Caption: A graph showing a comparison of the passbands (AKA transmission functions, AKA response curves) of the SDSS ugriz filters of the Sloan Digital Sky Survey (SDSS) and CFHTLS ugriz filters of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS).

Features:

1. Here we are only interested in the SDSS ugriz filters which are the thick curves on the graph.

2. The horizontal axis is wavelength in angstroms (Å): 1 m = 10**6 μm = 10**10 Å and 1 μm = 10**4 Å.

3. The vertical axis is transmission probably a fraction of perfect transmission (i.e., 1).

4. The SDSS ugriz filter transmission functions include the effects of whole optical system of the SDSS 2.5-m telescope and extinction through an air mass of 1.3 at Apache Point Observatory where SDSS has the SDSS 2.5-m telescope (see SDSS: Camera).

   Apache Point Observatory is located in Sacramento Mountains in Sunspot, New Mexico at altitude 2788 m.

5. The air mass at sea level toward zenith is defined to be 1. There are various ways of calculating (see Wikipedia: Air mass: Calculating air mass) and quantitatively determining what it means (see Wikipedia: Air mass: Air mass and astronomy). However, the bottom line is the more air mass, the less flux from an astronomical object and the more distortion of the astronomical object.

   The 1.3 air mass for the SDSS ugriz filters probably is a characteristic value for Apache Point Observatory and probably accounts for most of the reduction of the peak transmission of the the filters below a perfect 1. But we'd need an expert to explain it all accurately.

6. SDSS ugriz filters are characterized as follows:
   1. SDSS u filter: λ_center = 0.356505 μm, FWHM = 0.058228 μm, essentially a near ultraviolet (UV) 300--400 nm) passband, and hence its name u for ultraviolet.
   2. SDSS g filter: λ_center = 0.470033 μm, FWHM = 0.126268 μm, essentially a violet color band (∼ 0.380--0.455 μm), blue color band (∼ 0.450--0.495 μm), and green color band (∼ 0.495--0.570 μm) passband, and hence its name g for green.
   3. SDSS r filter: λ_center = 0.617448 μm, FWHM = 0.114952 μm, essentially a yellow color band (∼ 0.570--0.590 μm), orange color band (∼ 0.590--0.620 μm), and red color band (∼ 0.620--0.740 μm) passband, and hence its name r for red.
   4. SDSS i filter: λ_center = 0.753363 μm, FWHM = 0.123895 μm, essentially a red color band (∼ 0.620--0.740 μm) and near infrared band (∼ 0.750--1.4 μm) passband, and hence its name i for infrared.
   5. SDSS z filter: λ_center = 0.878169 μm, FWHM = 0.099439 μm, essentially a near infrared band (∼ 0.750--1.4 μm) passband. The name z seems to be because of N. R. Pogson (1829--1891).

7. SDSS has released many galaxy images constructed from gri photometry.

   Yours truly thinks that they mix the gri photometry in such a way as to create approximately true-color images---but yours truly CANNOT find any place where this is stated explicitly---just ignoring tricky points is the oldest game in the book.

   So in the gri images, the colors may be similar to what the typical human eye would see if its sensitivity were scaled way up.

   However, the image makers have probably also mixed the gri photometry to make stuctures stand out.

   In any case, there is nothing especially privileged about the human psychophysical light response which in any case varies a bit from person to person and varies strongly with light conditions from photopic vision (well lit condtions) to scotopic vision (dimly lit conditions).

8. In general one has to be careful about attributing true-color or false color to astrophotography.

   Nowadays, the image makers can make the colors anything they like and generally they will NOT tell you what they have done.