The summer is not just galaxy season, it’s also globular cluster season! Most globular clusters are located in the halo of the Milky Way and while a few can be found amongst the dense star fields of the Milky Way, most are well separated visually from our galactic arms. My favorite globular cluster is M 13 in the constellation Hercules. From the beginning of June, you can find M13 already getting high in the east, just north of due east, right along the edge of Hercules’s quadrangle.
M13 holds a special place in my heart as it was my first “deep sky” object that I ever had to find by star hopping with with my trusty red “Christmas Trash Scope“ decades ago now. Later, as my telescope collection grew in both quality and quantity M13 has been my benchmark target to judge the quality of my gear, both visually, and photographically.
Visually, M13 looks like crushed diamond with sparkling stars all throughout. It is bright, and the longer you expose, the larger it will appear, however you must take care not to over expose the core, else you will loose the ability to show the individual stars all the way in. Furthermore, the stars of M13 have colors! If you avoid over exposing, you can bring out the blue and amber star colors, which gives your image more character. All of my earliest attempts at imaging M13 showed a solid white snowball which matched my visual experiences and so I didn’t know any better. Look for those colors in your data — they are there!
The Hercules globular passes directly overhead for most of us in North America and is an excellent backyard target, as houses or the neighbor’s trees rarely obstruct it when it is highest and at it’s best. Furthermore, it is bright enough to stand up to imaging even in light polluted areas. The image above here was taken just outside Orlando Florida, and with a quarter moon in the sky! Just take plenty of exposures to stack down the shot noise from your sky glow.
As a good-sized target, M13 looks great for a wide range of fields of view. A wider field image of at least a degree will reveal two bonus objects. NGC 6207, a 12th magnitude galaxy just 28 arc minutes to the north east is often captured in wider views and with sufficient aperture can be spotted visually. A real prize is to capture the very tiny galaxy IC 4617, a 15th magnitude blip of a galaxy that is only an arc-minute across.
There is no better target I think to demonstrate an optics resolving power than a globular cluster, both visually and photographically. The view through the eyepiece can never be matched by a computer display (with today’s technology), but a fine optic and camera combination can come close to revealing that powdery essence of M13, plus bring out the colorful stars throughout it’s core and halo.