Photographing Solar Eclipses - Filters
Author: Bill Kramer
Last update: Saturday, 18-Apr-2015 18:53:23 EDT
Solar filters are made of very dark material and intended strictly for use visually or with a camera - not both! Another way to say this is don't use visual solar filters with your camera and do not use a camera or telescope solar filter without the camera or telescope. Because a lens is gathering more light it is very important to get a solar filter matched to your telescope or camera lens.
Solar filters should go in front of all other optics. Solar filters that attach to the eyepiece should never be used. Highly focused light could cause such a filter to over heat and crack. A proper solar filter fits on to the front of the telescope or lens absorbing the solar energy before it enters the remainder of the optical path.
Most solar filters appear mirror like or very shiney. That is to reflect most of the sun's light and only let a little bit through to your eye or camera. Before using a solar filter be sure to inspect for any damage. When observing the sun using a telescope you only get one chance at a mistake - extra caution is always advised. Double checking is not enough.
Another safety tip is to be very aware of the disposition of the filter when in use. Filters that fit on the front without any means of securing the filter to the tube can be dangerous when the wind picks up (or a little kid gets curious). Joe Cali presents an excellent solution in his article about mounting solar filters.
Some filters are made of a silvery material that is flexible (hence it can be punctured, so be careful with it). Others are made of glass. Glass filters often provide an orange color sun while the flexible materials are typically blue. Neutral density solar filters produce a pale white to yellow sun color when imaged. My preference is for a dark yellow color in photographs - which can be obtained using any solar filter and some digital manipulation.
Hydrogen -Alpha filters are a lot of fun during the partial phases as they can reveal surface texture and give you an advance peek at the prominences. HA filters only reveal the sun in the deep red color produced by hydrogen as it looses energy in the chromosphere of the sun. Images are deep red but often enhanced using image edit software to produce yellow-orange and red mixtures. During totality solar HA filters are useless, they block most if not all of the light.
O-III, polarized, and other sky filters do not show anything interesting either, at least in theory, I have never tried them during an eclipse. There just isn't enough time for such things. Stick to a wide field eyepiece, there is plenty to see.
Filters - Projection viewing
The safest and best way to view the partial phases safely is with a projection system. A simple projection system can be constructed by holding a pair of binoculars about one to two feet away from the floor or a light colored background such as a beach towel. A flat surface is best. Focus or move the binoculars until the solar disk is focused, there will be two of them on the ground or projection screen. A picture of this sort is a great reminder of the fever that is building as totality approaches. You can get people and the eclipse all in one shot with a regular camera using regular outdoor settings.