Baader planetarium b-ccd filter(blue!) stacked with baader planetarium 35nm hydrogen alpha filter . OOOOOOPS.

[Kitsunegari]

So  yea.   I added a baader planetarium hydrogen alpha filter to my baader planetarium blue ccd filter and  bada boom.  a ton of h-alpha light out of my blue filter!

 

How embarrassing, yet extraordinarily remarkable that nobody else noticed this.

 

  

[Whirlwind]

This isn't really a surprise.  Most filters have some out of band transmission (this is usually referenced as their optical density (OD) value).  Even the best filters transmit some light out of the desired passband even if it is a tiny fraction of a percent.  If you put the filter in front of a very bright source then you will inevitably see the leakage (a fraction of a high luminosity can still be a high value etc).  Even Baader don't claim a perfect filter (you can see some leakage in their filter profiles). Nevertheless for RGB filters you use these primarily to obtain colour data of a broad ban image so some limited leakage can be OK as it isn't the dominant component.  Also you can expose on G2V stars to get a 'correct' colour balance (for our eyes) anyway which would remove any leakage impacts.  

[Kitsunegari]

4 hours ago, Whirlwind said:

This isn't really a surprise.  Most filters have some out of band transmission (this is usually referenced as their optical density (OD) value).  Even the best filters transmit some light out of the desired passband even if it is a tiny fraction of a percent.  If you put the filter in front of a very bright source then you will inevitably see the leakage (a fraction of a high luminosity can still be a high value etc).  Even Baader don't claim a perfect filter (you can see some leakage in their filter profiles). Nevertheless for RGB filters you use these primarily to obtain colour data of a broad ban image so some limited leakage can be OK as it isn't the dominant component.  Also you can expose on G2V stars to get a 'correct' colour balance (for our eyes) anyway which would remove any leakage impacts.  

I understand that.  however this is a bigger mistake than it would seem.

 

The transmission graph on their website reveals zero leakage in the h-alpha range so it is a bit unusual;  almost as if this is intentional.   You can see the leaks all over the place as you say;  but this is pretty huge and  how could they miss h-alpha transmission with this large of a peak?  Especially for being "the best of the best".

   

[Whirlwind]

On 31/05/2021 at 15:17, Kitsunegari said:

The transmission graph on their website reveals zero leakage in the h-alpha range so it is a bit unusual;  almost as if this is intentional.   You can see the leaks all over the place as you say;  but this is pretty huge and  how could they miss h-alpha transmission with this large of a peak?  Especially for being "the best of the best".   

But it's not zero and this is where the problem lies.  Most astronomy filters have an OD value of 3 or 4 and for some science 5-6 (it's likely impossible to create one with no light leak).  As such if you have a very bright source (the flu tube) compared to the background then in an OD 3/4 filter enough is likely to be detectable.  On the other hand stars are much fainter in this regard so practically you may not need an OD 5-6 filter and is probably overkill for what we use it for.