5. Solar Filters
5. SOLAR FILTERS
Solar filters come in a variety of types. This is also an area where EXTREME CAUTION is required. One mistake could mean that your precious eyesight is damaged PERMANENTLY. So, unless your are knowledgeable about subject then FORGET IT UNTIL YOU HAVE LEARNT WHAT TO DO FROM AN EXPERT OBSERVER. It is the policy of the SPA Solar Section to use the projection method if you are new to solar observing. This chapter should only be read by persons who are more experienced in solar observing and are aware of the risks. Children should always be supervised when undertaking an observation, as they don’t see the same risk adults do.
The point of a solar filter is to enable you to look at the Sun directly through the telescope. The filter has to be capable of several things. It has to get rid of the Infra-Red (IR) radiation emitted by the Sun, reduce the amount of visible light to an acceptable level and must be securely mounted on the telescope without any risk of being displaced or falling off. The filter must also have the ability not to be damaged easily by handling or by the Sun’s rays (bleaching by ultra-violet (UV)). Just because the item is opaque to the visible part of the electromagnetic spectrum (the part of the spectrum we can see with our eyes) does not mean that the infra-red is stopped as well, often quite the reverse! The main property of the filter is reflection, using a surface which is almost mirror shiny will reflect most of the light and heat but allow a tiny proportion through, which will be safe to eyesight. Some of the heat is absorbed by the filter as well.
OBJECTIVE END FILTERS
Objective End Filters are made so it stops harmful light and heat before entering the telescope, so is safe to eyesight, and secondly, equipment.
Mylar is a type of aluminised plastic film. The aluminium coating being reflective, allowing safe viewing. The best of this type of filter is if the film is aluminised on both sides, with this coating reflecting light, UV and Infra-Red and let through a small amount in the blue part of the spectrum, hence the Sun’s image will not be yellow, but appear electric blue in colour. This plastic film is quite tough, but the coating may deteriorate in not handled correctly. As the coating is on the outside, fingerprints, scuffs and scrapes can take off the coating, which will allow more of the more dangerous IR and UV through. If mounted correctly on a board or other device of your choice it can give good views of the solar surface. It is quite cheap. But you must read all of the instructions which may have come with the filter material. Sometimes you may getting some Mylar (especially single coated Mylar) which may require you to use two single sheets (one on top of the other) to bring down the viewing to acceptable and safe levels. Check the Mylar beforehand and ensure there are none of the handling defects mentioned before, and look for ‘pinholes’, tiny holes in the aluminium coating. If there are a lot then DO NOT USE IT, and substitute another fresh piece of Mylar onto the equipment. Making a mask over the front of your telescope with a hole smaller then the diameter of your objective lens would be of value as it would cut down the amount of light entering the telescope. Your can change the colour of the solar image by using a yellow optical filter, and you can use your good eyepieces without the risk of damage, i.e., Othoscopic or Possl. Unfortunately it is not recommended to clean Mylar directly as it may degrade the coating.
Baader Astro Solar film is another filtering material which can be bought readily from good Astronomy shops. It is not Mylar as such, but is coated on both sides. It is used by many solar observers. It gives a whiteish colour and is very safe if handled correctly. It normally comes in A4 sheets but larger sheets are available. A lot of manufactured solar filters use this material.
Inconel Filters are factory made glass objective lens filters, and come in various types. The most well know maker of these filters are Thousand Oaks Optical in California, USA. These are reflective filters, the coating made on flat glass plates, normally inside aluminium fitting for your telescope. They have a nickel-chromium coating on one side which is possibly strengthened with stainless steel to make the coating less likely to be damaged. There are 3 types, the first two (Type 1 and Type 2) are used for visual and photographic, while Type 3 is only used for photography and never used for visual work. The image colour is yellow-orange, giving the appearance of a more natural looking Sun. These can still be damaged by rough handing, so heed the warnings given by the manufacturer instructions.
