Orion is not only the best known winter constellation, but it is also a great urban constellation. Thanks to its distinctive arrangement of rather bright stars, Orion shines through in heavily washed-out skies, even when few other constellations can recognized. It is also the home of the Great Orion Nebula (M42) big, bright, and easily visible even in washed-out skies. However, M42 is only the most famous of many interesting astronomical objects in Orion. And best of all for urban astronomers, quite a few of Orion's objects—like the constellation itself—can be enjoyed in light-polluted skies. Here is a run down...

M42 & M43

The Great Orion Nebula is the best emission nebula visible from the northern hemisphere. Even from a heavily light-polluted area such as the Washington, DC metro area, the brightest part of the nebula around the Trapezium shows its characteristic “mouth” shape clearly in a 90mm (3.5 inch) refractor. Looking through a 10 inch (250mm) Dobsonian, swirls and eddies are visible. The Orion Nebula responds well to narrowband filters, even with small aperture telescopes. Using an UltraBlock filter with a 90mm refractor really brings out M43 and starts to show some “texture” in the main nebula. With a 10 inch Dob, the UltraBlock lets you trace out streamers extending from either side of the “mouth” and other features in the “backside” of the nebula. An even narrower O-III filter brings out more details than the UltraBlock when used with a 10 inch scope, especially in the “mouth” and the area between M42 and M43. However, with a 90mm refractor I find the O-III filter shows less than the UltraBlock.

 
σ (sigma) Orionis

This multiple star system is often overshadowed by the Trapezium, but the mix of colors and range of magnitudes makes σ Ori my favorite multi-star system. The primary is a star of magnitude 3.7 (it's actually a double that is much too close to resolve). To one side you will see two companions (D and E) of nearly equal magnitude ~6.6; E is relatively far away at 42”, and D is much closer at 13” (D also looks fainter to me). The fourth component, C, is magnitude 10.3 and is located 11” away on the opposite side from D. All four stars are easily visible with my 4.25” and 10” reflectors, but my 90mm refractor only shows A, C and D easily: E is rather hard to see unless seeing is pretty steady. My reflectors show the stars’ beautiful colors: the primary is white, D is blue, E is deep blue, and C appears violet or lavender to me. This latter color is probably an illusion, but I see it consistently. The colors, however, don't come through in my 90mm refractor.

Trapezium

The Trapezium. is probably the most famous of all multiple star systems. It's location nestled within the glow of the Orion Nebula makes it quite beautiful. To me, it consistently appears as if the Trapezium is sitting in a little hollow within the larger nebulosity, although perhaps this is an optical illusion. The four primary stars are brighter than eighth magnitude and separated by at least 8", so they are easily seen even in small telescopes. The primary challenge to observing the Trapezium is not aperture, but rather the generally poor winter seeing conditions. Components E (mag. 10.3) and F (mag. 10.2) are in theory in easy reach of a 90mm refractor, but in practice poor winter seeing and the background nebulosity makes E and F surprisingly hard to see, with F being harder to see than E. With a 90mm refractor, it is hard to see E unless the seeing is very good, and only with exceptionally good seeing has the 90mm shown both E and F. With my 10 inch Dob, I can see E as long as the seeing isn't very bad, but glimpsing F still requires pretty steady seeing.

Rigel

Rigel has a 6.7 magnitude companion located 9” away. It's easy to see the companion in my 10” Dob unless the seeing is terrible; it's also easy in my 4.25” Newt as long as the seeing is decent; but it can be challenging in my 90mm refractor unless the seeing is pretty steady. The contrast between the bright, blue-white primary and the tiny speck right next to it makes for a beautiful view.

 
λ (lambda) Orionis

λ Orionis is a tight pair of blue-white stars, magnitudes 3.6 and 5.5, separated by 4.4”. They are easily split by my 90mm refractor, though they seem even closer than they actually are due to the noticeable magnitude difference. What makes this double outstanding is that λ Ori lies in a beautiful field of stars, surrounded by star cluster Collinder 69.

NGC 2022

Often overlooked, NGC 2022 is a magnitude 11.7 planetary located just a bit southwest of lambda λ Ori. About 25" in diameter, it appears clearly non-stellar at 130x. It appears brighter than the listed magnitude would suggest. It is easy to spot in a 10 inch Dob and I can spot it with my 4.25 inch Newt if I use an UltraBlock filter. However, in light-polluted skies NGC 2022 is very hard to see with a 90mm refractor. I have seen NGC 2022 with the 90mm refractor using an UltraBlock filter and 130x magnification, but only because I simultaneously had NGC 2022 centered in the 10 inch Dob and could use it to aim the 90mm to exactly the right spot.

What not to look for...

Of note, the other Messier object in Orion, M78, does not show up well at all in washed-out skies. I can see it with my 10 inch (250mm) Dob, but only very indistinctly and it is easily overlooked. M78 is primarily a reflection nebula, so narrowband filters don't help. I've not been able to see M78 with my 4.25 inch or 90mm telescopes in urban skies (but both easily show M78 from dark-sky locations).  Similarly, the Flame Nebula just west of Alnitak (ζ Ori) is pretty much invisible when observing under washed-out skies; I've not been able to see it even with my 10 inch Dob.