<p>Is there a difference between a K-correction (see Wiki <a href="https://en.wikipedia.org/wiki/K_correction" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/K_correction</a>) and simply &quot;resizing&quot; the bands, e.g. the 2-10 keV x-ray band, with the redshift? I've read that K-correction requires the assumption of a spectral shape. Why is resizing strictly speaking not enough?</p>

CC BY-SA 4.0
2020-12-17T16:11:16.343Z
Do you mean physically resizing the _filter_? In principle that would work, except you can't just resize a filter. You can build filters centered on various wavelengths, but redshift and hence K-correction is continuous, so you would in principle need infinitely many filters, and filter are expensive. Moreover, you would need to know the redshift _before_ observing the object through the appropriate filter. K-correcting is easier.
pela
CC BY-SA 4.0
2020-12-17T16:33:24.567Z
@pela no I mean converting observed flux density to intrinsic luminosity, using the luminosity distance and a factor (1+z) because it's a density
theWrongAlice
CC BY-SA 4.0
2020-12-17T22:26:44.350Z
The point of the K correction is that you observe a redshifted object in some band, and you then want to know the flux in the rest-frame of that band. E.g. you want to know the rest-frame _B_ magnitude of a galaxy at redshift z = 0.5, observing it in the _B_ band. But the flux you observe is not the flux that was _emitted_ in the _B_ band, since that light has been redshifted into the observed _V_ or _R_ band. [cont. below]
pela
CC BY-SA 4.0
2020-12-17T22:27:15.123Z
[cont. from above] Rather, it is the flux that was emitted somewhere in the UV and then redshifted to your _B_ band. In order to estimate the original _B_ flux, you have to make assumptions of the spectral shape (i.e. the slope of the continuum, and possibly nebular lines which in some cases can contribute quite significantly to the flux, I think).
pela
CC BY-SA 4.0
2020-12-17T22:33:38.540Z
I think _the_ reference for the K correction would be [Oke & Sandage (1968)](https://ui.adsabs.harvard.edu/abs/1968ApJ...154...21O), but David Hogg wrote a quite pedagogical (albeit non-refereed) paper on it ([Hogg et al. 2002](https://ui.adsabs.harvard.edu/abs/2002astro.ph.10394H)).
pela
<observational-astronomy><redshift><data-analysis><photometry><x-ray>
K-correction vs resizing photometric bands
theWrongAlice
# K-correction vs resizing photometric bands

(Asked by: username_0 on 2020-12-16)

Is there a difference between a K-correction (see Wiki <https://en.wikipedia.org/wiki/K_correction>) and simply "resizing" the bands, e.g. the 2-10 keV x-ray band, with the redshift? I've read that K-correction requires the assumption of a spectral shape. Why is resizing strictly speaking not enough?

**Tags:** observational-astronomy, redshift, data-analysis, photometry, x-ray

> **Comments:**
> **username_1**: Do you mean physically resizing the _filter_? In principle that would work, except you can't just resize a filter. You can build filters centered on various wavelengths, but redshift and hence K-correction is continuous, so you would in principle need infinitely many filters, and filter are expensive. Moreover, you would need to know the redshift _before_ observing the object through the appropriate filter. K-correcting is easier.  
> **username_0**: @username_1 no I mean converting observed flux density to intrinsic luminosity, using the luminosity distance and a factor (1+z) because it's a density  
> **username_1**: The point of the K correction is that you observe a redshifted object in some band, and you then want to know the flux in the rest-frame of that band. E.g. you want to know the rest-frame _B_ magnitude of a galaxy at redshift z = 0.5, observing it in the _B_ band. But the flux you observe is not the flux that was _emitted_ in the _B_ band, since that light has been redshifted into the observed _V_ or _R_ band. [cont. below]  
> **username_1**: [cont. from above] Rather, it is the flux that was emitted somewhere in the UV and then redshifted to your _B_ band. In order to estimate the original _B_ flux, you have to make assumptions of the spectral shape (i.e. the slope of the continuum, and possibly nebular lines which in some cases can contribute quite significantly to the flux, I think).  
> **username_1**: I think _the_ reference for the K correction would be [Oke & Sandage (1968)](https://ui.adsabs.harvard.edu/abs/1968ApJ...154...21O), but David Hogg wrote a quite pedagogical (albeit non-refereed) paper on it ([Hogg et al. 2002](https://ui.adsabs.harvard.edu/abs/2002astro.ph.10394H)).  

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## 0 answers

