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FILTER SAFETY

It is widely held in the public consciousness that there can be danger in observing the Sun. Before every eclipse, the media, professionals and others issue constant reminders of the need to protect ones eyes from Solar radiation, lest one do serious damage or, in extreme cases, go blind. These warnings are very valid and should be heeded meticulously. Unfortunately, some of the danger is insidious. Suppressing the visible light to an acceptable level is relatively easy. This can give a false sense of protection. Outside the visible spectrum, there is radiation that can be equally, if not more, damaging. Both the ultra-violet (UV) and the infra-red (IR) are problems to various parts of the eye from the cornea to the retina.

It is for the above reasons that Coronado Technology Group has always been, is, and always will be obsessed with the safety of its products. A proposed new product is not even assessed for marketability, price, manufacturability or technology until it has been fully proven that it can be made COMPLETELY safe for the user.

This is extremely important from another aspect. The astronomy products market is full of instruments and accessories from the beginners' models to the sophisticated, high priced instruments for the aficionados. In many cases, the less expensive products are simply reasonable copies of the better ones. It is this rich choice of instruments that allows so many people to take advantage of the wonderful hobby of astronomy. For the vast majority of these cases,- for telescopes, etc., used for nighttime observation, there are no consequences of safety. The only object in the sky that can damage the observer (apart from a miss-directed meteorite!), is the Sun. Consequently, the potential user of an instrument for Solar observation should take every precaution that it is safe before attempting to use it;- not just the first time, but EVERY time. One of the best ways to ensure this is to purchase the instrument from a reputable manufacturer who is able to provide all the necessary information and data relating to safety. Do not risk your eyes by buying 'knock-offs' as is prevalent in the rest of the astronomy field. The safety issues of filters are much more complicated than is obvious. Coronado Technology Group, through David Lunt's continual collaboration with outside ophthalmology experts, has spent over forty years dealing with these issues and can fairly say that they are the world's leading authority on the design and construction of Fabry-Perot etalon filters and their safety aspects, recognized as such by most of the professional institutions around the globe.

Coronado Technology Group can always provide customers and astronomers with any safety data they require, together with safety specifications that are accepted by eye safety organizations. DO NOT USE any instrumentation for which similar information is not available. You can always be absolutely at ease when using a Coronado product. As serious professionals and in our position as the leader in Solar instrumentation, we are also taking a prominent part in the establishment of standards for this rapidly growing section of the astronomy field;- standards to which all manufacturers will have to adhere for the benefit and peace of mind of the user.

From time to time, Coronado will issue new, updated data on safety issues, both on its web site and through dealers. This will include specifications for telescopes, filters and accessories. We will also provide information on questions and answers for this most important aspect of Solar observation so that the potential user can be satisfied that the correct information and specifications are being obtained.

Examples of Solar instrumentation and their specific safety issues

H-alpha Filters

Hydrogen alpha (or H-alpha) filters are specifically designed to isolate the principle wavelength of Hydrogen in the Solar spectrum, at 656.28nm, in order to provide observations of the Sun's chromosphere.

There are two main types of this filter. One is a front aperture filter (usually with some ancillary filters for the 'eye' end), which is mounted over the front of the telescope. Another type fits into the 'eye' end of the telescope, usually in the focusing tube. The latter will be provided with an additional energy rejection filter (ERF) that must be positioned at the entrance to the telescope. In order to see the chromosphere, in detail, the passband of these filters must be less than 0.1nm wide. All wavelengths, other than the H-alpha, must be suppressed to a level that is considered eye safe by ophthalmologists experienced in these matters;- laser safety, for instance. Although there are well-established levels of radiation safety and some countries have set guidelines through their national institutions, there are, as yet, no internationally established criteria to be used as standards. Coronado has initiated a programmed to establish such standards.

Typical levels that must be adhered to at various spectral ranges are as follows:

Ultra-Violet(250-400nm): Rejection of 1 x 10e-5 or <0.001%

Visible(400-700nm): Rejection of 5 x 10e-5 or <0.005%

Near Infra-Red(700-1500nm): Rejection of 1 x 10e-3 or <0.1%

Far Infra-Red(>1500): Rejection of 5 x 10e-3 or <0.5%

These values are recommended as minima. In reality and, as is the case with all Coronado filters & dedicated solar telescopes, the actual rejection should be and can be considerably lower. This is particularly important in the IR regions due to damage from these wavelengths being cumulative. The rejection in the visible region is, of course, defined by viewing comfort and this will vary with the observer. Actual rejection in this region for the average observer is ~1 x 10e-5 or ~0.001%. This, of course is an integrated value for the whole visible spectrum. In the case of an H-alpha filter, the transmission at the line is ~10% which, although seemingly high at first glance, is the value within the bandpass of the filter,- typically <0.1nm. However, this does mean that the rejection for the rest of the visible should be reduced accordingly to bring the average transmitted energy below the required level.

As has been mentioned, all Coronado filters exceed the above recommendations by a considerable amount and are perfectly safe to use at all times and over extended periods. Following, are examples of spectral transmission curves for Coronado filters. Although Coronado has the in-house capability of doing such testing and, in fact, keeps a constant check on all models, tests are also solicited from accredited, outside sources with the capability and expertise to perform such work, and provide additional knowledge of the parameters for eye safety. In this way, Coronado can always assure the user of the utmost credibility in these safety matters. It is for this reason that Coronado highly recommends that the potential purchaser of Solar instrumentation always demand such procedures and guarantees from any manufacturer or dealer.

As can be seen from the above graph of the transmission of a Solarmax filter, all radiation outside the H-alpha passband (very narrow, vertical line),- is within the noise of the instrument. (The IR detector is quite "noisy" compared with the UV - visible one). The actual values are as follows:

Coronado

Safe Levels

UV:

<0.00001%

<0.001%

Visible:

<0.0001% 

<0.005%

Near IR:  

<0.001%

<0.1%

Far IR: 

<0.001%

<0.5%

As can be seen, these values fall well below the required minima by a considerable factor, in most cases close to two orders of magnitude. The Data for the filter was independently provided by Dr. B. Ralph Chou of the University of Waterloo, in Canada;- an ophthalmologist and recognized authority on eye safety. (see Sky & Telescope,- February 1998, pp36-40)