Light pollution filters and photometry

Forums Photometry Light pollution filters and photometry

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    Kevin West

    It has been recommended to me to get a light pollution filter for CMOS photometry

    In particular this one. Optolong L-Pro Light Pollution Broadband Filter.

    Is this wise given that it’s job is to block some of the very photons you are trying to collect?


    Andy Wilson

    Hi Kevin,

    You are right to be wary of using a light pollution filter for photometry. These filters are designed to help imaging nebula.

    Looking at the specification it has high transmission in the nebula emission lines and blocks common narrow band light pollution. In photometry, the magnitude you measure depends on which part of the spectrum you observe. So the magnitude measured through a filter like this could not be compared with other observers.

    If you are just measuring the period of a variable target or looking for outbursts then this might be usable, but it may introduce problems.

    Another point to consider, these filters are designed to boost contrast of targets that predominantly emit light in the nebula lines. This is unlikely to be much use for stars or asteroids as they tend to emit across the visible spectrum.

    Best wishes,

    Kevin West

    Hello Andy,
    Many thanks for your reply to my post

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