29Jul2015

How To Observe The Sun Safely

 Pete Lawrence A white light filter reveals the majesty of our star without putting your eyesight at risk


A white light filter reveals the majesty of our star without putting your eyesight at risk. Cr. Pete Lawrence

The Sun is a very gratifying object to observe. It is dynamic, with features on its visible surface that sometimes appear to change over the course of just a few hours. Most people have never seen this sort of thing.

It is also rather convenient in that you can only observe the Sun during the day when it is generally warm – this is the comfortable side of astronomy!  Being so close, our star offers us a unique opportunity to study a stellar body in detail. However, this closeness also carries danger with it, so you need to be very careful when attempting to view the Sun through a telescope.

There are various ways to do this, but the safest is to fit a full-aperture white light filter over the front end of your telescope tube. The 
term ‘white light’ means that you’ll see 
the Sun as it is normally, but filtered 
and greatly dimmed to protect your 
eyes. The resulting view has good 
contrast and neutral colour.

These filters can be bought ready-made, but they are relatively simple to make yourself using sheets of solar film cut 
to size. Baader Planetarium’s AstroSolar film is available in two grades: OD 3.8 
is for imaging only, while OD 5.0 is suitable for visual observing and imaging. OD stands for ‘optical density’ – the 
higher the number the dimmer the 
image. Thousand Oaks Optical also 
supply solar film in sheets.

Creating the filter will take about an hour. In addition to the solar film, you’ll also need some thin card, sticky tape 
and double-sided tape. Once built, 
your solar filter will be able to convert 
a regular astronomical telescope into 
one suitable for white-light solar 
viewing. It’s worth checking the 
filter for pinprick holes and tears each 
time you’re about to fit it. To do this, simply hold it up to the Sun and inspect 
it visually. If you find any, discard the 
filter and make a new one.

When you use the filter, it’s important 
to remove or cap your telescope’s finder. This prevents it from being damaged by the Sun’s intense rays and removes the urge to look through it to line up the 
main instrument! Always make sure 
the telescope is pointing away from the Sun before fitting the filter. When you’re done observing, do the same – aim the telescope away from the Sun before removing it.

Keep it covered
If your telescope aperture is too big to entirely cover with solar film, you can 
use a mask made from stiff card to cover over it; then cut a smaller hole in this mask and cover that with solar film. Make sure that the mask fits over the entire aperture and that no light can leak around its edges. For telescopes with a central obstruction, 
such as reflectors or Schmidt-Cassegrains, cut the aperture hole off centre so the secondary mirror doesn’t block it.

Once the filter’s fitted, you’re ready to view the beauty of the white light Sun. With it, you’ll see dark sunspots and bright faculae embedded within the shaded edges of the Sun’s disc, a real 
effect known as limb darkening. Sunspots generally occur in groups or active 
regions. A typical sunspot has a dark 
inner area, the umbra, surrounded by 
a lighter one, the penumbra.

The visible surface of the Sun is called the photosphere. A 6-inch telescope should reveal it as a fine, rice-grain pattern called 
solar granulation. This represents the tops of vast energy-transferring convective 
cells working beneath the Sun’s surface.

Keeping a daily record of the Sun’s activity is a great way to create a connection with our nearest star. Over 
the course of a few days, you’ll start to reveal the dynamic changing nature of 
its ‘surface’ features and reveal just how these features appear to rotate across 
the Sun’s disc. Click here to see our step-by-step guide.

WARNING: Never look directly at the Sun with the naked eye or any unfiltered optical instruments


Tools and materials

  • Card: An A2 sheet of thin, bendable 
card cut into 50mm-wide strips forms the filter’s slip-on wall.
  • Ruler and pencil: Essential to enure you mark out the card and solar film to the correct size, making the filter safe to use.
  • Scissors: Make sure they are sharp so you don’t damage the solar film as you cut.
  • Sticky Tape: Both normal and double-sided tape are invaluable for easy construction.
  • Solar Film: The main filter material is generally available in A4 sheets, but is sometimes available in larger rolls.

Find out more
For solar film, check out www.baader-planetarium.com/sofifolie/sofi_start_e.htm and www.thousandoaksoptical.com.

For daily updates of what the white light Sun looks light and notification of important activity, visit www.spaceweather.com.  Source: Skyat Night

 

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