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Are filters bidirectional?

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#1 bhwolf

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Posted 15 November 2014 - 12:04 AM

OK silly question (possibly) -- I'm assembling my kit deciding how I'll image tonight, and I realize in one configuration I have a choice of which way to install an LP filter.  Visually I don't see a difference, but had me wondering if filters are bidirectional or are there typically an optimal direction for light to travel through the filter? 

 

Thanks!

Brian



#2 Scott in NC

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Posted 15 November 2014 - 12:11 AM

I'm reasonably sure that the direction doesn't matter, but all of my filters only have threads on one side, so can only be attached to an eyepiece or diagonal in one direction.



#3 GlennLeDrew

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Posted 15 November 2014 - 01:13 AM

All filters I've ever used--dyed and interference--have been bi-directional.



#4 jimthompson

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Posted 15 November 2014 - 01:59 AM

The basic function of the filter is the same regardless of the direction light passes through.  However, the reflected component may be different.  In some instances you may find ghost images when looking at bright objects are better (ie. less visible) for one particular filter orientation.  This varies with the quality of the filter (anti-reflective coating or not) and the type of filter.

 

Often filters made for scientific research are marked with the optimum light direction through the filter.  This is due to the desire to direct undesired light away from the sensor, most notably in IR blocking filters.

 

Regards,

 

Jim T.



#5 Scott in NC

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Posted 15 November 2014 - 04:40 AM

Good info, Jim.  Thanks for sharing that. 



#6 bhwolf

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Posted 15 November 2014 - 11:21 AM

Great info, thanks everyone!




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