Sky's Up Global Astronomy Magazine Volume III (September 2021) | Page 62

telescopes of 125 to 150 mm aperture can be used for high-resolution work . The sun as viewed thought objective filters can have a distinct coloration ( blue , yellow or white depending on the filter ). Solar Herschel Wedges are without any doubt the best way to observe / image the white light sun ( Continuum ). These devices absorb about 95 % of the incoming sunlight . The remaining 5 % have to be reduced using neutral density filters . Solar Wedges should always be used with a refractor telescope . Other filters can be used to improve the low contrast of many with light solar features ( e . g . Baader Solar Continuum , UV / IR , different Wratten filters ).
H-alpha and Ca-K : Narrow band H-alpha ( 656.3 nm ) solar filters are mainly of two types : front loading and end loading . The front loading filter uses a large diameter etalon ( an optical filter that operates by the multiple-beam interference of light , reflected and transmitted by a pair of parallel flat reflecting plates , based on the Fabry-Perot Interferometer ) over the entrance of the telescope . The end loading etalon is smaller and it ’ s placed inside the light path of the telescope . Each of these configurations has advantages and disadvantages . The narrower a filter ’ s bandpass or bandwidth ( the extent or band of wavelengths transmitted by a filter ) the greater is the contrast of the resulting image . In order to observe prominences in H-alpha a filter with a 10-angstroms ( 1 nm ) bandpass is needed . A narrower bandpass filter ( 0.15 nm ) will show a certain number of features but a sub-angstrom filter is needed to observe all the details
Figure 5- Solar telescopes
on the chromosphere . Filters for Ca-K ( 396.9 nm and 393.3 nm ) observing can also be used with excellent results . Compared to the H-alpha line , the H and K lines are broader and thicker in appearance : a filter having a bandwidth of 2-10 Angstroms is sufficient for Ca-H or K observations ( Figure 4 , Figure 5 ).
Solar Imaging The recent advent of CCD cameras that can be operated in a video mode , taking 10 or more images per second for periods of up to a few minutes , can be used with excellent results for high resolution imaging of the Sun . Webcams and astronomical digital video cameras are equipped with a color or a black & white CCD or CMOS . These cameras operated with different interfaces ( USB 1.0 , UBS 2.0 , USB 3.0 , FireWire and GigE ) capture several hundred to thousands of individual images ( frames ) in rapid succession storing them in popular video formats . This video file includes frames seriously degraded by seeing
COURTESY OF Pedro Ré
and others that are less affected . Amateur astronomers today regularly capture images of the Sun that rival those taken by professional astronomers . These images often constitute valuable scientific contributions . Specialized software align , sort and stack hundreds to thousands of images , automatically producing a low noise composite image . These images can be processed using aggressive image processing tools to bring out hidden detail ( Figure 6 ). Over the following pages , you will find several image processing tutorials that cover almost all aspects of solar astrophotography : ( i ) Full Disk imaging ; ( ii ) High-Resolution White Light ( Continuum ) imaging ; ( iii ) High- Resolution H-alpha imaging ; ( iv ); How to produce an artificial flat-field . More information also can be found at the following web links : pedroreastrophotography . com / pedroreastrophotography . com // image _ processing . html pedroreastrophotography . com // sun _ h _ alpha . html
n n n Pedro Ré is currently a Full Professor at the Faculty of Sciences , University of Lisbon with a PhD in Animal Ecology ( University of Lisbon – 1984 ). His main research interests deal with the Ecology of the Ichthyoplankton , Ecology of the planktonic phase of sardine and anchovy , Daily growth of fish larvae based on the microstructure of the otoliths , Rhythms of activity of fish larvae and Fisheries Biology . He is also the author of more than 150 scientific papers ( 65 peer-reviewed ) and 18 books ( author , co-author , or editor ), and about 150 outreach papers dealing with marine Biology , amateur astronomy , history of astrophotograhy and history of the telescope . More information at : www . astrosurf . com / re / index _ bio . html and pedroreastrophotography . com /
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