Safe viewing of the Transit Safe viewing of the Transit of Venus of Venus

Biman BasuBiman Basubimanbasu@gmail.com

1

Global visibility of ToV Global visibility of ToV 5/6.6.20125/6.6.2012

2

Unlike the transit in 2004, this year’s transit will begin long before sunrise in India. So only part of the transit will be visible.

Why the Sun is dangerous to Why the Sun is dangerous to look atlook at

• The Sun is an extremely hot and bright object.

• It emits 40% visible light; 58% infrared (heat) radiation; and 2% ultraviolet (UV) radiation.

• The visible light that provides us with colour vision represents just a small part of the electromagnetic spectrum.

• The UV and infrared radiations are invisible to the human eye, but can have dangerous effects on the retina.

3

The Sun in many coloursThe Sun in many colours

Sun in visible light

Sun in infrared

Sun in ultraviolet

4

The human eyeThe human eye

5

Structure of the retinaStructure of the retina

6

The risk of directly viewing the The risk of directly viewing the SunSun

7

Sun induced damage to the Sun induced damage to the eyeeye

• After just one direct viewing of the Sun, a person can attain a retinal ‘burn’ creating a blind spot known as ‘solar retinopathy‘, which often results in impairment or loss of vision.

• Solar retinopathy is damage to the eye's retina, particularly the macula, which is the region of keenest vision, from prolonged exposure to solar radiation.

• It usually occurs due to staring at the Sun, or watching a solar eclipse or a planetary transit without eye protection.

8

Solar retinopathySolar retinopathy

9

How the Sun damages the How the Sun damages the retinaretina

• Exposure of the retina to direct sunlight triggers a series of complex chemical reactions within the cells which damages their ability to respond to a visual stimulus, and in extreme cases, can destroy them.

• This photochemical retinal damage may be accompanied by a thermal injury - the high level of visible and near-infrared radiation causes heating that literally cooks the exposed tissue.

• This thermal injury or photocoagulation destroys the rods and cones, creating a small blind area.

10

Retinal damage is painlessRetinal damage is painless• Depending on the severity of the damage, an

affected observer may experience either a temporary or permanent loss of vision.

• The important thing to remember is that both photochemical and thermal retinal injuries occur without the victim's knowledge, because there are no pain receptors in the retina, and the visual effects do not occur for at least several hours after the damage is done.

• So it is always safer to take protection to avoid damage to the retina.

11

Damage to the corneaDamage to the cornea

• Staring directly at the Sun can also damages the cornea, leading to a condition known as ‘photokeratitis’, which is a painful inflammation of the cornea.

• Typically, the lens of the adult human eye absorbs some UV rays to protect the eye.

• But, prolonged exposure to the Sun overwhelms these natural defences, often leading to photokeratitis.

• This condition usually heals on its own in about a week.

12

PhotokeratitisPhotokeratitis

13

Safe viewing of the transit of Safe viewing of the transit of VenusVenus

• A safe solar filter should transmit 0% of the harmful UV radiation (315 and 380 nm), less than 0.003% of visible light (380 to 780 nm), and no more than 0.5% of the near-infrared radiation (780 to 1400 nm).

• Filters made from certified aluminised Mylar sheets, black polymer sheets, or dark welder’s glass (no. 14) are safe for viewing the progress of the transit.

• Use of pinhole camera or small mirror, although good for viewing solar eclipses, may produce fuzzy image of the Sun in which Venus may not be clearly visible.

14

Unsafe solar filtersUnsafe solar filters

• Unsafe solar filters include all colour films, black-and-white film that contains no silver, photographic negatives with images on them (x-rays and snapshots), smoked glass, sunglasses (single or multiple pairs), photographic neutral density filters used in photography, and polarising filters.

• Most of these transmit high levels of invisible infrared radiation which can cause a thermal retinal burn

15

Spectral response of solar Spectral response of solar filtersfilters

16

Safe solar filtersSafe solar filters

17

CAUTION! Never put on the eclipse goggles and then look

through binoculars or a telescope!

Watching through a Watching through a telescope telescope

When watching the transit through a telescope, the Mylar filter must be fixed in front of the objective. Never use "eyepiece" solar filters!

18

The projection methodThe projection method

The best way for amateur astronomers to observe the position of Venus during transit is to project the image of the Sun on a white card using a small telescope. CAUTION! NEVER LOOK AT THE SUN DIRECTLY!

19

DON’Ts while observing transit DON’Ts while observing transit of Venusof Venus

• Don’t attempt to observe the Sun with naked eyes.

• Never look at the Sun through a telescope or binocular without a proper filter, placed in front of the objective; not behind the eyepiece.

• Don’t use smoked glass, colour film, sunglasses, exposed black-and-white film, or polarising filters, as they are not safe.

• Don’t look at a reflection of the Sun from water, even coloured water.

20

21

Watch the transit safely!Watch the transit safely!

It’s a once-in-a-lifetime It’s a once-in-a-lifetime opportunity to see one of opportunity to see one of

Nature’s rarest spectacles.Nature’s rarest spectacles.

HAPPY VIEWING!HAPPY VIEWING!

Thank You!Thank You!

22