<p>Doing photometric reduction (from image to brightness) includes the following steps:</p>

<ul>
<li><p>image cleaning (bias + dark + flat + etc.)</p></li>
<li><p>identify/measure brightness of the object of interests from the cleaned image (using e.g. sextractor). Let's say the image was taken in U filter with the brightness (mA_U_image) in any arbitrary magnitude given any arbitrary zeropoint (ZP).</p></li>
<li><p>identify/measure brightness of standard objects (which are objects that we know their brightness) using U filter and the same ZP. Let's say the brightness is mB_U_image.</p></li>
<li><p>Find transformation: mB_U_real(AB) = mB_U_image(arbitrary with zeropoint = ZP) + correction where mB_U_real(AB) = magnitude of B in U filter in AB system (or in other magnitude system as identified by the database of B given U filter).</p></li>
<li><p>Apply the transformation: mA_U_real(AB) = mA_U_image(arbitrary with zeropoint = ZP) + correction</p></li>
</ul>

<p>Note: this method is simplified because both science objects and standards were assumed to be taken from the same U filter.</p>

<p>So, to answer your question:</p>

<ul>
<li><p>Typically, there are two frequently used magnitude systems: Vega and AB. You might need to get the Vega to AB correction if the transformation involve changing from one system to another.</p></li>
<li><p>UGRIZ and ugriz: I found that sometimes UGRIZ is used to notify that the magnitude is expressed in Vega system and ugriz is in AB system, or vice versa. For an example, <a href="https://arxiv.org/abs/1605.01054" rel="nofollow noreferrer">https://arxiv.org/abs/1605.01054</a></p></li>
<li><p>U and u and u' are slightly different (check their response curves from the telescope handbooks). So, transforming from one filter to another might need to include more terms in the transformation.</p></li>
<li><p>If standards and science objects were taken from different filter/telescope/setting (even in the case of U and u), you might need to also include a color correction term in the transformation.</p></li>
<li><p>Eq. 1 is for calculating chi-sq to be used in the optimization problem. I believe that the equation is expressed in u-g color index as an example. You should be able to use any color index pair. If you really want to use u-g index, but you have UBVR, here is what I think you should do: find u = U + correction, and g = B + correction.</p></li>
</ul>

CC BY-SA 4.0
2019-07-17T01:08:55.173Z
2019-07-17T01:08:55.173Z
Kornpob Bhirombhakdi
<p>I know there's a difference between the ugriz and u'g'r'i'z' systems of photometric magnitude, but I also sometimes see UGRIZ written. Is there a difference between a UGRIZ magnitude and a ugriz magnitude?</p>
<p>I'm trying to apply the method from the paper <a href="https://iopscience.iop.org/article/10.1086/322132/pdf" rel="nofollow noreferrer">Photometric Redshifts of Quasars</a> (Richards et al. 2001, ApJ 122, 1151-1162, Equation 1) to my data, which contain data in U, B, V, R, etc. Do I need to do any transformations to my data in order to apply their equation?</p>
<hr />
<blockquote>
<p>First, we construct an empirical color-redshift <span class="math-container">$\chi^2$</span> (for each redshiftÈ indicated by the subscript z) is computed as</p>
<p><span class="math-container">$$\chi_z^2 = {\left((u'-g')-(u'-g')_z \right)^2 \over \sigma^2_{u'-g'} - \sigma^2_{(u'-g')_z}} + C_{gr} + C_{ri} + C_{iz}  \tag{1}\label{eq1}$$</span></p>
<p>where <span class="math-container">$(u'-g')$</span> is the measured <span class="math-container">$u'-g'$</span> color of the object, <span class="math-container">$(u'-g')_z$</span> is the color from the median color-redshift relation at a given redshift, <span class="math-container">$\sigma_{u'-g'}$</span> is the photometric error in the <span class="math-container">$u'-g'$</span> color, which is given by <span class="math-container">$(\sigma_{u'}^2+\sigma_{g'}^2)^{1/2}$</span>, and  <span class="math-container">$\sigma_{(u'-g')_z}$</span>  is the 1 <span class="math-container">$\sigma$</span> error width of the median color-redshift relation as a function of redshift.</p>
</blockquote>

CC BY-SA 4.0
2019-07-17T00:40:18.177Z
Where have you seen “UGRIZ” (that wasn’t just a typo or something like that)? I’m unaware of any such system.
Peter Erwin
CC BY-SA 4.0
2019-07-17T00:41:06.830Z
Oh, so maybe ugriz and UGRIZ are actually the same?
Jim421616
CC BY-SA 4.0
2019-07-17T00:47:47.640Z
No, I mean there is no “UGRIZ” system as far as I know. (Of course, you go on to mention “U, B, V, R, etc.”, which is a completely different system from ugriz or u'g'r'i'z', so it’s a bit confusing...)
Peter Erwin
CC BY-SA 4.0
2019-07-17T01:14:48.687Z
The UGRIZ seems to be a typo/incorrect capitalization of the standard _ugriz_. Hopefully you've also found the [SDSS photometric transformation](https://www.sdss3.org/dr8/algorithms/sdssUBVRITransform.php) details - the first one listed (Jester et al. 2005) is claimed to be suitable for quasars
astrosnapper
CC BY-SA 4.0
2019-07-18T02:32:30.113Z
I hope you don't mind the pedantic edit, feel free to roll back or edit further.
uhoh
<photometry>
Photometry systems
Jim421616
# Photometry systems

