Graphite pencils on white paper produce negative astronomical drawings. That means that any rendered starlight appears dark against a light background. If you plan to scan and publicize your sketch, you have the option to leave it as is or invert the image to create a realistic portrayal of the eyepiece view. Often the brightness of the object becomes the deciding factor.
A negative sketch can make difficult-to-see details stand out more clearly. I’ll use NGC 2090, a spiral galaxy in the constellation Columba, to demonstrate. This object has a magnitude of 11.3, a low surface brightness, and is 4.9′ by 2.4′ in size. It’ll be a challenging target for observers in the Northern Hemisphere due to its low altitude. To locate it, point your telescope toward the southern sky near midnight. It lies 1.5° east of Alpha (α) Columbae.
The author sketched NGC 2090 using a 16-inch f/4.5 reflector on a Dobsonian mount with a 13mm eyepiece for a magnification of 141x. She used graphite on white paper and a blending stump, and adjusted “Curves” with Photoshop CS6. Both sketches have north at the top and west to the right.
For those of you using a 6-inch telescope, the galaxy will have a slight north-northwest to south-southeast elongation with a central brightness at a magnification of 100x. Through a 10-inch scope, you’ll spot a 12th-magnitude star 1.5′ from its center. Through a 16-inch instrument, the core is 1.5′ long and you may see portions of the spiral arms within the faint halo if you use averted vision. A magnitude 7.7 star lies 18′ southeast of the galaxy.
The faint structure within the halo would have been virtually invisible had I inverted the drawing. So instead, I left it as a negative sketch and adjusted “Curves” (an image editing tool to correct tone and contrast) in Photoshop to accentuate its details. Before making any adjustments like this, you should check your computer monitor’s calibration for correct shade and color reproduction.
The artist captured NGC 2362 using a 10-inch f/4.7 reflector with a 13mm eyepiece for a magnification of 93x. He used graphite on white paper, scanned and inverted the sketch, and added color and star glow with GIMP.
At the other extreme, bright jewels in the sky practically beg for drawings to be inverted to bring out their luminance. NGC 2362, a spectacular young open cluster in the constellation Canis Major, fits the bill. Spanning 8′, it’s rich and compact, nearly centered on 4th-magnitude Tau (τ) Canis Majoris. A 7th-magnitude star lies 7′ east of the cluster’s center, and you’ll find two 9th-magnitude stars at a similar distance on the opposite side. A northern arc of 10th-magnitude stars connects the three.
NGC 2362 is a treat through any telescope, but make sure to catch a low-powered view to soak in the bluish glow radiating from Tau. With larger telescopes and moderate power, you’ll be able to resolve 50 to 75 stars and split Tau into a triple.
Jay Eads sketched this object, then scanned and inverted it before he digitally added luminance and color. By using descriptive observation notes as a guide and taking advantage of image-processing software, he was able to render an impressive resemblance of the eyepiece view.