Direct observations through filters

Paweł Rudawy, Dominik Gronkiewicz

More advanced solar observers could apply objective filters for observations of the Sun. How to use objective filters for solar observations?

WARNING. Solar observations with optical telescopes of any kind are potentially dangerous and could cause a high risk of severe inures. One should applied extraordinary safety measures and procedures, especially in order to avoid eye damages leading even to complete and unrecoverable blindness! All observers of the Sun in visual domain should strictly follow common safety rules. The observations of the Sun with objective filters should be applied only by skilled and experienced amateur astronomers.

WARNING. The Mylar foil filters as well as special solar glass filters should be installed over the front aperture of the telescope (in simply words: before the telescope, between the Sun and the telescope).

WARNING. Never look straight on the Sun through any telescope, binoculars or any other optical device, if they are fitted with some not tested and not certified “solar filters” or “dimmers”, especially if installed on the eyepieces. An abrupt break of the overheated filter is very likely, resulting in severe damage of the eye or even in an unrecoverable blindness. If you are not sure about safety of the filter you're using, ask filter's manufacturer about its suitability to solar observations.

WARNING. Never use home-made solar filters: smoked glass, photographic film, floppy disks etc. Even if the solar disk appears dim, infrared and ultraviolet radiation will irreversibly damage your retina causing blindness. Use only certified solar filters provided by astronomical shops. Welding mask filters are safe for naked-eye viewing (e.g. during eclipse or planetary transits) since they are opaque to ultraviolet radiation.

An enormous glare of the solar disk, if focused in a human eye, will cause very severe damage or complete and unrecoverable blindness instantly! It is because a brightness of the solar disk is hundreds of thousands of times higher that the brightness of the full Moon. The enormous glare of the solar disk could be efficiently dimmed using special Mylar foil filters or much more expensive glass filters, designed for installation over the front aperture of a telescope (before the main objective in refractors and before the main mirror in reflectors). While the transmittance of the Mylar foil filters (as well as of special solar glass filters) is very small, of the order of 0.0001-0.00001, they let through a fraction of a percent of the Sun's light inside the telescope only. The weakened light beam could forms an image of the Sun in exactly the same way as images of all other astronomical objects during the night observations and it could be contemplated by the observer through the average eyepiece. The optimal attenuation of the Mylar filter should be ND5 (it is 100 000 times) for visual observations and 3.5D (3 000 times) for photography.

If you're not certain about your filter's safety, the best way is to ask manufacturer. They will provide you with any safety informations and certificates.

Even further improvement can be reached by combining Mylar filter with a color filter, especially green or yellow. This extra filter will limit the effects of atmospheric turbulence on image as well as reduce or eliminate chromatic aberration in lens telescopes (refractors). However, if such filter is applied, that should be noted in observation report.

Of course, the Mylar filters are not ideal and posses flaws: the foil is not resistant to scratches (thus should be applied with a great care) as well as it should be installed in a frame to stay flat and to be easily attached to the telescope. Of course, the frame can be easily prepared using appropriate cap of a jar or a piece of cardboard tube. There are also dedicated foil mounting cells available for all popular sizes of amateur telescopes. Either way, foil should be attached to the cell using strong two-sided tape applied to entire perimeter of the cell. A plastic glue may also be used; however, some glues seem to work poorly with mylar filter causing it to detach after a few weeks. Solar heat causes the cell to expand slightly; this should be taken into account during assembly of the filter, it should be slightly loose in room temperature. A good idea to deal with thermal expansion of the cell is to assemble the filter during its natural working conditions, e.g. during hot sunny summer day. Solar foil should not be torn nor pinched or scratched. Any damage requires immediate replacement. A well prepared solar filter is visible in the picture.

Solar filter in painted cell

Despite the great comfort of the observations with the objective filter, an application of the Mylar or glass filter cancels also in instant all problems caused by overheating of the telescope by direct solar light. The Mylar foil is very cheap and could be purchased in any good store offering astronomical instruments and accessories.

Special solar glass filters are also available. They are slightly more expensive (due to necessary high quality of the filter). Glass filters are indeed more durable than Mylar foil. However, image quality has not been reported to be any better than using Mylar foil. One should be particularly careful when using glass filter for photographic measurements. While mylar foil seems to be approximately gray in the visible spectrum, many glass filter have their intrinsic color, therefore filter transmission characteristic must be known.

Solar telescopes

Dedicated solar telescopes available on the market have been getting cheaper over a few last years, placing them in financial reach of many amateur astronomers. They are equipped with narrowband filters that allow observations in strong spectral lines (usually H-alpha or calcium K line). Such telescopes (see picture) are usually assembled with filtration system inside the telescope, therefore no extra filters are needed.

H-alpha or Ca-II telescopes observe in very narrow spectral bands, therefore are able to image a higher layer of solar atmosphere that heat strongly during solar flares – chromosphere. If a big active region is visible from Earth, flares may be visible in Ha band even a few times a day. Their appearance may range from small brightening to large systems of ribbons, expanding and cooling for hours (see photo). Thus, one gets much higher chance of success than during white light observations. However, the price of H-alpha telescopes (which is mostly the price of the Ha filter) is much higher than a regular telescope.

Another advantage of H-alpha telescopes is that observations of dynamic phenomena are not limited to flares, one can also register evolution of prominences – a giant clouds of cool plasma – including prominence eruptions. Lucky observer may see the rising of the prominence followed by a flare.

H-alpha telescopes allow advanced observations

IMPORTANT NOTE. Calcium K line is centered at 393.3 nm, which falls into near ultraviolet part of spectrum. Not only we are blind for this wavelength, but also ultraviolet is highly unsafe for human eye, so observations with such filter can ONLY be performed via photographic means.

Most advanced H-alpha filters may be purchased separately to be installed in any amateur telescope. One must remember, that typical narrowband filtration system consists of two parts: energy rejection filter (ERF) that partially dims solar light and the proper H-alpha interference filter. ERF or H-alpha filter alone will not be sufficient to prevent your eye from excess of light.

NOTE. In any of the cases mentioned above, if you have concerns about safety of the configuration you're about to use to observe the Sun, contact telescope's or filter's manufacturer and list in details all the elements of your setup. They should you provide your with safety information.