Now that the big event is over, we’ve shared our eclipse stories and relived the glory over and over, it’s time for some of us to finish up the last of our image processing. By now I think everyone is fairly burnt out over eclipse images which have been flooding the internet for the past 2 weeks. For myself , since this was my very first eclipse experience, I wanted to share the moment with friends etc using the best single image of totality I had. But this is just one image. For imaging total eclipses, it’s all about the corona, that “ring of fire” as my wife calls it, that seems to explode in the instant of totality. No camera can record the sheer immensity and beauty of it such as the human eye can see it all at once. No wonder we were told to ditch the equipment during totality and just watch! The corona is that “aura of plasma surrounding the Sun that extends millions of kilometers into space” (Wikipedia). Temperatures in the millions of degrees! The physics of it are incredible in itself. We can only see it during totality (scientists can use an instrument to study it called a coronograph which blocks out the sun’s disc) However to photograph it is very challenging because it has such a wide range of brightness. The inner corona is very bright while the outer corona is very dim. One exposure is not going to capture it all. Hence the concept of “bracketing exposures” during totality has become well known to eclipse imagers. You automate a series of different exposures, in my case 4, and repeat the sequence again and again until totality ends. Different cameras may be able to take up to 7 different exposures automatically. This depends on the camera type (i.e Canon vs Nikon, full frame sensor vs APC or cropped sensor). Exposure during totality generally can range roughly from 1/1000 down to about 1 second or so. After the images are obtained you take all of the images from each exposure setting and combine them to create a single image, a process called “stacking” which increases the signal to noise ratio in the final image, improving resolution. At the end of this procedure you are left with let’s say 4-7 “master” images, each showing different parts of the corona by virtue of the different exposure times. In my case unfortunately the 4th series or slowest exposure time was way overexposed and not usable. That’s why I would change my ISO to 200 from 400 at the next eclipse, as was suggested to me!
Now the next step is the most critical. Combining , in my case 3, master exposures into one image which captures the entire corona in all of its detail. How the heck do you do that? Actually this is the easiest part! For myself I am reasonably experienced at deep space image processing and I use a program called Pix Insight. While the program’s features are well beyond the scope of this post, what I can say that is interesting about it is that it is “open source” and anyone can potentially contribute any image processing script or other process to it if it is deemed useful. There are a lot of smart folks out there and thankfully a process was created called “HDR composition” which does exactly what we need! Now get this: you open the program, go to Process> HDR composition and you get the pop up shown below. Click on “add images”. Click on the round blue sphere at the bottom and Voila! That is totally all you do! I should say you can if you want to, fuss with any or all of the parameters shown but the default values work very well. How this accomplishes the seemingly impossible task of merging images of different exposures to capture the entire brightness range in a single image , all in 1 click, is quite beyond the scope of this discussion! But trust me on this , it works!
Final notes: I was puzzled by a couple of things. One was the sort of pink bias in the single images and the other was the brightening of the Moon’s limb. You can see this brightening on many of the single exposures as well as the pink hue . This is not a processing artifact or enhancement of any kind. My hypothesis on this that the presence of high cirrus clouds during totality which we all forgot about in the moment I believe affected the images perhaps causing this phenomena. It is also possible in the case of the Moon’s limb brightening there could have been a hint of Earthshine getting in there! To my wife’s chagrine I decided not to artificially darken the Moon’s disk. She wanted to have it look like what we saw but as I was saying, this is NOT about what we saw! What the camera saw is going to be different. I was careful not to overprocess the coronal detail. This is very easy to do! As I am sure you have seen, there are a lot of way overprocessed coronal images out there. I thought this was just about right, preserving some of the impressive brightness of the corona as well as the structure without making it look like Saran Wrap!
However, as with any image, terrestrial or celestial, the beauty or lack thereof is ultimately in the eye of the beholder!
Until next eclipse!
Thanks for reading!