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!
That's a very easy Google search:
The usual advice in the literature is to use Katrin Jordi's transformations, though they all give very similar values.
You can try combining two relations (g+r and r+i), or do your own new derivation with some set of stars like the M67 standards, Landolt stars, etc.
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
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...
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, 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?)
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.