Solar filter
A sun filter is a special neutral density filter and an accessory part of an optical instrument that, when used correctly, makes it possible to safely observe the sun . The devices for observing the sun, in which solar filters are used, are mostly astronomical telescopes or binoculars , in surveying also theodolites and in navigation marine sextants .
In order to avoid damage to the eyes, the sun must never be observed without a suitable filter, which also applies to the free-eyed observation of solar eclipses . A telescope pointed at the sun should also never be left unattended.
General
In order to be able to observe the sun with a telescope or binoculars, you need a filter that reflects and absorbs most of the light so that the observer's eye is not damaged. Suitable solar filter films can be purchased from specialist retailers, which can be stretched in front of the opening of the telescope. It reduces the incident light to 0.001% (1 / 100,000). In addition to foils, lens solar filters made of (coated) glass are often used.
Forms of solar filters
Attachable solar filters are offered in the following forms:
- Lens sun filter made from a metal vapor-coated foil
- Lens solar filter made of an optically polished plane-parallel glass plate coated with chrome or aluminum
- Eyepiece sun filters made of plastic or glass: They are dangerous and unsuitable (see below).
- Herschel wedges , which direct a large part of the light into a trap, can be safely used in refractors in conjunction with a good gray filter.
Fold -out glass filters are used on flight or marine sextants , which are usually designed in two filter strengths (for hazy and bright sun).
Further forms of protection and observation options
In addition to using a filter, an additional option for attenuating the light is to use a Herschel wedge or penta prism near the focal point. In contrast to the filter film, however, the latter methods are only suitable in connection with an additional filter - or for projection purposes - due to insufficient attenuation. They are also significantly more expensive, but usually also deliver significantly better images. This is also a safe way to look at the sun .
With this method, in addition to sunspots , the granulation and sun flares are also visible . (Additional) color filters can be used to increase the contrast .
Protuberances , flares and other details are best visible with amateur astronomical means with an Hα filter. This filter only lets light in a narrow range around the wavelength of 656.28 nanometers through and is only used on refractors. The extremely expensive Hα solar telescopes are already equipped with a built-in filter by the manufacturer and can only be used for observing the sun.
Color and Optical Density
In the case of glass filters, a chrome layer creates a yellow-orange image of the sun, while an aluminum layer creates a blue-white image. In the case of foil filters, a blue and white image is also usually produced.
The optical density (OD) or extinction indicates what fraction of the incident sunlight passes through the filter. The optical density of the metal coatings can be varied depending on the application. As a rule, these are OD = 3.0 (1 / 1,000), OD = 3.8 (1 / 6,309) or OD = 4.0 (1 / 10,000) for photographic purposes or visually with an additional filter that determines the optical density of the entire system is reduced to OD = 5.0 (1 / 100,000), and OD = 5.0 (1 / 100,000) for visual applications without additional filters and also limited for photographic purposes.
Possible applications
A lens solar filter can be used with almost any type of telescope. The only exceptions are the Schiefspiegler according to Kutter or the Yolo .
Danger of blindness when replacing filters
Self-made filters (soot-blackened glass, rescue blankets, welding goggles and the like) are unsuitable because there is no control over whether the equally harmful but invisible ultraviolet and infrared components of sunlight are filtered out. If no solar filter is available, one should switch to an observation using an ocular projection , in which the observation time should, however, be kept short and interrupted by breaks, as parts of the optics can become very hot. Also, eyepiece solar filters, which can be included with inexpensive department store telescopes, must not be used for visual observation: Due to the heat generated, they can burst after a short observation time, which can result in irreparable, massive eye damage or even blindness within a fraction of a second.
Herschel wedges are the only filters that can be used on the eyepiece. They direct a large part of the light (around 95%) into a light trap. However, another neutral density filter must be used to ensure safe use. With some Herschel wedges, the light trap is not closed, so there is a risk, especially with children, of reaching into the trap or even looking into it. Therefore caution should be exercised when using this method. In addition, Herschel wedges may only be used with refractors, since this is an eyepiece filter and the secondary mirror would be unprotected and could be damaged in the case of mirror telescopes. Herschel wedges provide a very good and high-contrast image of the sun, which is not always guaranteed when using lens solar filters.
The only safe way to observe the sun through binoculars or telescope is to attach a suitable sun protection filter (foil, glass) in front of the opening of the telescope, never on the eyepiece . For reliable visual solar observation, the optical density must be at least OD = 5.0 (a maximum of one 100,000th of the sunlight is allowed through). The sun then still appears four times as bright as the moon .
Damaged lens filters must no longer be used as adequate protection is no longer guaranteed. Particular attention is paid to the filters made of solar filter film (scratches, holes, cracks, aging).
A viewfinder on the telescope must also be equipped with a solar filter in front of the opening for solar observation in order to avoid eye damage, burns or damage to the viewfinder (heat). If necessary, it must be sealed light-tight or dismantled.
In case of doubt, the advice of experts (e.g. opticians, astronomers) should be obtained before the observation.