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Baader F/2 Filters on an F5 scope?

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#1 cal1mero

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Posted 30 June 2020 - 11:56 AM

I will be buying teh Baader Narrowband F/2 filters for a RASA. Based on Baader's description, the filters are specially made for F2 scopes because of special treatment of the high angle light impact. They are mention that they will not work on higher F scopes (F4+).

 

As anybody tried to use them on slower scopes?  what is your experience compared to standard narrowband filters?

 

Thanks

Eric

 



#2 sanbai

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Posted 30 June 2020 - 05:59 PM

Thin film filters are designed for light rays coming at specific incident angle (usually 0° from axis). The interference will change when the angle changes, and so will do the band passing the filter. The bandpass of visual filters is usually broad enough to cover the targeted wavelength any at any incidence angle up to f/3 scopes. Imaging filters have narrower bandpass, so that's more challenging.

My guess is that those filters are very narrow band but only effective for the outer part (large angle) of the light cone of very fast scopes (f/1.8-3.5 they say). The inner part of the cone (which is the cone of longer f/ scopes) will filter out the targeted wavelength, so it won't be effective for the purpose in regular, slower scopes.

Edited by sanbai, 30 June 2020 - 06:01 PM.


#3 cal1mero

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Posted 30 June 2020 - 09:35 PM

Sanbai,

Thanks for your reply. Your explanation is clear and indeed the filtering may not be working properly when the ray hits perpendicularly the filter.



#4 Eddgie

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Posted 30 June 2020 - 10:16 PM

I have filters that are sold as f/3 filters and they work fine at f/5.

 

Now I may be wrong, but when other companies sell filters with a speed, that usually means that it will work at slower speeds, but past the "rated" speed, the bandwidth shift will cause the outer part of the field to be at a different band than the center.

 

If the filter only worked at f/2, then the rays entering from the center of the cone (shallower angle) would be diminished so you would not be in band at the center of the field which is typically where you want to place your subject.

 

Maybe I am wrong though, but my filters rated for f/3 works perfectly well at slower speeds. 



#5 sanbai

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Posted 30 June 2020 - 10:38 PM

I have filters that are sold as f/3 filters and they work fine at f/5.

Now I may be wrong, but when other companies sell filters with a speed, that usually means that it will work at slower speeds, but past the "rated" speed, the bandwidth shift will cause the outer part of the field to be at a different band than the center.

If the filter only worked at f/2, then the rays entering from the center of the cone (shallower angle) would be diminished so you would not be in band at the center of the field which is typically where you want to place your subject.

Maybe I am wrong though, but my filters rated for f/3 works perfectly well at slower speeds.

I guess that's because the bandpass of your filter is broad enough and it's actually calculated to just work at f/3 (may be not 100%) and to be optimal in slower scopes.

Baader says very explicitly that these filters are only for f/1.8 to f/3.5. They don't publish the spectrum because it would look awful, although it works for the actual purpose.

In a former thread here at CN there were some spectrum posted from regular filters measured tilting the filter some degrees. The change was obvious. A simple test with your eyes also shows roughly the effect of changing incidence angle.

Edited by sanbai, 30 June 2020 - 10:41 PM.



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