make a white light fil­ter

Sky at Night Magazine - - SOLAR OBSERVING -

One of the safest ways to view the Sun is to fit a white light fil­ter over the front of your tele­scope. The re­sult­ing view has good con­trast and neu­tral colour. These fil­ters are rel­a­tively sim­ple to make us­ing sheets of so­lar film cut to size. Baader’s AstroSo­lar Safety 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’, with higher num­bers giv­ing you dim­mer im­ages. Check your fil­ter for holes and tears each time you’re about to fit it. If you find any, dis­card the fil­ter and make a new one.

It’s im­por­tant to also 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.

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.

White light fil­ters let you gaze upon the Sun’s pho­to­sphere di­rectly

Is your aper­ture too big for so­lar film? Cover it in card with a smaller, film-masked hole in­stead

It’s im­per­a­tive that you take care when seal­ing the fil­ter in place so no light can get in

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