M103 - Open Cluster in Cassiopeia... With a Twist!

Updated: Jan 13, 2020

How do you make a boring cluster more interesting?

Messier 103 is a pretty small open cluster in the constellation of Cassiopeia, about 9,000 light-years away from Earth. It is about 25 million years old and contains about 172 stars.

M103 was discovered in 1781 by Pierre Méchain, who is also to thank for many other objects in the Messier catalog.

We initially did not intend to photograph M103 that night, but we decided to include it before packing up after having imaged two nebulae, the Pacman Nebula and the Helix Nebula. We used our Orion 8" Astrograph f/3.9 telescope and our ZWO ASI 1600MM-Pro camera and only spent a total of 35 minutes on it. We would have preferred imaging it for a couple of hours, but being tired and knowing we still had 30+ minutes of packing and an hour of driving ahead, we decided to stop it there.

As you can see on our photo below, there is a colorful artifact coming from the top left. This is because we accidentally framed our target just to the edge of the very bright star Ruchbah (it is actually still far from the edge, but we're talking about a magnitude 2.65 star). Ruchbah is part of the Cassiopeia constellation, and we talk about this star a little more in depth later in our "How to Locate M103" segment.

The way the final image looks is… interesting! We were going to crop the flare out before saving the final image, but decided to upload it to our Patreon's page before making that decision, and the comments all said to keep it all in there! We appreciate the feedback and are now glad we did not crop it out.

Messier 103 (LRGB), with the ASI 1600MM


Camera: ZWO ASI 1600mm Pro Mono

Telescope: Orion 8" Astrograph f/3.9

Mount: Atlas EQ-G motorized Mount

Guiding: ZWO ASI 290MM Mini

Acquisition: ZWO ASIAIR

Power: Pegasus Astro Pocket Powerbox

Processing: Pixinsight


Total Exposure Time: 35 minutes

Exposure Time per frame: 30 seconds

Filters: L (20 minutes) / R (5 minutes) / G (5 minutes) / B (5 minutes)

Gain: 75


Being very close to Cassiopeia's "W" shape, M103 is pretty easy to find and can even be spotted with binoculars.

It is small (barely 1/5th of the diameter of the moon) but bright enough to be seen with any small instrument. To locate it, aim your binoculars or telescope at the bright star Ruchbah (the bottom left star in the "W" shape of Cassiopeia) and drift just a little bit in the direction of the extreme-left star of that "W".

You should spot M103 with ease, but do not expect it to look incredible due to its small size and small number of members.


We imaged Messier 103 with 4 filters:

  • Luminance (L)

  • Red (R)

  • Green (G)

  • Blue (B)

M103 is a typical small open cluster and it does not have many stars, so processing was a breeze.

We went through the linear phase pretty fast using Automatic Background Extraction, Deconvolution, and Atrous Wavelet Transform on PixInsight. The non-linear phase was the same we used for pretty much all our images, minus HDR Multiscale Transform which we don't use for star clusters.

We sadly did not keep a single shot image to show you guys, but the 30-second photo looks almost like the end result, just dimmer.


Messier 103 is a very easy target to photograph and process. Although pretty small, it is not a busy cluster and, unlike another target like M52, is not in the vicinity of any nebulous gas to worry about. A little over half an hour is low, but still yields a beautiful final image.

Make sure to follow us on Facebook, Instagram, and YouTube to stay up to date with our work!

Clear Skies,

Galactic Hunter



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