Imaging the Sun

 
Taking an image of the Sun can reveal fine detail on the surface of our local star

Taking an image of the Sun is a fascinating astronomical pursuit that gives you the opportunity to study a star close-up. There are other advantages to solar imaging: it can only be done during the day when the temperature is normally quite pleasant and, with plenty of light around, you can say goodbye to fumbling around with red-light torches. Of course, there’s a real danger from the intensity of light – it’s one of the only times that astronomy can pose a risk of physical injury. In this course, we’ll look at how to image the Sun safely.

MASTERCLASS

Imaging the Sun with a white-light filter

Imaging the Sun in its natural ‘white’ light is an inexpensive way to get into solar photography. When you pay attention to the safety issues, it can be a very rewarding way to monitor our nearest star.

One of the most basic methods is to use solar projection. For this you’ll need a small refractor, ideally mounted on a driven equatorial mount. With your scope pointing away from the Sun, fit a non-plastic, low-power eyepiece and ensure the finder is removed or capped.

Watching the scope’s shadow on the ground, turn it to point directly at the Sun. A piece of stiff white card held behind the eyepiece will catch the Sun’s image, while a tweak on the focuser will bring it into sharp relief.

A card shield taped to the objective end of the tube may help improve contrast if the projection is difficult to see.

Now set your camera to automatic and take a shot of the image on the card screen. If the image comes out too bright, try moving the screen away from the eyepiece or, if you have manual controls, try under-exposing the shot.

This basic method is capable of showing the photosphere, limb-darkening, sunspots and faculae – it is, however, only suitable for refractors.

A more sophisticated method, also limited for use with refractors, is to use a device called a Herschel Wedge inserted in the eyepiece holder.

The wedge basically blocks most of the harmful heat and light from the Sun, reducing its intensity to safe viewing levels.

A more universal method that is suitable on any type of telescope is to use a white-light filter such as Baader AstroSolar Film or Thousand Oaks Solar Filter.

AstroSolar Film is available in A4 sheets. It costs around £15-£20 and comes in one of two types: one with a neutral density of 5.0 for visual work or a slightly brighter neutral density of 3.8 for imaging.

Larger 100x50cm sheets are also available. See the step-by-step below for instructions on fitting a solar filter for imaging.

With a filter fitted you can image the Sun just as you would the Moon. In fact, the same constraints apply because the Sun’s light is just as susceptible to our turbulent atmosphere.

Stills cameras such as DSLRs are good for low-power shots, but webcams or preferably high frame-rate planetary cameras are more suited for close-ups.

For optimal results, however, screwing a solar continuum or green imaging filter onto your camera’s eyepiece may enhance contrast in sunspot detail and solar granulation.

STEP BY STEP

How to set up and capture solar images

1. Fitting the filter

With the scope pointing away from the Sun, remove its lens cap and fit the solar filter; remove or cap the finderscope too. Make sure everything is securely fastened. If required, use a bit of low-tack electrical tape to hold the main filter in place securely. This is especially important on a windy day.

2. Line up with the Sun

Lining the telescope up with the Sun without the use of a finderscope isn’t as hard as it sounds. Without looking along the tube towards the Sun, roughly align the scope and then look at its shadow. As the scope approaches the correct alignment, so the tube shadow will reach minimum size.

3. Keep it in the dark

If you find it hard to see detail on your laptop’s screen in sunlight, you’ll need a dark enclosure. A simple one can be made by putting a blanket over your head and the computer, but for something more sturdy, try placing the laptop in a closed cardboard box with a slit cut in it to see the screen.

4. Insert your camera

If you have one, screw a solar continuum or green imaging filter onto your camera’s nosepiece. Insert the camera into the eyepiece holder of your scope and fine-tune the scope’s position so that an image can be seen on your computer’s screen. Locate the Sun’s edge and focus roughly.

5. Settings and focus

Using the highest frame-rate, reduce gain and then exposure until the image is correctly exposed and contains no white. If you can’t, you may need to use a neutral density filter. Rotate the camera so that any spots visibly move horizontally across the frame while slewing in RA. Finally, fine-focus the image.

6. Capture

For low image scale (magnification) setups showing all, or at least a large portion, of the Sun’s photosphere, aim to capture 500-800 frames. Increase this up to around 2,000 frames for larger image scales. If your camera offers it, reduce its gamma slightly to make granulation and spot detail easier to pick out.

TECH TALK

Finding your focus

Focusing is a critical skill to master in any form of astronomical imaging; without it, you’ll get poor results.

If you’re just starting out, accurate focusing can be quite hard to get to grips with, which can make the whole imaging experience rather frustrating. There’s no real reason why this needs to be the case, so here are a few tips on how to get your images as sharp as possible.

First, make sure your camera is securely locked into your telescope’s eyepiece holder and that the focuser tension adjustment is set firm.

You want to be able to move the camera back and forth quite easily, but you also want it to stay where you’ve put it. Locate a high-contrast part of the Sun.

For white-light imaging, the best target is the Sun’s edge. For more exotic filters, the Sun’s surface is normally sufficiently detailed for you to lock onto that.

Even here, though, it’s good practice to choose a sunspot group or perhaps a dark hydrogen-alpha filament to give you a better focus target.

With a gentle grip on the focuser, move in towards focus; getting slower as you appear to be reaching the critical point.

When you do reach this point, keep going, coming out of focus again on the other side. Then, reverse direction again, passing slowly through focus, this time from the other side.

Do this a few times until you’re confident that you can recognise the real focus position; then adjust the focuser until you’re in that position.