M4 was discovered in 1746 and catalogued by Charles Messier in 1764. It's easy to find due to its proximity to Antares, the brightest star in the constellation Scorpius. You can see this in context on my Antares Prime Focus page.
M4 (also listed in the NGC catalog as NGC 6121) is one of the sky's better globular clusters, but it tends to get overlooked due to all the other spectacular objects in the same area. The SEDS page on M4 suggests it is about 7,200 light years away, and also points out it's reddish appearance is due to it being obscured by "heavy clouds of interstellar matter." You can see this in context on my Antares Prime Focus page here. Because I set the background to be neutral (close to black) in this image the subtle red glow visible in other images is not present here. That has the effect of reducing the reddish appearance mentioned.
This image is from my Spring 2016 Davis Mountains trip. It's composed primarily of (only) 5 * 5 minute exposures. I also blended in another 3 hours of exposure to help control the noise in the dark area. I used inverse mask of the image to perform the blend in the areas between stars, blending in the low noise dark parts while leaving the bright stars alone. I also kept the dark levels low, which removed some information but makes the noise nearly invisible.
Takahashi TOA 130 telescope with flattener plus the Nikon D810A camera, riding on the AP 900GTO mount. My image processing techniques are discussed here.
You can see some of the Orange Antares Cloud on the lower left, and some of the h-alph red glow from Sigma Sco, above. Again, you can see this in context on my Antares Prime Focus page here. This image is kind of a mash-up using 5.5 hours of exposures through the FSQ-85 and TOA 130 telescopes.
Here's open cluster M4 from 110 minutes exposure with the FSQ-85 and QHY8 , a crop shot at 328mm. This is sourced from the top image on my Antares Prime Focus page.
This is a full-resolution crop of globular cluster M4, taken from the base Antares Prime Focus image shown on this page.
Antares Area, Prime Focus Pictures
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