Photographing the Sun: The Safety Rules, Then the Technique

The Sun is the most available target in astrophotography. It is up during the day, it is up all year, it does not care about your light pollution, and something on it changes every single time you look.

It is also the only target with a genuine capacity to injure you.

So this starts with the part you cannot skip, and I would rather over-explain it than have someone skim a paragraph and lose an eye.

The rules

A filter goes on the front of the telescope. Always the front.

Screw-in “solar” eyepiece filters were sold for decades and they are the single most dangerous accessory in the hobby. The telescope concentrates the full unfiltered beam onto a small piece of glass an inch from your eye. That glass heats, and it cracks, and it cracks without warning. If you own one, throw it away — do not sell it, do not give it away.

Use a filter made for the purpose, from a manufacturer who says so.

Baader AstroSolar film, Thousand Oaks glass, a purpose-built Herschel wedge, or a dedicated hydrogen-alpha telescope. Those are the categories. The filter must cover the entire aperture and be mechanically secure — taped, clamped, or threaded, such that wind or a knock cannot shift it.

What is not a solar filter: sunglasses, welding glass below shade 14, exposed film, smoked glass, CDs, mylar balloon material, two pairs of eclipse glasses, or a neutral density filter of any strength. Photographic ND filters block visible light and pass infrared, which is invisible, which means you will not blink.

Check the filter every time. Hold it to a bright lamp before it goes on the scope. Any pinhole, scratch or separation and it is finished. Solar film degrades and it is cheap to replace.

Cap or remove the finderscope. It is a small unfiltered telescope pointed at the Sun, and it will happily set fire to whatever is behind it — including the side of your head as you lean in.

Never leave a solar setup unattended. Not for a minute, not with children or animals nearby.

None of this is anxiety. It is that the failure mode is instant and permanent, and every experienced solar imager follows the same list without thinking about it.

Two entirely different Suns

Once you are safe, there is a fork in the road, and it is worth understanding before spending money.

White light shows the photosphere — the visible surface. Sunspots, the granulation that covers the disc like cell structure, and faculae, the bright veined regions near the limb. This is what a solar film or a Herschel wedge on an ordinary telescope gives you, and it is inexpensive: a sheet of film is the price of a meal.

Hydrogen alpha shows the chromosphere, a layer above the surface, at one specific wavelength. Entirely different Sun: prominences arcing off the limb, filaments snaking across the disc, plage, spicules, and flares that visibly change within an hour. This needs a dedicated instrument with an etalon — a Lunt, a Coronado, a Daystar — and starts at several times the cost of a small telescope.

Most people start in white light. Some never leave, because a large sunspot group in good seeing is genuinely spectacular.

Why it is lucky imaging again

Same problem as the Moon and planets, only worse.

You are imaging in the middle of the day, through air the Sun itself is heating. Seeing during the day is turbulent in a way night-time seeing rarely is, and it is often worst in the afternoon after the ground has had hours to warm.

So: record video, grade every frame, keep the best few per cent, stack those. Thousands of frames, hundreds kept.

Two things help more than any equipment:

Image in the morning. An hour or two after sunrise, before the ground has dumped its heat into the air. The difference between 9am and 3pm is not subtle.

Do not shoot over a roof, a car park, or a driveway. Heat rising off any of those goes straight through your light path. Grass is dramatically better than tarmac.

Exposure and focus

Exposure is short. You are looking at the brightest object in the sky through a filter designed to make that survivable. Milliseconds, and watch the histogram — keep the disc bright but well clear of clipping, because a clipped photosphere loses the granulation you came for.

Focus is brutal. There is nothing crisp to focus on in white light except a sunspot’s umbral edge or the granulation itself, and both are being blurred by the very seeing you are fighting. Focus on a spot’s edge if there is one, refocus often, and accept that focus drifts as the tube heats — which, pointed at the Sun, it does faster than at night.

Turn off any automatic anything. Auto white balance, auto exposure and auto gain will all chase the disc around and ruin a capture that was otherwise fine.

Processing, and the colour question

The stack comes out grey, because that is what the sensor recorded. Every orange Sun you have ever seen was coloured in processing.

This is not a cheat and it is not controversial — hydrogen-alpha is a single wavelength, so a monochrome record is the honest one and any colour is a rendering choice. It is worth doing deliberately rather than by accident.

Akastroid’s solar mode offers three: white light, a warm near-neutral matching what the eye sees through a safe filter; hydrogen alpha, the deep orange-gold the wavelength is conventionally shown in; and as recorded, no tint at all, one channel in and one channel out. The last one exists because some people want the measurement rather than the picture, and hiding that option would be a small dishonesty.

Two other things the solar path handles that ordinary deep-sky processing does not:

Limb darkening. The disc is genuinely dimmer at the edge than the centre, because you are looking through more solar atmosphere at a shallow angle. Left alone, a stretch that suits the middle crushes the limb. Correcting it flattens the disc so detail survives from centre to edge.

Prominences. They are far dimmer than the disc, so nothing that renders the disc well shows them at all. They need their own treatment and then recombining — the same problem, structurally, as a landscape and a sky in a nightscape.

Wherever you are

The Sun is the one target with no seasonal or hemispheric argument. It is equally available from Chile, Norway, Kerala and New Zealand, and unlike everything else in this hobby, living in a city costs you nothing at all.

If you are somewhere with persistent cloud at night — or somewhere the summer sky never really gets dark, which rules out deep-sky work for months at high latitude — solar is the branch that keeps working.

The short version

• Filter on the front of the telescope, covering the full aperture, mechanically secure. Never at the eyepiece.
• Inspect it against a lamp before every session. Cap the finder.
• White light is cheap and shows sunspots and granulation. Hydrogen alpha is expensive and shows a different Sun entirely.
• Record video and keep the best few per cent. Daytime seeing is worse than night.
• Image in the morning, over grass, not over tarmac.
• The colour is a choice you make, not a thing you recorded.