One of our Southern Hemisphere gems is the globular cluster 47 Tucanae. It is the 2nd brightest globular cluster after Omega Centauri and contains millions of stars. It is in the southern constellation of Tucana (the Toucan) and, with a Declination of -72 degrees, it is quite near the South Celestial Pole. As such, like Omega Centauri, it is only visible in the Southern Hemisphere. At an apparent diameter of 50 arc mins, it actually extends beyond the 30 arc mins field of view of my OTA, camera combination, although as you can see the outer reaches are getting diffuse. Since the apparent diameter of the Moon is also 30 arc mins, this will give you a good idea of the size of the globular cluster. It is about 120 light years across.

Apparently in a telescope one can see about 10,000 stars and it is a glorious sight to behold. It is visible with the naked eye and, again for those of you in the Southern Hemisphere, can be readily identified by its proximity to the Small Magellanic Cloud (SMC). However, given the distance to 47 Tucanae is ~ 13,000 light years, the SMC is 15 times further away.

I captured 90 subs of 30 secs duration in RGB, but ended up rejecting 10 - 20 subs per colour channel for star elongation. I did have some guiding challenges and may have been better to have imaged with no guiding. However, the majority of the images were good and I am happy with the star shapes right across the FOV.

As to colour…..I went a number of iterations on this. I had several goes with Arcsinh but wasn’t particularly happy with the results. It usually seems to mess up the centres of stars. A Histogram Transformation (HT) stretch alone seemed to result in very subdued colours, so in the end I did a series of stretches with HT, with an intervening saturation lift with Curves Transformation. I was also careful to avoid saturating the stars at the core, yet clearly the core of the cluster is very bright, so I finished the stretch with CT, where the outer reaches of the cluster could be brightened without blowing out the core.

I had no issue with the influence of the Moon and DBE effectively removed any gradients. 

There is a preponderance of yellow stars. I used the ColorMask script, to generate a yellow mask and used that to increase the saturation of said stars, as I felt the initial yellow colour was a bit insipid. Interesting a few “Blue Stragglers” can be seen and a small number of red stars came through as well. Initially, the majority of the stars had a blue tinge. Given the age of the cluster and after looking at some reference images, I pulled the blue back to give a more white hue to the majority of the stars. There is still a slight bluish caste to the dimmer background stars, but I felt this was alright. As always it is hard to judge this.