A solar filter with optical density 5, covering the whole objective lens from the front (on binoculars, one on each side), chosen to match the diameter of your tube, firmly attached and with the finder covered. The “photographic” density of 3.8 is not for viewing: its own maker says so. You’ll have it on for more than two hours: in Málaga, from 09:42:09 to 12:02:35 (Spanish time, CEST, UTC+2), with 1 min 49 s of totality in the middle, the only time it comes off. Where the filter goes and why is in binoculars and telescopes during the eclipse.
Density 5 for viewing; 3.8 for the camera only
Optical density tells you how much light the filter stops. The density 5 film from Baader, one of the makers of this material, “reduces the intensity of incident sunlight by a factor of 100,000”. The same maker sells another film with density 3.8 and describes it on its page like this: “Not suitable for visual observation”, “imaging only” and “only intended to be used by well-experienced Astro-imagers”. The manual for its mounted filters repeats it: the 3.8 ones “must not be used for visual observation, permanent eye-damage may occur”. The same manual sums it up: density 5 is for viewing and for photography; 3.8 is for photography only.
What to check when you buy: that the filter states its density and that it is suitable for visual observation. If it says “photo”, “imaging” or 3.8, it isn’t for your eye.
The glasses standard isn’t the filter standard
Eclipse glasses are checked against EN ISO 12312-2, but that standard doesn’t cover filters for instruments. The AAS (American Astronomical Society) explains: “The ISO 12312-2 international standard does not apply to solar filters for optics”. Even so, those filters can be considered safe if they meet that standard’s requirements for transmission, uniformity and the quality of the material and the surface, if they are firmly attached to the front of the instrument and if they are used as their supplier directs. The AAS adds that only experienced observers should use them.
Baader says the same about its film, with one more warning: it is not approved “for naked eye solar observation or production of solar viewers”. So it is neither for looking at the Sun with the naked eye nor for making your own glasses. For viewing without an instrument, use EN ISO 12312-2 glasses or a viewer, which you can check with our guide to certified glasses.
Film or glass
Glass with a metallic coating, aluminised polyester film or black polymer: according to the AAS, all three work. Film, Baader explains, takes knocks better than glass, but its coating is ruined if it gets scratched or rubbed, and a filter with damaged film “should be destroyed immediately”, so that nobody uses it by mistake. And watch out for imitations: B. Ralph Chou’s eye-safety text on NASA’s eclipse website calls for aluminised film “manufactured specifically for solar observation” and rules out “space blankets” and gardening film.
The size: your tube’s
A full-aperture filter covers the whole objective, not part of it. To know which one fits, you need to measure two things: the diameter of the objective lens, which is the aperture, and the diameter of the tube it will be attached to. In Baader’s manual, each filter states the aperture it is designed for and its clamping range, which adjusts “to fit the inner or outer tube diameter”.
On binoculars it only attaches from the outside: Baader’s binocular filters “cannot be clamped to binoculars from the inside”, because the lenses sit too close to the edge. If you make your own holder from loose film, the same rule applies: it must cover the whole objective and leave no gaps.
Firmly attached, against knocks and wind
The AAS asks for the filter to be attached so that it won’t come off “if your instrument is bumped or the wind suddenly gusts”. Baader asks you to check the fit before each session and, if necessary, secure it with tape. And in the manual for its mounted filters it warns that a strong gust or a loose screw can dislodge even a well-fitted filter, which is why they come with safety straps. Inside the path, the filter also comes off when totality begins and goes back on before it ends; the exact second is in when to take off eclipse glasses in totality.
Binoculars: two filters. Telescope: cover the finder
Binoculars have two objectives, and Baader reminds you to “be sure both objectives are securely covered”. On a telescope, the finder (the small scope used for aiming) needs covering too: “Unprotected viewing through the finder would have the same catastrophic consequences as viewing through the main telescope”, and pointed at the Sun it can burn your skin. The AAS gives three options: cap it, remove it or give it its own filter.
Before each use, and with children around
Baader asks you to check the film for damage or scratches before each session, and never to leave the telescope unattended outdoors in daylight, so that nobody, least of all a child, looks through it without protection. It also states that its solar products are not intended for children under 14.
For setting up the instrument: in Tarifa, at the time of maximum eclipse (10:47:25), the Sun will be 38.1° high and to the east (azimuth 95°), according to our calculations, so find a spot with a clear view on that side. To photograph it, the camera has its own rules: solar filters for cameras and phones.
For naked-eye viewing during the partial phases, you need 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: Baader — AstroSolar Photo Film OD 3.8 (not suitable for viewing; photography only); Baader — AstroSolar Safety Film 5.0 instructions (PDF) (100,000 times less light; both objectives; finder covered; fitting and tape; damaged film destroyed; not for the naked eye or for making glasses; the glasses standard doesn’t apply; never leave the telescope unattended); Baader — Instruction Manual: Baader Solar Filters (PDF) (5 for viewing, 3.8 never; aperture and clamping range; binoculars from the outside; wind and straps; children under 14); AAS — Solar Filters for Optics (types of filter; attachment; finder); AAS — Suppliers of Safe Solar Viewers & Filters (ISO 12312-2 and filters for optics); B. Ralph Chou — Eye Safety During Solar Eclipses, on NASA’s eclipse website (film made for the Sun; no space blankets); times, altitude and azimuth of the Sun: eclipseando data (skyfield engine + DE440s); totality duration in Málaga: IGN.
If you would like something to keep from the eclipse, the shop prints T-shirts, mugs and posters with the name of your town, the time of its maximum and its seconds of totality. See the merchandise →
Remember: never look at the Sun without proper eye protection (filters made to EN ISO 12312-2). The official reference for safe viewing in Spain is the IGN (in Spanish): eclipses.ign.es.