A star as wide as the Moon: the Sun with a phone

On 12 August the Moon’s shadow ran across Greenland, Iceland and northern Spain, and for a couple of minutes an unknowable number of European phones stared at the Sun. Then the eclipse ended and everyone went back to photographing dinner. The Sun stayed where it was. It is the one astronomical target that asks for no darkness, no drive into the hills, no new Moon and no three in the morning alarm. It asks for exactly one thing instead, and there is no talking it down.

The first rule is not a photographic one

NASA puts it in plain words: looking at any part of the bright Sun through a camera lens, binoculars or a telescope without a special-purpose solar filter fitted over the front of the optics will cause severe eye injury instantly. The corollary catches almost everybody out. Eclipse glasses certified to ISO 12312-2 are made for the naked eye, and behind an optic they protect nobody, because concentrated sunlight burns through the film faster than you can react.

A phone adds a second casualty, which is its own sensor. NASA’s photography people are blunt about that too: pointed straight at the Sun it can be damaged like any other sensor, and all the more so with a clip-on tele lens concentrating the light in front of it. Note what the certification actually covers, as well. ISO 12312-2 applies to viewers for the eye, and it certifies no filters for camera lenses at all. The ND filters in your bag are a different animal again: they cut visible light while letting infrared and ultraviolet straight through, which is the part you never see coming.

What you need is a real solar filter, film or glass, mounted in front of the lens rather than behind it. Cover every active camera, because a phone will switch lenses on its own whenever it feels like it, and clamp it so a gust cannot lift it. Fit it before you point. Take it off after you have turned the phone away.

A star as wide as the Moon

With the tedious part out of the way, the numbers are far kinder than the planets ever were. The NASA fact sheet gives the Sun an apparent diameter of 1,919 arcseconds at one astronomical unit. The Moon measures 1,896. Those two figures sit so close together that total eclipses exist because of it, and nothing else in the solar system repeats the trick.

For a phone the consequence is immediate. On the main wide camera, where one pixel covers roughly 74 arcseconds, the solar disc lands on twenty-six pixels: a pinhead in the middle of an empty frame. On the 120 mm equivalent tele, at 15.3 arcseconds per pixel, it spreads across a hundred and twenty-five, which is very nearly what the Moon does on the same lens. Shoot at full resolution and both figures double. Thin for a portrait, plenty for a clean disc with a hard edge.

Sunspots, counted in pixels

A blank disc gets old quickly, and what you are really hunting is spots. NASA describes the dark core of some sunspots as larger than Earth, and explains why they look black: inside a big spot the temperature drops to around 4,000 degrees C against roughly 5,400 for the visible surface, so it emits less light than its surroundings and contrast does the rest.

The solar radius is 695,700 km, so the diameter is close to a million and four hundred thousand. Earth manages twelve thousand seven hundred. Scale that against the 1,919 arcseconds of the disc and a spot as wide as our planet works out at seventeen and a half arcseconds: on a phone tele that is one pixel, maybe one and a half, which is to say nothing. A group three or four times Earth’s width reaches about sixty arcseconds, so four pixels at twelve megapixels and ten or so at full resolution. That one shows: a dark dot on the disc, unmistakable, and it has moved by tomorrow.

Hence the honest forecast. Big groups yes, small ones no, photospheric granulation not in this lifetime. Spot numbers rise and fall along the eleven year cycle, so there are stretches when the disc is a blank white plate for weeks and stretches when you find four groups in a single morning.

Exposure works backwards here

Every other night spent shooting the sky is a scrap for photons. Here you have too many, and the craft turns inside out. With a solar filter in place the correct exposure lands on very short times at base sensitivity, noise drops out of the conversation entirely, and the tripod earns its keep on framing more than on stability.

The automation fails in a predictable way. Metering reads the whole frame, and the frame is black apart from that little disc, so the phone decides the scene is dark and opens up, and the Sun comes out as a white blob with no edge to it. Two gestures fix it. Lock exposure on the disc, then wind the compensation down until the limb turns into a circle cut with a razor. After that you touch nothing, and you shoot raw.

Focus changes character too. In the dark, autofocus has nothing to grab and has to be walked out to infinity by hand; with a filter fitted, the edge of the disc is a violent contrast step, exactly the kind of detail autofocus is good at. Tap the limb on screen, let it lock, then hold it there.

Midday air is against you

There is a reason solar observers get up early. The worst seeing of the twenty-four hour cycle, as Sky & Telescope points out, arrives in the afternoon, once the ground has spent all day banking heat and starts handing it back as columns of rising air in front of your lens. The best hours are the ones just after sunrise, before the landscape warms up. The Sun is available for twelve hours a day and two of them are any good.

Against trembling air you use the planetary trick: record video, keep only the frames born in the instant the air column settled, stack those. Lucky imaging works on the Sun for the same reason it works on Jupiter, with the bonus that light here is so abundant that very short exposures cost you nothing.

Buying scale, with a telescope or a piece of card

A hundred and twenty-five pixels stay a hundred and twenty-five pixels, and the only way to get more is to put magnifying optics in front of the phone. Afocal shooting applies as it does to the planets, with one difference that governs everything: the filter goes on the telescope’s aperture, at the front, covering the full diameter. The sums on useful magnification are unusually comfortable. At a hundred times the scale falls to 0.15 arcseconds per pixel and the disc would come out thirteen thousand pixels wide, three times outside the frame. Somewhere around thirty times, the Sun fills the long side of the image and still fits. For once the target is too big.

There is also the route that puts no eye at risk: projection. Point a small telescope or a pair of binoculars at the Sun without looking through them, hold a white card half a metre behind the eyepiece, and the disc appears on the card with its spots, as large as you care to make it. The phone photographs the card, an ordinary lit subject like any other. Two hard rules: nobody goes near the eyepiece while the instrument is aimed, and the instrument does not sit there cooking for half an hour.

Two weeks to cross the disc

The Sun turns on its axis in 609 hours according to NASA, a shade over twenty-five days. Seen from Earth, which is going round it the same way in the meantime, the apparent period stretches to just under twenty-seven days, so a spot needs a fortnight or so to travel the lit face from one limb to the other. One frame a day for two weeks, always at the same hour, and you have a film of a star rotating, shot from your balcony.

One last number, for anyone framing tight. The Sun drifts at the same rate as the stars, fifteen arcseconds per second, and crosses its own diameter in a little over two minutes. At a thousandth of a second there is no motion blur to speak of. On the tele, though, your framing runs away at a pixel a second.

If all that sounds like a long preamble for a small white dot, keep the date free: on 2 August 2027 the Moon’s shadow returns to Europe over southern Spain and carries on into North Africa, and near Luxor totality will run 6 minutes 22 seconds, the longest over land until 2114. Anyone who turns up with the filter already fitted and the exposure already rehearsed will have spent the previous year photographing the same star on the days nobody bothers with it.