Sky's Up Global Astronomy Magazine Volume II (April 2021) | Page 27

What is an Energy Reduction Filter ? ERF is the first part of an H-Alpha system . It cuts down the intensity of light almost equally for the entire spectrum . This filter is usually placed in front of a refractor telescope of aperture ratio f / 10 or higher .
What is Fabry-Perot Etalon ? Named after its inventors Charles Fabry and Alfred Perot , this special kind of arrangement of two optical discs is the heart of this system . It allows only specific sections of spectrum to pass through . Etalon is a stack of two planeparallel optical surfaces with a tiny space in between . With proper and careful placing , this stack can cause interference of light that comes on it . And then it gives out a series of narrow bandwidth ( Near 1 Å ) spikes of light components in its output . One of these spikes comes with 6562.8 Å wavelength H-Alpha line at its center . But this output is not yet ready for observation as it is still a mix of many light components of different frequencies from the full spectrum .
What is blocking filter ? This third item is a semi narrow bandpass filter that allows small portion of spectrum around H-Alpha line while blocking all others . When the series of spikes coming out of Etalon is passed through this BF , it blocks all but allows one spike only with 6562.8 Å wavelength at its center . This final narrow band light coming out the BF carries the sharp signature of the hydrogen rich chromosphere or solar surface .
What is 2nd Stack of Etalon ? To make the image further sharper , even narrower bandwidth is needed . To ensure that , a second stack of etalon is added to the system . When properly tuned , the second etalon marginally cuts down the bandpass allowed by the first one . With a carefully maintained and tuned double etalon system , we can have a
COURTESY OF Debasis Sarkar
Above , diagram of H-Alpha telescope system ; left , sunspot
COURTESY OF Debasis Sarkar and Janmejoy Sarkar . ( Sky Watchers Association of North Bengal )
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