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Light pollution filter and DSLRs photometry?

javi_goagc's picture
javi_goagc
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Joined: 2012-01-28

Hello dear partners at AAVSO forums,

This is the first time I write to this forum, so first of all I want to thank AAVSO and its support stuff for the unvaluable work they do in helping amateurs around the world to learn about variable stars, enroll into observing projects and perform usefull measures. 

I´m an amateur astronomer from Canary Islands, Spain, who I´m just beginning in the field of the variable star photometry with DSLR.

I own a Canon 450D model and, from what I have read in the Citizen Sky project documentation and forums, I understand that the default raw image capture behaviour of these Canon models is such that the green filter in the bayer matrix almost corresponds to the standard scientifically-valuable V filters, so after extracting only the green channel in debayeurization process and afterwards applying a certain transformation coefficient we can obtain a very approximate result to that of an V filter.

My question is: unfortunately I watch from a light polluted urban area, and up to now I´ve traditionally used a light pollution filter designed specifically for these Canon Models (an Astronomik CLS EOS Clip filter). They do a great job for general (non-scientific) astrophotography purposes.

Has any of you had experience with using this or similar light pollution filters for photometry?

At first I thought that, as long as the green wavelength is not cutted through the filter, the important photons will be captured.

But after thinking of it twice and reading the product specifications for this filter, the brand states this for the case of a 450D unmodified (my case):"DSLR photography (original): Good, colour balance shifted but contrast enhanced". So, if colour balance is shifted I think it automatically means that photons that I would be interested in being passing by the green filters of the bayer matrix will be entering through another filter, so after debauyerization and filtering by the green channel those photons will be irreversibly lost.

So, I have a strong suspect that it is absolutely no possible to use this filter for photometric measures, but just in case I wanted to ask you more experienced partners.

Thanks in advance for your help!

Javier

Light pollution filters for photometry
SXN's picture
SXN
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Joined: 2010-03-12

Javier,

Your suspicions are well founded. No, you absolutely cannot use a light pollution filter and perform accurate photometry. 

Mike

Thanks!
javi_goagc's picture
javi_goagc
Offline
Joined: 2012-01-28

Ok, really thanks for your clarification and quick response! 

Filter unclipped from the camera body ;-).

Best regards from Spain and clear skies,

Javier

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