# Photometry systems

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 (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.

• Where have you seen “UGRIZ” (that wasn’t just a typo or something like that)? I’m unaware of any such system. Jul 17 '19 at 0:40
• Oh, so maybe ugriz and UGRIZ are actually the same? Jul 17 '19 at 0:41
• 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...) Jul 17 '19 at 0:47
• The UGRIZ seems to be a typo/incorrect capitalization of the standard ugriz. Hopefully you've also found the SDSS photometric transformation details - the first one listed (Jester et al. 2005) is claimed to be suitable for quasars Jul 17 '19 at 1:14
• I hope you don't mind the pedantic edit, feel free to roll back or edit further.
– uhoh
Jul 18 '19 at 2:32

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.