This month I made two attempts at imaging the Jewel Box open cluster, more formally identified as Kappa Crucis, or NGC 4755. The first result was mediocre at best, while the second turned out quite nicely, and that means we get to do another “Watch me learn astrophotography” posting!
As you’ll know from earlier posts along this topic, I’ve become a believer in data. The more data, I’ve been teaching, the better the result because you’re giving your stacking algorithms more to work with. But sometimes “more” is the wrong approach, and that was my mistake in the first effort. You see, my telescope is a factory standard Celestron C8 Powerstar PEC which was pretty high-tech in its day and still performs admirably for regular viewing, but is a bit basic for astrophotography by today’s standards. The mount has a clock drive, to track the motions of the stars in right ascension, but no drive at all for the declination axis, and the whole thing is operated with a few buttons on the base of the mount, and a very basic controller paddle. So no computerised database of objects, no automatic alignment, and no way of attaching a guide-scope. And while the Periodic Error Correction (PEC) feature is pretty nifty in theory, it can’t keep up with the actual periodic error in the clock drive, so it doesn’t actually improve anything.
So when I try to shoot long exposures, the telescope will suddenly drift off target to the east then slowly drift back. This takes about 10 seconds, and happens every minute or so. This smears out the stars into streaks, which is not normally what you want in deep sky images. And the longer the exposures, the higher the chance of any particular frame having this problem. My usual way of dealing with this has been to simply shoot more frames, so as to leave me with more useable data. I’ve been known to shoot upwards of three hundred frames, selecting only the best third. This was the approach I had planned to use on the Jewel Box.
So the first attempt, I planned on capturing 100 frames of 30 seconds each, at ISO 1600. I don’t have a lot of experience with open clusters, but I figured that since there was no nebulosity, and since I wanted to capture as many stars as possible, I didn’t have to worry about contrast and could get away with the high ISO rating. The expected high levels of noise would be dealt with by stacking the large number of frames. Unfortunately, two things went wrong: light gusts of wind buffeting the telescope, causing streaking in practically all the shots, and clouds closing in over the target after about 65 frames. In the end, I only had about 20 usable frames, so that the final result still had plenty of noise. On top of that, many of the brighter stars were almost saturated, destroying the colour information. As you can see in the image on the left, the result is noisy and blurry, and the distinctive blue-with-a-few-bright-red-stars colouring of the cluster is almost completely lost.
So what did I do differently the second time around? Most importantly, I shortened the exposures. This meant capturing less light, but since the main stars of the cluster are quite bright, this didn’t matter much. It also meant that individual frames were more likely to fall within those moments when the telescope was properly motionless, and so yield good data.
I also shot at ISO 400, so as to have less noise to deal with, also giving better quality data. I only captured 31 frames (of which almost all were usable), but the difference is marked. A median stack virtually eliminated all the background noise, and because the stars were far from being saturated, the image withstood 2×2 binning well, reducing image size and yielding brighter, more point-like stars. For the final touch, I applied the ASINH algorithm which makes the brighter stars a litle fuzzier and exaggerates the differences in colour between them. Finally, I muted the colours once more by tweaking the saturation downwards, just to make things look a little more natural.
While I’m still not confident enough to claim that this is a great example of astrophotography done right, I’m still quite pleased with the results. It’s definitely one of my best shots and I’m proud to show it to people. Which is why I’m repeating it full-sized below!