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For any object on Simbad so lets take a random one,

http://simbad.u-strasbg.fr/simbad/sim-id?Ident=ton%20618 (Ton 618)

I'm trying to get a layman's explanation of Flux.

What do the letters stand for,

Does V stand for Visual and is it therefore the Apparent Magnitude of the object? I see some pages on wikipedia saying "Apparent Magnitude (v)" and seemingly refer to the value in the Simbad page.

I look up in Google e.g. "Flux Filter-name" and don't get an explanation.

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Each letter represents a filter that passes certain wavelengths of light and blocks others. The flux of the light that passes the filter is measured and that can be converted to a magnitude.

Some letters are mnemonic, so U is ultraviolet R, G, B are for Red, Green and Blue, and V is for Visual. I is a near-infrared filter, but more infrared filters were added and assigned letters J K L M and N, then Z Y and (randomly) H were inserted, resulting in the current rather jumbled system. More details can be found at the Wikipedia article

Lower case letter are just a variant, the specific meaning of a filter may vary from telescope to telescope, and while you can get a general idea of whether an object is brighter in the UV or IR, you would need to read the original paper to find out exactly the characteristics of the filter used.

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  • $\begingroup$ I might be wrong, but generally speaking, lower case filter names are often attached to narrower photometric bands. In this case, the Gunn system. But that might also be an accident. $\endgroup$
    – ProfRob
    May 29, 2022 at 15:53
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In the documentation of Simbad, it is written: The magnitudes in Simbad are not always given in a single system. For instance, the "K" band includes K K' and Ks. For more details check the bibliographical reference and the SVO profile filter service : http://svo2.cab.inta-csic.es/theory/fps/.

Magnitudes are given, when possible, in the Johnson's UBV system. Otherwise, the (AB) flag is given if it cames from a AB system.

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