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  • Writer's pictureJames Paulson

Imaging Tips: Globular Star Clusters

Updated: Sep 30, 2022

Some Background on Globulars

Globular star clusters are some of the finest gems in the night sky. These conglomerations of tight gravitationally-bound stars can often contain up to a million stars in their realm. They are found in the halo of the galaxy, above and below the plane of the galaxy itself and are some of the oldest stars in the universe, containing lower abundances of heavy elements. These stars are not surrounded by gas and dust such as in some of the open star clusters. As a result, no known globular clusters contain stars undergoing active star formation.

I’ve often wondered what it would be like to live on a planet in a star system inside of a globular cluster. One would have to think that it would be rather bizarre even if it were achievable because with the presence of so many stars around you, the universe itself might even be imperceivable. Your conception of space would be rather limited.

And globular clusters are not just limited to our own galaxy. In fact, globulars have been seen to be a part of all known galaxies making them unique. Another interesting phenomenon that they exhibit is that they are in a retrograde orbit, at least around our own galaxy, going against the direction that the galaxy is rotating. Could this mean that they are remnants of ancient galactic collisions? Much more needs to be studied in this area.

Imaging Globulars with Amateur Instruments

Whatever their origin, they are certainly fine objects to photograph for us amateur astronomers, and often times they are among the first objects we might set out to capture. At least for me, living in the northern hemisphere, the return of M13 each year in the constellation of Hercules marks the right of passage of the end of winter and that summer and warm nights will soon be with us.

Everything about imaging globular clusters will go against conventional wisdom of long exposures though. The tendency with long exposures is that you will blow out some of the stars by over-exposing them and this is an issue. My advice is to take your exposure time down, perhaps do 30 second subs, or even 15 second subs, and do lots of them to stack.

You can continue to keep your total integration time longer to collect more fine detail, but by stacking shorter duration exposures you should get better results. And results might vary from cluster to cluster as well, so always experiment. You could try stacking say 60x30 second exposures for a total of 30 minutes of integration time, and then do the same thing with 120x15 second exposures to see what your results will look like. One thing is certain, doing 15x120 second exposures will result in some blown out stars and you will see a marked difference. It is all about well depth on your sensor, and if you decide to do this to experiment, don’t forget to shoot the corresponding darks and flats to compliment your stacking. You always need those calibration frames.

People with great results note that between 90 and 120 minutes of stacked results deliver excellent images, so plan accordingly. More data results in less noise and delivers more pinpoint star images when done properly.

No special filters are needed to image globular star clusters except for a UV/IR cut filter. Look for the colors of the stars as well as the overall composition of the stars in the final image. Experienced imagers claim that adding more subs reduces the noise in the image as well as tends to make the stars look more precise. Best advice is to experiment.

Plate scale is also an issue in imaging globular star clusters. Short focal length fast refractors always deliver great images, but you want to be between 750mm and 1500mm in focal length to achieve a decent image scale. This is where fast imaging reflectors with apertures above 150 mm and even SCT’s equipped with focal reducers and off axis guiding can come to life. Always consider the aperture and focal ratio of your imaging scope together. Larger aperture scopes will gather more light and faster scopes will let you collect the light with shorter subs. Some people report maxing out on their imaging abilities after about 90 minutes but this is seen with slower instruments like Maksutov’s and Schmidt Cassegrains working at longer focal ratios. Again you just need to experiment.

A lot of the same techniques and advice can also apply to open clusters, depending on the stellar density in the cluster itself. That is what is going to limit your ability to integrate your images with good results. I’ll deal with open clusters in a separate article.

Also, because you are doing shorter exposures, you can save more subs due to space debris passing through star fields while collecting light, as well as limit the amount of subs that you would lose due to guiding errors. My suggestion is to work towards about 2 hours of total data to stack (integrate) to create your final image. If your subs are short enough and your polar alignment is good, you might even get away without autoguiding your images altogether. You will just have to experiment.

Good luck and don’t forget to have fun doing this hobby. With practice and data collection, you will quickly build a beautiful image library of the finest globular star clusters our Milky Way can offer. Be sure to start with the Messier catalog and work your way forward from there. You won’t run out of targets, and you might just get hooked on having fun with star clusters.

M13 was taken by myself using an ASI533MC camera on a Meade SN6 in 2021.

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