Using an objective end filter may be better if you have a telescope which has more plastic in its construction and therefore less likely to be damaged if used for solar projection. If you have a Maksutov/Schimdt-Cassegain or other compound telescope then an objective end filter is a must!
Any filter which allows infra-red and/or Ultra-Violet radiation through is DANGEROUS!
Eyepiece held Solar filters. In the 1960’s /1970’s and 1980’s you could very easily buy a telescope, mainly made in Japan, which varied in optical quality and the amount of accessories you could get with it. They came in all forms of names but were principally the same telescope, and they all came with the same accessories. One of these accessories was the Sun filter (and was normally marked SUN on the ring). This was a small round disc of very opaque glass in a metal filter ring which screwed into the rear of an eyepiece. By placing the eyepiece in the focusing mount you could then view the solar disk directly. However, they appeared alright, but no one, especially those new to Astronomy; use to think what was really happening. The heat and light was absorbed by the dark glass, and the viewing levels appeared to be safe, but the heat did not go away, it was just heating up the filter! As it was near the focus point the levels of heat is very high indeed. Glass will expand slightly when heated and if the metal filter ring was a fairly tight fitting the glass expanded, took up all the available room, expanded further, deformed and then cracked or shattered, which will then allow unfiltered light to enter the eye, which will be damaged beyond all help!
These filters are, in some cases, still being sold with some telescopes. They are extremely dangerous and must not, under any circumstances, be used by solar observers. They will crack or shatter if used.
Polaroid. This is material which will darken if two pieces are placed in line of vision and rotated. It is common in sunglasses because of the ability to considerably reduce glare. This material will only darken visible light. It will not stop infra-red. So making a solar filter with this material or even looking through darkened Polaroid glasses with the naked eye at the Sun is dangerous.
Non-specific manufactured filters. There have been instances were a ‘trader’ started selling a solar filter which was nothing more than a piece of yellow plastic. It brought the visible light down, but did not stop infra-red or ultra-violet and was optically useless. Nevertheless, they were being sold for around £40, and people did buy them. One was tested with a infra-red television remote control and it changed channels on the TV (through the filter) at 20 feet! Only buy solar filtering material or equipment from a reputable astronomy equipment dealer. If you have any suspicions regarding the safety of a filter DO NOT USE IT! Send it to the Director for testing and evaluation.
Herschel Wedge (Solar Diagonal). This was a piece of equipment which looked very much like a Star Diagonal (90 degree prism) with the exception of having a mirror or prism to reflect the image into an eyepiece, it has a piece of wedge shaped glass (to prevent reflections) which is unsilvered, and the housing has a large hole which allows most of the solar image to pass through into the outside. They were common as a Japanese telescope accessory in the 60’s and 70’s. This wedge only allows around 5 per cent of the light to be reflected up to the eyepiece where a neutral density filter brings down the light to acceptable levels. Most of the heat is sent directly out of the telescope. These should not be used without a filter near the eyepiece (NOT a solar filter mentioned above, they would be too opaque). You can buy specially made versions from reputable suppliers but they are quite expensive. Never use a Solar Diagonal by itself without the correct filter. The more common types of Japanese made solar diagonals are rare now and it is unlikely you will find one easily.
NO FILTER IS 100 PER CENT SAFE, ALWAYS ASK ADVICE BEFORE USING IT, ESPECIALLY IF YOU ARE UNSURE. ALWAYS READ THE INSTRUCTIONS.
DO NOT USE
- SMOKED GLASS
- NEUTRAL DENSITY FILTERS
- FOGGED COLOUR OR BLACK AND WHITE FILM
- ANY TYPE OF OPAQUE PLASTIC
- ANY PLASTIC MATERIAL COATED WITH METALLIC INK OR GLITTER
- ANY EYEPIECE HELD SOLAR FILTER
- ALUMINISED MYLAR
- INCONEL SOLAR FILTERS
- BAADER ASTROSOLAR FILM
By using Solar Projection, you really do not need any special equipment other than a piece of white paper. It can give, on most occasions, better image quality and above all, it is very cheap.