Technical Tid-Bit #4 – Telescope Solar Filters

     By Aaron Clevenson, AL-Cor

 

By now I am sure that all of you have heard, but it bears repeating…  The only dangerous activity that astronomers participate in is solar observation.  With solar eclipses and the recent Venus Transit it is critical that solar observations be done in a way that protects our eyes.  Looking at the sun can quickly and permanently damage your eyes.  It has happened to some rather famous astronomers.  Looking at the sun through unprotected binoculars or a telescope will permanently damage your eyesight IMMEDIATELY.  Don’t do it.

 

Last Month we talked about how Welder’s Glass can be used to look at the sun safely without magnification.  This time, let’s look at how you can do this through your telescope.

 

The first consideration is buy a good full aperture solar filter from a reputable dealer and manufacturer.  They come made of mylar and also coated glass.  For my eyes, the glass ones are the only way to go.  They are more expensive, but my eyesight is priceless.  I personally think that plastic sheeting is just too easy to damage.

 

The next consideration is to check it EVERY TIME BEFORE you use it.  Even a tiny hole in the plastic or the coating can let dangerous levels of light through. 

 

The last consideration is to buy the type that is right for what you are planning to do with it.  Solar filters come in different types.  A filter that blocks 99.99% of the incoming light is fine for photography, but NOT for visual observing.  The most common type of solar filters block 99.999% of the sun’s light and work for visual observation.  This is the type that telescope manufacturers typically offer.  But there are also filters that block 99.9999% of the light and are very safe.  (This is the same level of protection as the #15 Welder’s Glass from last month’s article.)

 

Why full aperture?  Well there are two reasons.  First, do you want all that intense solar radiation focusing on small areas inside your telescope?  It can cause serious damage.  And related to that, but much more serious, it is not uncommon for the image of the sun to get so hot that it will crack the solar filters that are not full aperture.  If that happens while you are looking through it, you will instantly permanently damage your eye; not even enough time to react.  Always go FULL APERTURE!

 

Have fun, be safe.