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Filters for Solar Photography (an addendum)

This is a summary of Mapug-Astronomy posts with most of the information provided by Chris Frye and John Hooper. However, any errors can be attributed to me.

Recent posts on Mapug-Astronomy have mentioned a method for photographing the sun in the light of the Calcium H and K emission lines. It seems that Martinez in his book Astrophotography II suggests that a filter which passes these two lines in the 390 to 400 nm region can be made using a Wratten 18A and a Wratten 2B filter together. Together the two band edges of these filters have an overlap such as to give a passband that has a peak at 396 nm. This passes the Calcium H line at 393 nm and the K line at 396 nm The idea is that with the hydrogen emission cut out the "faculae" on the sun become visible.

Inspecting the filter passbands actually reveals that the 18A filter has two windows of transmission. One peaks at 360 nm and the other at 750 nm. One is in the near ultraviolet and the other in the near infrared. The combination of ultraviolet and infrared passed may give this filter an apparent very dark green tinge. But it is essentially opaque in the visible region of the spectrum. It is the ultraviolet passband that is combined with the 2B filter which passes light longer than 390 nm. Thus the 2B filter takes out the ultraviolet below 390 nm and the 18A filter takes out wavelengths above 400 nm. This leaves a passband at about 396 nm.

However, the 2B filter passes all of the infrared and the 18A has a transmission window in the infrared. Thus an infrared rejection filter has to be used to remove the infrared which leaks through the basic filter combination. The 18A filter comes in glass according to the Kodak reference information. Glass passes all wavelengths longer than 320 nm. The infrared filter required would be a so called "hot mirror" such as is used for any filtered imaging with a CCD imager. If film is used, the film must be insensitive to infrared or be used with the "hot mirror" as well. Tech pan for example is quite infrared sensitive.

The above filters are of course used with a regular solar filter as well to cut down the intense light from the sun. The pass band characteristics of this "pre-filter" must also be taken into account. Some give an orange-yellow image and some give a blue image. It is probable that the lighter series 3 filter from Thousand Oaks would be satisfactory since the filter combination shows very great attenuation even in the "pass band." Note that I have not seen any publication of the actual bandpass curves for either the glass (orange image) or the mylar film type (blue image) solar filters. If either of these rejects the 400 nm region, it would not be suitable for the Calcium lines of course. It must be noted that an energy rejection filter supplied with the H alpha type filters is not suitable since they generally pass just a range of wavelengths between 640 and 740 nm.

It has also been pointed out that there are two filters in the Special Filters section of the Schneider Optics (Kreuznach, Germany) catalog that are of interest. One is the #420 which starts to pass wavelengths longer that 387 nm and the # 403 which passes wavelengths below 407 nm. These in combination also give a window at the wavelengths of the Calcium lines. The combined transmission is about 2.5% at 397 nm.

I have not personally experimented with these filter sets. But it looks like an interesting area for experimentation.

While discussing solar filters, it should be mentioned that there are also very narrow band filters (of the order of 2 to 0.4 angstrom widths) for the Hydrogen alpha wavelength of 6563 angstroms and also for the Calcium line at 3934 angstroms. Thousand Oaks and Day-Star make such filters. These filters are described elsewhere on this web site.

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