observer, there are other more difficult challenges than that. Using 10 inches of aperture, an UHC filter is adequate and the task is not very difficult. Taking a big step upwards, in an 18-inch telescope and with H-Beta filter the full figure of the Horsehead is readily observable, including the snout.
Disregarding the telescope, the observability of the Horsehead Nebula is strongly dependant on sky conditions; and if you have once detected it, doing so again is not any longer that difficult.