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Category: Reaching F/30
Why do I need F/30?
Posted on Apr 23, 2011, 1:21 am Author: Jen Winter

F/30 light is needed for all narrow bandpass filters to perform properly.

Bandpass fitlers are of the class of filters called "interference filters"  These filters require that light hits them at a perpendicular angle.   We call that "normal".   If light reaches an interference filter at an angle, it will pass light that is shifted off the desired wavelength.  - specifically, blue wing shifted. You can see this sometimes with other filters when you tip them and they change from pinkish to purple or blueish.  This is the same effect.

Light from an objective is by nature, angled.  It comes to focus at angles that affect the performance of a filter.  Sunlight coming to a telescope arrives nearly parallel to the front objective, so other manufacturers who install their solar filters in front of the objective do not have this requirement.

The inside, central area of a light cone is generally quite strait, but the outer edges are angular.   That means that the outer area of the aperture will hit at an angle and cause wing shift, with the inner area operating on-band.   That causes an effective widening of the FWHM bandpass of the filter.   It would perform like a wider filter (and wing shifted blue) if the light cone is still angled from a fast focal ratio.   

After the filter, users can focal reduce if desired back down to a smaller image size.  However, for best performance, about F/30 is needed to accomplish the narrow, 0.7Å, 0.5Å or lower bandpasses that DayStar can manufacture.


Calcium and other wide bandpass filters need not operate at F/30.  They are wider bandpasses of 2.0 or 5.0Å so they are free to operate at faster F/15-F/20 focal ratios. 

The F/ratio issue is not associated with a specific line (like H alpha or He D3 vs Calcium) but rather associated with FWHM bandpass such as 0.7Å, 0.5Å FWHM.

Tagged Keywords: bandpass, focal, ratio, f/30, normal, wing, shift, blue, angle, hydrogen, alpha

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