Open Cluster in Scorpius

Designation:     Caldwell 75; NGC 6124
Magnitude: +5.8
Apparent size:39 arc-minutes.
Diameter:19 light years.
Distance:1,700 light years.
Exposure:12 x 170 seconds = 34 minutes.
Field of View:1.56 x 1.03 deg. Up is 81.1 degrees E of N

An interesting open star cluster in the Scorpius constellation, about 14° south of Antares. The main components are magnitude +9 to+11, so it is they are not visible to the naked eye.

Described by The Night Sky Observers Guide vol 2 as a “bright rich cluster of seventy-five 9th to 12th magnitude stars irregularly distributed over a 35′ area”.

This exposure covered just over a half hour.

Telescope Details
SkyWatcher Esprit 120 mm apochromatic 3-element refractor.
840 mm focal length @ f/7 with field flattener.
Baader L-Booster UHC-S light pollution filter 2458276.
SkyWatcher EQ6-R Pro mount.
Camera: ZWO ASI 071 MC Pro (CMOS 28.4mm 16 Mpx)
ZWO ASI120 guide camera, using PHD2 software.


  1. Beautiful photo. Can I ask you a question about it? You wrote that it’s too dim for the naked eye, with stars out to mag +11. You list it at +5.8 in your description of the cluster, though, which is just barely inside what the naked eye can handle, if the skies cooperate. I don’t recall ever seeing it with the naked eye, so I’m inclined to think it’s too dim, but can you clarify? Thanks! I hope all is well with you.


    1. Hi Scott,

      Good point, thanks for pointing it out. I worded that rather sloppily and have now made a slight amendment.

      The individual stars are way fainter than the unaided eye can see – but the summation for the entire cluster across 39 arc-minutes is listed as +5.8 in each of three sources I checked.

      Assuming the faintest stars visible in a truly dark sky are magnitudes up to (down to?) +6.5, then I guess it is theoretically possible from a dark sky site under good conditions to see some fuzz, although it is more than likely too spread out to be noticeable.



      Liked by 1 person

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