Ex­plainer

Our near­est star can pro­vide some of the most out­stand­ing astro­nom­i­cal views

Sky at Night Magazine - - CONTENTS - Lyn Smith is di­rec­tor of the Bri­tish Astro­nom­i­cal As­so­ci­a­tions So­lar Sec­tion. The BAA’s web­site is up­dated reg­u­larly with images of so­lar ac­tiv­ity. For more de­tails visit www.britas­tro.org

As­tron­omy is some­thing you usu­ally as­so­ciate with the dark, so the Sun can be eas­ily over­looked as an astro­nom­i­cal ob­ject for ob­ser­va­tion. How­ever, with the right equip­ment and a sim­ple safety rou­tine, so­lar ob­serv­ing can be one of the most visu­ally re­ward­ing ob­ser­va­tion projects in am­a­teur as­tron­omy.

One of the eas­i­est things to view is the sunspots on the Sun’s vis­i­ble layer, the pho­to­sphere. It’s im­por­tant to never look di­rectly at the Sun through a tele­scope di­rectly, as you can dam­age your eye­sight. Like­wise, you should never look di­rectly at the Sun with sun­glasses or dark­ened glass as these do not block out the most harm­ful wave­lengths of light.

How­ever, you can safely project the Sun’s im­age through a tele­scope. It’s usu­ally a re­frac­tor as the in­tense heat can dam­age some other types of tele­scope. With the dust cap on your tele­scope to stop harm­ful stray light, place a square of white card be­hind the tele­scope eye­piece and ad­just the scope un­til the in­stru­ment’s shadow is rounded on the card; only then re­move the dust cap and the Sun’s im­age

should be pro­jected onto the card. If not, ad­just the tele­scope slightly un­til it ap­pears but re­mem­ber to never look down the eye­piece di­rectly.

Us­ing so­lar flters

White light ob­serv­ing is gen­er­ally the study of the so­lar pho­to­sphere where dark sunspot groups are found and bright patches called “fac­u­lae” are seen more clearly near the so­lar limb. As an al­ter­na­tive to so­lar pro­jec­tion, you can also equip your tele­scope with a glass white light flter which will al­low you to look di­rectly through your tele­scope at the pho­to­sphere to see sunspots in fne de­tail. Such flters can be pur­chased from any rep­utable as­tron­omy re­tailer and ft on the ob­jec­tive end of your tele­scope. If cost is an is­sue you can make your own, though it’s vi­tal to use cer­tifed so­lar safety flm and make sure the fnal flter has no holes or gaps in it.

How­ever, it is the so­lar at­mos­phere or chro­mo­sphere where the real ac­tion takes place. This can be seen by us­ing a spe­cial so­lar flter which only al­lows light through to­wards the red end of the spec­trum known

as a hy­dro­gen/al­pha flter. Here you can see so­lar promi­nences, fla­ments, plage and fares.

So­lar promi­nences ex­tend from the so­lar limb out into the dark­ness of space and are vast emis­sions of hy­dro­gen trav­el­ling along mag­netic feld lines. Some­times, the feld lines can loop back down into the chro­mo­sphere or break, eject­ing plasma into space. Promi­nences can be short- or long-lived fea­tures but the re­ally fas­ci­nat­ing ones are the short-lived erup­tive promi­nences that can al­most grow and twist as you watch them in real time. Dark fla­ments can also be seen in this wave­length, streaked across the so­lar disk. These are ac­tu­ally promi­nences but seen as dark fea­tures across the bright so­lar disk rather than bright fea­tures seen against the dark back­ground of space hence an­other name to dif­fer­en­ti­ate them.

The only down side is that hy­dro­gen/al­pha flters can be very ex­pen­sive to pur­chase. You can either buy a spe­cial­ist so­lar tele­scope, or a kit to ft your ex­ist­ing tele­scope. These have both an en­ergy re­jec­tion flter that will re­duce the heat en­ter­ing your tele­scope and a block­ing flter for the eye­piece. Coron­ado and Daystar pro­duce the cheap­est hy­dro­gen/al­pha tele­scopes, around the £700–£800 mark.

A re­frac­tor can be used for pro­ject­ing the Sun’s im­age onto a white card

A hy­dro­gen/al­pha flter al­lows you to en­joy amaz­ing views of so­lar fea­tures in the chro­mo­sphere

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