As the sun is now in active period, metal coated filers are sold instead of the classical sun glass
The classical sun filers for oculars are feared to be broken by the heat from the sun.
While the new type of metal coated filter
which are to be set in front of the objective lens are free from those
kinds of dangers.
I have investigated the characteristic of these metal coated filter and found the followings.
As the visible rays are decreased to the ratio of 1 divided by 10000,2 or 3 percent of the infrared and heat rays pass through the filers.
The radiation from the sun is consist of visible and infrared on even ratio ,so the passed infrared ray through the filter may heat and do damage to your eye .
I ordered KENKO OPTICS to make the HA-15 filter glass to install in the filter cell for the telescope ocular .
HA-15 is a heat extinction glass which is made by HOYA GLASS for industrial heat cut filters.
photo 1 (click to enlarge)
the above photo 1 shows the parts of the filters.
The size of the filer for 2 inch size oculars are equal to the camera filter .So the filter cell is for camera.（Kenko is the filter maker for the camera)
I made the filter for 1 1/4 inch size ocular using the cell for the moon glass.
photo 2 (click to enlarge)
photo 2 shows the filter assembles
To absorve ultra violet ray ,I also add green filter (PO-1)
3 (click to enlarge)
Above photo 3 shows the filter on the eyepiece.
This system needs to be used with sun filter for the telescope sold in the market.
Wrong way of seeing the Sun may cause serious damage to your eyes.