Stellar Cluster NGC6231

We had some clear nights arrive, although with the full moon, there was a massive amount of light in the sky. The moon was so bright it was casting shadows like the noon-day sun - with dark-adjusted eyes it was painful to look at it directly.

So, what does an aspiring astro-nerd do? They look to targets away from the moon as far as possible. In this case, something like 180 degrees away from it. Of course, the downside is that most of the good targets (Eta Carina, Southern Jewel Box Running Chicken, etc), are below my field of view.

I stumbled across this one - NGC 6231 (aka ‘Caldwell 76’ or ‘The Northern Jewel Box Cluster’). With a sky full of light pollution and moon-light, I needed a bright target to capture - trying for nebulae or galaxies would have been an exercise in futility.

So, in the end, I captured a total of 180 images, each 10 seconds long (to compensate for the amount of light around). I stacked these with three different tools, and (as always) achieved three different results.

In all cases, the final images are crops from the original 6000x4000 pixel stacked image.

I initially thought that the Affinity-stacked image would be the best overall, but was quite surprised to find that the SiriL+Histogram image looked the best (or at least the most visually appealing) when compared to the other images.

---

Firstly, I tried the ‘Astrophotography Stack’ tool in Affinity Photo. This gave me the best stack (in terms of star quality) and a good, if somewhat dark base to work from.

---

After Affinity, I turned to the Windows program ‘Sequator’ (which works perfectly under ‘CrossOver’ on my Hackintosh). It gave me a brighter output than Affinity, but the stars were over-exposed and had obvious purple / magenta fringes. A quick bit of work in Affinity to tame that, and we have our second image.

---

Then I turned to SiriL…

This image had the ‘Histogram’ adjustment added to it, after the usual background removal, colour calibration, and ‘Asinh’ stretching. Adding the Histogram stretch quite often blows out the noise in the background of the image - a particularly murky red-brown sludge colour. This noise can be a pain to remove in Affinity.

However, in this case, the noise was not too bad. So I was able to remove it fairly easily. However, the stars of the cluster appear to be over-exposed and should to be toned down.

---

The final image is the result of stacking in SiriL and not applying the Histogram stretch. This one gave similar results to Affinity, but required some careful post-processing to bring the stars of the cluster out from the background.

---

It’s hard to say which one I like the best. The images with the not-so-bright stars are closer to reality and what you would see if looking at them through a telescope. Yet the (to my mind) over-processed and over-bright stars looks the best on the computer screen.

• Astro