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  • Writer's pictureJames Paulson

Imaging Tips: Open Star Clusters

Some Background on Open Star Clusters

Open star clusters vary considerably from globular star clusters. For starters, where the stars in globulars are primarily population II stars, and thus very old, less luminous, cooler, and metal poor, the stars in open clusters tend to be young, and thus are population I type stars.

These loose associations can have from a few tens to a few hundred, even thousands of stars associated with them. The stars tend to be the same age and of the same distance and similar makeup. Most of these stars formed from nebula, and often times the remnants of that nebula are still visible with that cluster, for example the Pleiades and its associated blue nebulosity.

These open clusters are often times spectacular. Take for example the Double Cluster in Perseus. These two cluster named NGC884 and NGC869 are located about 7640 light years distant and are about 14 million years old. Compare that with the stars in M13 which lie 22,000 light years distant and are about 12 billion years old. Quite a stark contrast between the two types.

Imaging Open Clusters with Amateur Instruments

Some have said that open clusters are the low hanging fruit of astrophotography. If that is the case then they make ideal targets to learn on, and we all have to start somewhere. Even so, no matter what level you are at in your imaging abilities, it is always fun to capture these gem like objects. You can actually follow a lot of the same recommendations that you would use for globular clusters. Pick an imaging scope with around 600 to 1200mm of focal length and start there.

Keep your subs short so you are not blowing out the stars, or as we say from a technical perspective, exceeding the well capacity of the sensor. Follow all your usual recommendations, that is, cool your camera and let it stabilize, move to your target, use your autoguiding if you have it, and shoot 30 second exposures. Shoot a couple of hours worth, sort through your frames and stack the best ones. Use dithering as you shoot in order to randomize where the image lands on your sensor. Use calibration frames like flats and darks like you always would do.

When you stack these images, and especially with a couple of hours worth of captures, you are going to have a lot of detail. I will run them through Astro Pixel Processor because I like my final stack to be fairly close to what my image will look like. I then will move it into Adobe Photoshop, do some basic levels adjustments to set the black point, do a touch up on the curves, usually run the HLVG plugin to clean up the green tint in the image, then go in and adjust the vibrance and the saturation. Remember to treat all of these things like you would be salting your meal, a little is good, too much will ruin it. Think of it as polishing your image.

With some time it won’t take you long to build a nice collection of open star clusters to add to your collection. Always remember to have fun doing it.

Here is one I shot last Fall of M45. I want to return and redo this one much better with everything I have learned along the way.

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