The Digital Solargraph: Virtual Analog

What is a daily solargraph?

Solargraphs are long-exposure images, usually taken over weeks or months. The approach here is different, see more details below. But the gist of it is that I take a very underexposed image every minute for the sun trail, and separate calibration images every 30 minutes. What you see is a combination of the two, the sun trail built from the underexposed images and superimposed onto the regular 30-minute calibration images.

Why the separate exposures?

When you take a regular photo with the sun in it, the sun is usually way overexposed and blown out. To capture a sharp, clean sun trail similar to what you'd expect in a solargraph, you have to underexpose the image by quite a lot. I initially just took those underexposed images, but then I wanted to cover more of the sky and added a second camera.

To stitch the images together into complete panoramas, I need to calibrate the stitching. That requires images that are exposed like more normal images, the dark frames have very little detail, making calibration mostly impossible.

How are these images captured?

To get a wide enough view that covers much of the sun's sweep across the horizon, I have two GoPro Hero 12 Black cameras set up at a 90° angle from each other. That gives me about 250° horizontal coverage, and nearly 100° vertically.

As a result, I have to stitch a lot of images before I can composite them. In the summer, the each camera takes 800 pictures per day, plus 27 bright calibration shots. Even with GoPro's very efficient RAW format, that's 12GB per day of just images. I download and process them automatically at the end of the day. If everything goes smooth, the day's images and video should be up by 10pm PDT.

If you care about more details, the dark sun trail images are captured at the darkest setting I can manage on the GoPros, 1/2000s at ISO 100 through a 10-stop ND filter. Calibration shots are taken with automatic exposure at ISO up to 800 (the ND filter stays on, so I have to crank up the ISO). All images are downloaded as raw files, not only because I need the dynamic range and the JPEG compression completely destroys the dark shots, but also because the JPEGs get partly dewarped (even in wide mode), which makes it impossible to stitch them together.

Why are some days missing?

Some of them haven't happened yet… but also, I was tinkering with settings early on and lost a couple days of calibration exposures. Those show up as gaps here too — the accumulated exposure just skips them and picks back up on the next captured day. I might fill them in eventually from the day-by-day view.

Why digital solargraphs?

Flexibility, mostly. I can do a lot of things you can't do with an analog capture that takes months, like make a website like this. I can create partial solargraphs for individual days, weeks, or months, and — as you can see above — composite them all into one image as the season goes.

You can take it even further and simulate solargraphs from solar irradiation data! Not only does that save you six months of waiting, you can also draw it for many different places on the planet (assuming there's data for them). The line between photography and data visualization gets kind of fuzzy here.

This is very much a work in progress. The shots I use for these videos were initially only meant to help me recalibrate the panorama stitching (which doesn't work so well with the underexposed shots). But they've turned out to be a lot of fun to use as the background for partial solar trails, and to see how cloud cover impacts the solargraph.