Photographing the Sun: The Safety Rules, Then the Technique

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

It is also the only target that can injure you.

So this starts with the part you cannot skip. 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 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 cracks, without warning. If you own one, throw it away. Do not sell it or 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) so 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. You cannot see infrared, so you will not blink.

Check the filter every time. Hold it up 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 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, and especially not with children or animals nearby.

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

Two different Suns

Once you are safe, there is a choice to make, 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 solar film or a Herschel wedge on an ordinary telescope gives you, and it is inexpensive. A sheet of film costs about as much as a meal.

Hydrogen alpha shows the chromosphere, a layer above the surface, at one specific wavelength. It looks like a 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 a fine sight.

Why it is lucky imaging again

It is the 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. Daytime seeing 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 percent, and 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 large.

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 far 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. Use 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 hard. In white light there is nothing crisp to focus on except a sunspot’s umbral edge or the granulation itself, and both are blurred by the same seeing you are fighting. Focus on a spot’s edge if there is one and refocus often. Focus drifts as the tube heats, and pointed at the Sun it heats faster than at night.

Turn off anything automatic. 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 color question

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

That is not a cheat, and nobody seriously argues about it. Hydrogen alpha is a single wavelength, so a monochrome record is the faithful one and any color is a rendering choice. It is worth making that choice deliberately rather than by accident.

There are three defensible options. 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. As recorded: no tint at all, one channel in and one channel out. The last one matters because some people want the measurement rather than the picture, and a tool should not hide that option.

The solar path also handles two things that ordinary deep-sky processing does not.

Limb darkening. The disc really is dimmer at the edge than in the center, because near the limb 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 center to edge.

Prominences. They are far dimmer than the disc, so any rendering that suits the disc does not show them at all. They need their own treatment and then recombining. Structurally it is the same problem as a landscape and a sky in a nightscape.

Wherever you are

The Sun has no seasonal or hemispheric catch. It is equally available from Chile, Norway, Kerala and New Zealand, and living in a city costs you nothing.

If you are somewhere with persistent cloud at night, or far enough north that the summer sky never gets properly dark and deep-sky work stops for months, solar keeps working.

Summary

• 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.
• Record video and keep the best few percent. Daytime seeing is worse than night.
• Image in the morning, over grass, not over tarmac.
• The color is a choice you make, not something you recorded.