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Category: Pre-purchase Questions
Bandpass (FWHM) Selection
Posted on Apr 25, 2011, 3:27 pm Author: Jen Winter
 

Bandpass Choices....
Should I buy a .7Å or a .5Å or something in between? 

Because DayStar Filters are sold by bandpass instead of aperture, the user will need to make a choice about which he/she would like to own.  DayStar does not double-stack filters to accomplish narrower bandpass.   The coatings on the etalon crystal dicate the bandpass of each filter.  Some are wider and some narrower.   DayStar Filters manufactures filters from 0.2Å wide (FWHM) to over 5Å wide for different applications.   Narrower filters are more expensive by nature.  

Hydrogen Alpha filters are offered in 0.8Å down to 0.2Å

Don't assume that narrower bandpass is best just because it's more expensive. 
Each observer has his own personal favorite just like flavors of ice cream. 

Some observers love prominences.  Some love chromosphere.  It is difficult to say what is best because each observer has his own taste.   Rather, think about what your observing goals are and what features you are excited about seeing.  Certainly, it is interesting to explore the sun and find out more than you saw before.  But you probably have already seen some pictures of the Sun in Hydrogen Alpha and have some desire to see more of one kind of feature or another.



A 0.7 filter will show big fat prominences and maybe a little bit of surface.


A 0.6 will show a really good mix of both proms and some surface contrast.  Many customers are happy with .6 filters for their whole life.


A 0.5 will show slightly smaller prominences and lots of harder contrast on the surface.


A 0.4 is all about chromosphere.   This specific bandpass shows the soft fuzzy boundary layer of the sun's chromosphere.  Prominences get thinner in a .4


A 0.3 is very very contrasted, with fine line details on the surface.  prominences are pencil thin but it really sharp contrast.

I try to explain that prominences are like tornadoes.  They move across the landscape rather slowly, while inside, their movement is very fast.  Fast enough to have the Doppler shift we use in predicting tornadoes or the Doppler shift of red and blue wing shifted light.   That means that a wider filter will see the wing shifted fast moving prominence light but a narrower filter will see less of that because it only passes a narrow bandpass range. 
If you're excited to see prominences, try a 0.7 or 0.6.   If you want to see the swirling detail of the active regions on surface detail, then save up for the .5  Usually hard core imagers and researchers buy the 0.4 and 0.3 filters. 

Observers may also consider that a DayStar can be upgraded at a later date with a narrower etalon.  Sometimes a new owner can only afford a wider, 0.7Å filter; but is later able to save up to upgrade to a 0.5Å filter.   There is no need for buyers' remorse in a bandpass selection with a DayStar Filter. 


Tagged Keywords: Å, advice, guide, selection, filter, daystar, choice, FWHM, bandpass

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