From the October 2014 issue

Star-rich views

Celestial sketcher Erika Rix gives tips and techniques as to how best to sketch pairs of open clusters.
By | Published: October 27, 2014

“You want me to sketch a pair of open clusters? But look at all those stars!”

Why shy away from tackling those magnificent glistening views? In fact, you might draw them with ease by following a few simple guidelines. I’ll explain how starting with a popular duo in the constellation Gemini, M35 and NGC 2158.

The larger of the two, M35 has a diameter of 28′ and lies 2.3° northwest of magnitude 3.3 Eta (η) Geminorum. With a magnitude of 5.1, it’s detectable to the unaided eye under dark skies. This diffuse young cluster is probably best known for its bright chain of stars curving northeast to southwest, led by a striking yellow and blue double whose components shine at magnitudes 7.6 and 8.9.

M35’s companion, NGC 2158, lies just to the southwest. It glows at magnitude 8.6 and measures 5′ across. When I use a wide-angle eyepiece in an 8-inch scope, both clusters fit in the same field of view at 125x. NGC 2158 is a remote, compact cluster that provides suitable contrast to its neighbor, but don’t be fooled by its high star density. It looks dim and small by comparison, but it’s not

Double Cluster
The author completed this sketch of the Double Cluster using a 4-inch f/9.8 refractor and a 13mm eyepiece, which yielded a magnification of 77x. She chose Rite in the Rain paper for its ability to hold its integrity during humid conditions. She also used a super-fine black felt-tipped artist pen, a No. 2 pencil, a 0.5mm mechanical pencil, a 6-inch circle template, and a blending stump to produce the soft glows around the brightest stars. She then scanned, inverted, and processed the sketch in Photoshop, orienting it so that north is up and west is to the right.
Erika Rix
The first step in sketching this scene is to lay a foundation. Start by plotting the reference points, a handful of the bright stars referred to as “anchors.” Add distinguishing patterns within the clusters next, such as the curved chain in M35. Use a blending stump rubbed in graphite to draw the nebulous glow of NGC 2158. Then all that remains is completing each cluster in small sections before finishing the surrounding star field.

Done? Great! Now you’re ready to move up to the big leagues. Perhaps the most famous cluster pair of all is the Double Cluster in Perseus, NGC 869 and NGC 884. The view is spectacular, but you’ll need a wide-field, low-power telescope/eyepiece combo to see them together. Each one is similar in size to M35 and almost as bright, with a separation of only 0.5°. They lie in the Perseus OB1 association. Astronomers think they originated from the same star-forming region.

The western cluster, magnitude 5.3 NGC 869, is a few million years younger than its partner. Using an 8-inch scope at 125x, you’ll resolve nearly 100 of its stars, and you’ll pick out another 50 in NGC 884, which shines at magnitude 6.1.

M35 and NGC 2158
To capture M35 and NGC 2158, this astrosketcher used an 8-inch f/5 Newtonian reflector and an 8mm eyepiece, which provided a magnification of 125x. The naked-eye limiting magnitude of his observing site was around 5.7. He used a graphite pencil on white paper. He then scanned, inverted, and processed the sketch in Photoshop, orienting it so that north is up and west is to the right.
Michael Vlasov
When sketching this crowded scene, it’s easy to lose your place among the vast number of stars. The trick is to focus on small sections around the anchors first and adjust the size of the stars (representing magnitudes) as needed to differentiate them. Then, it’s just a case of working in wedges until the entire field of view is complete.

If fatigue sets in, do yourself a favor — end the session and continue it another night. My sketch of the Double Cluster took 4½ hours over the course of two observing periods.

Now that you have useful guidelines, sharpen those pencils. Soon you’ll be plotting stars by the cluster … cluster pairs, that is!