venerdì 18 maggio 2012

Annular Solar Eclipse of May 21 2012: how to observe and take pictures

Path of the eclipse taken from NASAwebsite. 
Next Monday, 21 May 2012 there will be a annular solar eclipse visible from Japan, part of China and USA. It will be the first of a few notable astronomical events and will be followed by the passage of Venus (link in italian) in front of our star on the 6th of June. 

Time and locations for observation: in Tokyo - for instance -  it will start at 6:19:02, reach maximum at 7:34:30 ending at 9:02:37. For time and location look here or on the NASA website (be careful that NASA times are in UT/Greenwich time and therefore the day is 20 may on those charts). Also this site has a lot of information.

What is an annular solar eclipse? It is an almost total eclipse, leaving only a ring of light on the borders. It is not total because the eccentricity of Moon's orbit around the Earth that made the so-called supermoon (link in italian) a few days ago  makes it this time slightly smaller than the Sun. 

How to look at it? Never look at the Sun without protection. Sunglasses are not enough: buy the correct filters from the shops. Never look at it through a telescope even with filter protection: there is too much light. If you do not have any filters use a mirror to project the light of the Sun inside the house and look at it on the wall. 

How can I take pictures at the Sun?  Never look at the Sun directly through a digital camera: the entrance pupil of the camera can be larger than the eyes and gather more light, like a telescope. If it is a compact camera you can look at it through the digital display, if it is a reflex never look at it in the mirror. 
Images of the Sun taken with a Canon SX30IS with CHDK 
software 1/3200s f11.  A plane crosses from right
to left. Below, on the left you can see sunspot 1476

OK, but I asked how to take pictures at the Sun...  Buy a ND (neutral density filter), at least ND 128. Better buy two and stack them if necessary. Be warned that the camera can be damaged if too much light comes through it. 
Use the shortest aperture available (1/1000th second or less) and smallest aperture (large number) possible to decrease the light.
If you do not have a ND filter you can use the glasses you get from the 3D movies. If they are of the polarizing filter variety you can superimpose them (breaking the glasses) and rotate the two filters to achieve maximum darkness. 

But I have no camera filters! If  you have a Canon camera you can dowload CHDK  an amazing software that allows to expand the possibilities of many cameras. You just copy the correct version on your SD card and you can - for instance - reach an aperture of 1/100000 of a second. With CHDK you can shoot at the sun even without filter (but the camera can still overheat by being exposed at the Sun. 

In all cases experiment in the next days well in advance, do not try to take the first pictures at the Sun during the eclipse or you will spoil the event and likely get not so good results. 
Images of the Sun with UV filter + polarizing filter
1/3200s f11. Note the many sunspots on the surface