(Asked by: username_0 on 2019-07-16)

I know there's a difference between the ugriz and u'g'r'i'z' systems of photometric magnitude, but I also sometimes see UGRIZ written. Is there a difference between a UGRIZ magnitude and a ugriz magnitude?

I'm trying to apply the method from the paper [Photometric Redshifts of Quasars](https://iopscience.iop.org/article/10.1086/322132/pdf) (Richards et al. 2001, ApJ 122, 1151-1162, Equation 1) to my data, which contain data in U, B, V, R, etc. Do I need to do any transformations to my data in order to apply their equation?

---

> First, we construct an empirical color-redshift $\chi^2$ (for each redshiftÈ indicated by the subscript z) is computed as
>
> $$\chi\_z^2 = {\left((u'-g')-(u'-g')\_z \right)^2 \over \sigma^2\_{u'-g'} - \sigma^2\_{(u'-g')\_z}} + C\_{gr} + C\_{ri} + C\_{iz} \tag{1}\label{eq1}$$
>
> where $(u'-g')$ is the measured $u'-g'$ color of the object, $(u'-g')\_z$ is the color from the median color-redshift relation at a given redshift, $\sigma\_{u'-g'}$ is the photometric error in the $u'-g'$ color, which is given by $(\sigma\_{u'}^2+\sigma\_{g'}^2)^{1/2}$, and $\sigma\_{(u'-g')\_z}$ is the 1 $\sigma$ error width of the median color-redshift relation as a function of redshift.

**Tags:** photometry
> **Comments:**
> **username_1**: Where have you seen “UGRIZ” (that wasn’t just a typo or something like that)? I’m unaware of any such system.  
> **username_0**: Oh, so maybe ugriz and UGRIZ are actually the same?  
> **username_1**: No, I mean there is no “UGRIZ” system as far as I know. (Of course, you go on to mention “U, B, V, R, etc.”, which is a completely different system from ugriz or u'g'r'i'z', so it’s a bit confusing...)  
> **username_2**: The UGRIZ seems to be a typo/incorrect capitalization of the standard _ugriz_. Hopefully you've also found the [SDSS photometric transformation](https://www.sdss3.org/dr8/algorithms/sdssUBVRITransform.php) details - the first one listed (Jester et al. 2005) is claimed to be suitable for quasars  
> **username_3**: I hope you don't mind the pedantic edit, feel free to roll back or edit further.  

---

## 1 answers

### Answer by username_4 on 2019-07-17. Score: 1 | Accepted answer

Doing photometric reduction (from image to brightness) includes the following steps:

* image cleaning (bias + dark + flat + etc.)
* identify/measure brightness of the object of interests from the cleaned image (using e.g. sextractor). Let's say the image was taken in U filter with the brightness (mA\_U\_image) in any arbitrary magnitude given any arbitrary zeropoint (ZP).
* identify/measure brightness of standard objects (which are objects that we know their brightness) using U filter and the same ZP. Let's say the brightness is mB\_U\_image.
* Find transformation: mB\_U\_real(AB) = mB\_U\_image(arbitrary with zeropoint = ZP) + correction where mB\_U\_real(AB) = magnitude of B in U filter in AB system (or in other magnitude system as identified by the database of B given U filter).
* Apply the transformation: mA\_U\_real(AB) = mA\_U\_image(arbitrary with zeropoint = ZP) + correction

Note: this method is simplified because both science objects and standards were assumed to be taken from the same U filter.

So, to answer your question:

* Typically, there are two frequently used magnitude systems: Vega and AB. You might need to get the Vega to AB correction if the transformation involve changing from one system to another.
* UGRIZ and ugriz: I found that sometimes UGRIZ is used to notify that the magnitude is expressed in Vega system and ugriz is in AB system, or vice versa. For an example, <https://arxiv.org/abs/1605.01054>
* U and u and u' are slightly different (check their response curves from the telescope handbooks). So, transforming from one filter to another might need to include more terms in the transformation.
* If standards and science objects were taken from different filter/telescope/setting (even in the case of U and u), you might need to also include a color correction term in the transformation.
* Eq. 1 is for calculating chi-sq to be used in the optimization problem. I believe that the equation is expressed in u-g color index as an example. You should be able to use any color index pair. If you really want to use u-g index, but you have UBVR, here is what I think you should do: find u = U + correction, and g = B + correction.


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