Transformations of SDSS magnitudes

Unione Astrofili Italiani (Italy) (UAI)
Mon, 08/10/2020 - 19:09

Goodmorning everyone!
I would like to know if there is a formula to determine Johnson's magnitude (V) from magnitudes ('g) and ('i) "SDSS".
Thanks everyone for your help!

Massimiliano Martignoni

Unione Astrofili Italiani (Italy) (UAI)
Transformations of SDSS magnitudes

Thank you \Brian

I read the suggested page, but I have just ('g) and ('i) SDSS magnitudes, not ('r)!

American Association of Variable Star Observers (AAVSO)
Lupton Transforms Used By Sequence Team for BVRI from SDSS Data

The Sequence Team uses Lupton for BVRI from SDSS Data

Lupton (2005) as listed below

These equations that Robert Lupton derived by matching DR4 photometry to Peter Stetson's published photometry for stars.

B = u - 0.8116*(u - g) + 0.1313; sigma = 0.0095
V = g - 0.5784*(g - r) - 0.0038; sigma = 0.0054
R = r - 0.1837*(g - r) - 0.0971; sigma = 0.0106
I = r - 1.2444*(r - i) - 0.3820; sigma = 0.0078

Tim Crawford

Sequence Team


American Association of Variable Star Observers (AAVSO)
I'm wondering if such match

I'm wondering if such match has been tried by using only Landolt standards? The amount of Landolt standards is not too great, I know, but e.g. SDSS Stripe 82 should cover quite a number of Landolt areas with wide colour index range, in addition there are several higher-declination Landolt areas...

Best wishes,

American Association of Variable Star Observers (AAVSO)
I came here to ask about…

I came here to ask about sequences for other Johnson Filters - this response makes me think the Johnson Magnitudes for stars that are already part of the V sequence for a particular star are already available (possibly as transformations from SDSS).


So how can we access the sequences for these other filters? Through VSX, I only ever see B and V (in the advanced options).

American Association of Variable Star Observers (AAVSO)
VSP magnitudes - Photomtry table

I guess you mean VSP not VSX.
You can select U, B, Rc and Ic too from the advances options menu at the bottom and those magnitudes will be displayed along with the V magnitude column in the photometry table if available.


American Association of Variable Star Observers (AAVSO)
Of course, such transforming

Of course, such transforming formulas will probably be useful only temporarily.

All the new astronomical catalogues are expressed in Sloan or Sloan-like (e.g., Pan-STARRS) magnitudes, so far as I can tell. Which will drive new photometry to Sloan; Johnson-Cousins will be relegated to anachronism, rather like inches and feet in a world of meters, or insisting on expressing stock prices in 1/16 dollars. One may persist in converting back and forth, but eventually one asks: why bother?

AAVSO technical leadership should probably be discussing how AAVSO will make its inevitable wholesale transition to Sloan, perhaps a couple of years from now. I've certainly bought my last Johnson-Cousins filter.

Amen to that.  Alternatively,

Amen to that.  Alternatively, Gaia Grp, a visual surrogate which has the advantage of having a broad bandwidth.  Broader filter bandwidth = Higher SNR for a given exposure time = greater photometric precision.

(or, perhaps, both?)

American Association of Variable Star Observers (AAVSO)
I agree, that photometric

I agree, that photometric systems of massive surveys done with one instrument have certainly momentum and (may have) certain benefits over photometric systems that are based on rather limited amount of stars.

From stellar astrophysics point of view, the benefit of SDSS filters is IMHO not that much better compared to Johnson-Cousins one. One important and clearly positive aspect is definitely SDSS u' vs Johnson U, latter is unfortunately not very well positioned respective to stellar spectral features (e.g. Balmer jump). But besides that - instrumental passbands from instrument to instrument are as variable and as difficult to transform as e.g. Johnson-Cousins ones. There is just more rather good but faint compstars in certain areas in the sky.

SDSS optimizes throughput and cures some issues at ends of optical atmospheric window, but otherwise IMHO it is even a bit less sensitive to stellar parameters other than Teff.

I'm keeping my fingers for final data from Gaia low-resolution spectrometers. Current summed up Rp and Bp or, even more, G are just better than nothing. Hopefully, using final N channel spectrophotometric data, it would be possible to construct any types of synthetic filters (including SDSS and Johnson-Cousins and many others) that really would allow for vast amount of detailed studies.

Best wishes,

American Association of Variable Star Observers (AAVSO)
HelloYes, you can estimate…


Yes, you can estimate Johnson's V magnitude from SDSS g and i magnitudes using color transformation equations. The Johnson V-band magnitude is a standard photometric system commonly used in astronomy, and SDSS g and i magnitudes are measurements in a different system. Color transformation equations help convert magnitudes from one system to another.

The formula to estimate Johnson's V magnitude (V) from SDSS g and i magnitudes is:

V = g - 0.5784*(g - i) - 0.0038

Here, "g" represents the SDSS g-band magnitude, and "i" represents the SDSS i-band magnitude.

Please note that while this formula provides a reasonable approximation, it might not be accurate for all types of stars or under all observing conditions. The accuracy of the conversion depends on the characteristics of the star's spectrum and the quality of the observations.

It's also worth mentioning that color transformations can vary between different studies and sources, so if you're working on a specific project, it might be a good idea to consult relevant literature or data reduction pipelines to find the most appropriate color transformation for your specific dataset.

Thank you

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American Association of Variable Star Observers (AAVSO)
What transformations does the Sequence Team use?

Hi All

Over the last few years since 2018 there's been some forum posts about what transformation system does the sequence team use to compute Rc and Ic magnitudes from the Sloan r and i magnitudes out of the APASS database.  I've seen Lupton (2005), Jester (2005) and Jordi (2006) referenced.

Could someone from the sequence team clarify what is currently used?



American Association of Variable Star Observers (AAVSO)
APASS R and I values

APASS has SR and SI data and offers R and I using the Jester transforms.

eg. In VPhot:

                    ' use Jester 2005 to compute R and I
                    Dim i, i_e, r, r_e As Double
                    If sr > 0 And si > 0 Then
                        r = v - 1.09 * (sr - si) - 0.22
                        r_e = Math.Sqrt(v_e ^ 2 + 1.09 ^ 2 * (sr_e ^ 2 + si_e ^ 2))
                        i = r - (sr - si) - 0.21
                        i_e = Math.Sqrt(r_e ^ 2 + (sr_e ^ 2 + si_e ^ 2))
                    End If