Binoculars and telescopes at the eclipse: filter on the front, never on the eyepiece

With binoculars or a telescope, the solar filter goes over the front lens, firmly attached, and on the telescope’s finder as well. Never on the eyepiece, and never wear eclipse glasses while you look through the instrument. It comes off only during totality and goes back on before totality ends: in Tarifa, off at 10:45:11 and back on before 10:49:38, the end time given by the IGN (Spain’s National Geographic Institute); in Málaga, between 10:48:08 and 10:49:51. All times are Spanish time (CEST, UTC+2).

Why on the front and not behind

Binoculars and telescopes gather far more light than the eye and concentrate it. The IGN compares it to a magnifying glass setting a piece of paper alight: looking at the Sun through an instrument can cause instant, irreparable damage, and that applies to the partly eclipsed Sun just as much as to the whole Sun.

The AAS (American Astronomical Society) gives two reasons for putting the filter on the front: it protects the instrument itself from the light and heat, and it stops anyone from looking through it unfiltered by mistake. A filter fitted on the eyepiece receives all that light after it has been concentrated. The glass filters that came with some cheap telescopes, screwed into the eyepiece, often crack from the heat without warning, and Ralph Chou’s eye-safety report for the AAS says the retina can be damaged before you have time to pull your eye away. His advice is to throw them out.

Eclipse glasses and binoculars, never together

Wearing eclipse glasses while you look through unfiltered binoculars won’t protect you: the light concentrated by the instrument can burn through the filter in the glasses and reach your eye. NASA, the AAS and the American Academy of Ophthalmology all warn about this, and the Trío de Eclipses website, the official eclipse site of Spain’s Ministry of Science, calls it “the fatal mistake”.

With a proper solar filter on the instrument, on the other hand, NASA explains that you don’t need glasses as well, because the instrument’s filter already does that job. Glasses are for naked-eye viewing, and the instrument’s filter has to be made for the purpose: Chou insists that no filter should be used on an instrument unless it was designed for it and fits over the front lens.

The finder, the fitting and the type of filter

If the telescope has a finder, the small scope mounted on top for aiming, the AAS says to cap it, remove it or give it its own filter, just like the main tube. Binoculars have two front lenses, so they need two filters, one on each side.

The filter has to be secure, so that a knock or a gust of wind won’t send it flying, but not so tight that you can’t remove it easily when totality begins. That’s why, with a telephoto lens inside the path of totality, the AAS advises against screw-on filters, which are hard to take off and put back quickly without dropping them. Magnetic ones are faster, although you may not find one that fits your lens.

There are glass filters with a metallic coating, aluminised polyester film filters and black polymer filters. According to the AAS, all three work; they differ in the colour they give the Sun and in price. Instruments with a built-in filter are fine for the partial phases, but if the filter can’t be removed they are no use for totality.

During totality: filter off, and back on in time

According to Chou’s report, the totally eclipsed Sun can be viewed through an instrument without a filter, as long as you take care not to be looking when totality ends and the light returns. The AAS adds that magnifying instruments follow different rules from naked-eye viewing and recommends asking an astronomer beforehand.

NASA describes the cue for covering it again: at the end of totality, the edge opposite the diamond ring you saw at the start begins to brighten, and that is the signal to stop looking directly. The filter must be back on before the first flash. Each town has two times for the end, ours and the IGN’s, and the IGN’s comes first in all 105 towns in the path. For taking the filter off, use the later of the two start times:

Mainland Spanish time. The naked-eye signs and the full table are in when to take off your eclipse glasses during totality.

If your totality is short, think twice before taking filters on and off. At the edge of the path some towns get very little time, such as Rubite, a village in Granada province, with 11 s according to the IGN. The AAS recommends that anyone seeing their first total eclipse should not try to photograph or film it and should simply watch.

No filter for the instrument? Project the image

If you don’t have a filter made for your binoculars or telescope, the IGN suggests projecting the Sun’s image onto a screen. That gives a brighter image than a pinhole in a piece of card, with two extra dangers: the instrument, and above all the eyepiece, heats up and can be damaged, so you have to let it cool down every few minutes; and someone, perhaps a child, may lean in and look through it. That’s why the IGN advises putting the screen on the ground, at right angles to the beam of light.

To look with the naked eye during the partial phases you need EN ISO 12312-2 glasses. On Order your glasses you can ask for ours: cardboard, with the EN ISO 12312-2 certificate, along with the phone filter and packs for groups. We reply with the price and the delivery time.

Sources: AAS — Solar Filters for Optics: Telescopes, Binoculars & Cameras (the filter always on the front; finder capped or filtered; fitting; screw-on or magnetic filters; dangerous eyepiece filters; types of material; built-in filters); AAS — How to View a Solar Eclipse Safely (no eclipse glasses with an unfiltered instrument; with magnification, ask an astronomer); NASA — Eclipse Viewing Safety (instant damage without a filter on the front; with the filter on the instrument, no glasses needed); NASA — What to Expect (the cue that totality is ending); B. Ralph Chou — Solar Eclipse Eye Safety, technical report published by the AAS (eyepiece filters that crack; only filters made for the instrument, on the front lens; totality without a filter, with care at the end); AAS — Imaging & Video (at your first total eclipse, better just to watch); IGN — Cómo observar los eclipses (in Spanish: the instrument as a magnifying glass; projection with binoculars or a telescope and its dangers); Trío de Eclipses — Salud visual (in Spanish: glasses behind the eyepiece, “the fatal mistake”); American Academy of Ophthalmology — Solar Eclipse Eye Safety; IGN durations and contact times: eclipseando data (skyfield engine + DE440s) checked against the IGN.