How to safely observe the Sun

How do you safely observe the Sun?

With such a wide variety of telescopes, cameras and filters available, anyone can capture breathtaking images of our host star in different wavelengths, writes Mark Townley.

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Observing and imaging the Sun has never been easier or more accessible, thanks to an increasing number of solar telescopes and filter systems.

This kit isolates specific wavelengths of light coming from the Sun – white light, calcium K or hydrogen-alpha – at the same time as blocking the other wavelengths, making solar observing safe and easy.

Here we look at what these three different views will show you of the Sun’s internal structure and features; things that would be otherwise hidden by our star’s overwhelming brightness.

 


 

White Light

An image of the Sun in white light, captured by James Robertson from Somerset. James used a Canon EOS 750D DSLR camera and a Sky-Watcher Esprit 80 ED triplet refractor with Baader solar white light filter.
(Credit: James Robertson)

 

What you can see

When observing in white light you are actually viewing a range of wavelengths across the visual spectrum.

This allows you to see a layer of the Sun called the photosphere, where you can observe familiar sunspots, the darker and cooler regions where magnetic field lines are concentrated. With their darker central umbra and lighter surrounding penumbra, sunspots are the easiest feature to see. Smaller dark spots without developed umbra and penumbra are known as pores.

In active regions we can observe faculae: brighter patches best seen from an oblique angle near the limb of the Sun where limb darkening allows them to stand out easier.

Through larger apertures and at higher magnifications polygonal granules are visible, which are giant convective cells of plasma.

Solar transits, where an object passes in front of the Sun, can also be seen. Commonly these are birds or planes, but on rare occasions the International Space Station will also pass over.

 

How to see it

Any telescope can observe the Sun in white light by using a suitable front-mounted glass or solar film filter.

Be sure to get these from a dedicated astronomy retailer rather than attempting to improvise a filter yourself – the safety of your eyes is paramount.

A green or continuum filter at the eyepiece end will allow a higher contrast view or image to be achieved by reducing the effects of the poor daytime seeing.

For the highest contrast view, a Herschel wedge used in conjunction with a continuum filter, an ultraviolet/infrared-cut filter and a refractor is the best option.

 

Imaging advice

Imaging the Sun in white light is easy and a wide range of cameras can be used.

With a dedicated solar filter over the front of your telescope, results can be captured and viewed instantly by holding a camera phone or compact camera afocal to the eyepiece, with a DSLR camera at prime focus or even by projecting the image of the Sun onto a white screen.

The best results are achieved using a Herschel wedge and digital CCD camera, where many hundreds of individual frames are captured in video format.

Using freely available software that can be downloaded from the internet, such as RegiStax or Autostakkert, the sharpest frames are automatically selected and digitally stacked into one final image with less noise and better quality than the individual frames. This can then be coloured to individual taste using freeware such as GIMP. 


 

Calcium K

Credit: NASA/Marshall Space Flight Center

 

What you can see

When we observe the Sun in calcium K light, we are viewing our star at 393nm, the ultraviolet end of the spectrum. This reveals the solar chromosphere, a layer some 1,000km above the photosphere at a temperature of about 12,000 Kelvin.

Most people struggle to see anything at this wavelength visually, so it is better suited to imaging.

Some of the features seen in white light are visible, such as sunspots with their associated umbra and penumbra, and also faculae, but now we can also see the bright white plage; areas of hot, magnetically frothy plasma that extend upwards above the photospheric faculae.

In calcium K, the influence of magnetic fields gives a bright white appearance, with the exception of the very strong magnetic fields around sunspots, which appear dark. Rarely, solar flares can be seen in calcium K, and rarer still are darker filaments, which can be seen on the disc.

 

How to see it

Observing or imaging in calcium K requires a refractor, and better results are achieved with longer focal ratio scopes as they are less susceptible to spherical aberration, which can cause a softening of the image at calcium K wavelengths, particularly in budget telescopes.

A calcium K filter is needed and a number of different sizes are available from Lunt, plus a blocking filter

Be sure to check with your dealer which size of blocking filter is best suited to your telescope. If observing visually it is best to try different eyepieces from your collection as the coatings on these can affect light transmission and image brightness.

 

Imaging advice

Calcium K imaging is a form of narrowband imaging, as only a very narrow spike of light at a certain target wavelength or colour is allowed to pass through the filter.

A mono CCD or CMOS camera is the best choice for capturing images; with a colour camera, only a quarter of the Bayer matrix (the blue component in this case) that covers a colour chip would be utilised, resulting in a loss of image resolution. Freeware program FireCapture is a great choice for capturing images. 

Try to keep exposure times fast, as shorter calcium K wavelengths are more susceptible to poor seeing and shorter exposure times allow the moments of best seeing to be captured. Use the histogram function in the capture software to avoid overexposing the image and losing precious detail.


 

Hydrogen-alpha

Bill McSorley imaged the Sun in hydrogen-alpha from the Derbyshire Dales. Bill used a QHY5L-II colour planetary camera, a 2.7-inch TeleVue refractor, a Celestron altaz mount and Coronado hydrogen-alpha filters.
Credit: Bill McSorley

 

What you can see

Hydrogen-alpha gives the best all-round views of the Sun, being easy to see in the eyepiece and showing an ever-changing view of various features that never ceases to amaze: from flame-like prominences on the limb to long, snaking dark filaments that hover above the disc, both of which are clouds of plasma held aloft by intense magnetic fields.

Again we are looking into the Sun’s chromosphere in this wavelength, albeit at a slightly different temperature and height above the photosphere than calcium K.

Sunspots and the bright plage around active regions are still visible, with solar flares noticeably brightening and then fading over a period of minutes to hours. In the best conditions a layer of spicules can be seen around the solar limb; a fine layer of dancing, hair-like jets of hot plasma that shoot out of the chromosphere.

 

How to see it

Hydrogen-alpha offers the largest choice of equipment available to suit all budgets, from front-mounted etalon filters that fit on existing refractors or compound telescopes used in conjunction with a blocking filter, to dedicated hydrogen-alpha telescopes or rear mounted filters that go before an eyepiece or camera.

Options such as double stacking during processing provide higher contrast views. The different sizes of blocking filters can seem bewildering to a novice, so it is best to seek advice from your local astronomical society or dealer to see if you can try different options before buying.

 

Imaging advice

As in calcium K imaging, it is better to image with a mono CCD or CMOS camera to get the best results.

Often early in the morning is the ideal time to image; before the heat of the day builds and makes the seeing conditions blur the finer details.

Always make sure you have a crisp focus by observing the limb or a high contrast feature like a sunspot or a dark filament. Exposure times can take a while to master in hydrogen-alpha, as too much can make the prominences on the limb stand out, but at the same time can wash out disc detail.

Conversely, too little exposure can give nice contrast on the disc, but then the prominences are underexposed. Capture separate images of the disc and prominences then combine them into a composite to get the best of both worlds.


Mark Townley is solar astrophotographer and expert on solar telescopes, outreach and all things concerning our host star.

This guide originally appeared in the August 2016 issue of BBC Sky at Night Magazine.
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