eLSST Magnitudes

LSST Current Filter Information

The filters currently planned for LSST are u,g,r,i,z,y passbands. These are similar to SDSS passbands, but there are some differences, including the addition of a y band.

For more information on the filters (including plots) and instructions on how to download the filter throughput curves, please go to More Filter Info.

Calculating magnitudes

For most magnitude calculations, simply calculate you can simply multiply the SED (F_{nu<\sub>(Lambda)) by the Phi(Lambda) of each
bandpass.}

mag = -2.5*log₁₀(F/F_o)

with F_o=3631 Jy for AB magnitudes, and F is the integral \int(f_nu(lambda) * Phi(lambda) * dlambda).

For your convenience, we provide a set of text files of the filter transmission, and some python classes to deal with these telescope throughputs and your source SED --- you can use this class to calculate magnitudes for your object, along with magnitude errors.

Sed.py and Bandpass.py: Two python classes (Sed and Bandpass) to deal with the telescope throughput files and the SEDs for your objects. Will calculate magnitudes, let you add reddening, redshift the SEDS, etc.
The throughput files, download as described above (using subversion).
example.py: An example of some of the Sed and Bandpass functionality. More examples available here.
exampleSED.dat and exampleBandpass.dat: A sample SED and Bandpass to run through example.py.
Test this by : downloading Sed.py, Bandpass.py, example.py, exampleSED.dat and exampleBandpass.dat to the same directory. Then run 'python example.py'.

If you have any questions about the above code, please contact Lynne Jones.

Calculating Magnitude Errors

For Point Sources

The expected error of a magnitude measurement in N observations can be computed from

sigmaN = sigma1 / sqrt(N)

where the single observation error, sigma1, is

sigma1**2 = sigma_sys**2 + sigma_photom**2

Here sigma_sys is the systematic photometric error and sigma_photom is the random photometric error:

sigma_photom**2 = (0.04-gamma)*x + gamma*x**2
  with x = 10**[0.4*(m-m5)]

where m5 is the 5-sigma limiting magnitude depth for point sources, described here, and gamma depends on the sky brightness, readout noise etc.

gamma
u	g	r	i	z	y
0.037	0.038	0.039	0.039	0.040	0.040

We expect that sigma_sys will be 0.005 mag or smaller, once self-calibration is complete. A suitable pessimistic value may be sigma_sys = 0.01 mag.

Other Photometry References

Additional information on Phi, the reasons for using Phi rather than a percent transmission function, and how to apply this to magnitude measurements, can be found in the following papers:

Solving the global photometric self-calibration problem in LSST
Jones, R. Lynne; Padmanabhan, Nikhil; Ivezic, Zeljko; Axelrod, Timothy; Bartlett, James; Burke, David; Cinabro, David; Cr ze, Michel; Popescu, Bogdan; Saha, Abhijit, 2010

SDSS Standard Star Catalog for Stripe 82: the Dawn of Industrial 1% Optical Photometry
Zeljko Ivezic, J. Allyn Smith, Gajus Miknaitis, Huan Lin, Douglas Tucker, et al. (SDSS Collaboration), 2007

Precise Throughput Determination of the PanSTARRS Telescope and the Gigapixel Imager using a Calibrated Silicon Photodiode and a Tunable Laser: Initial Results
Christopher W. Stubbs, Peter Doherty, Claire Cramer, Gautham Narayan, Yorke J. Brown, Keith R. Lykke, John T. Woodward, John L. Tonry, 2010

Preliminary Results from Detector-Based Throughput Calibration of the CTIO Mosaic Imager and Blanco Telescope Using a Tunable Laser
Stubbs, C.W.; Slater, S.K.; Brown, Y.J.; Sherman, D.; Smith, R.C.; Suntzeff, N.B.; Saha, A.; Tonry, J.L.; Masiero, J.; Rodney, S, 2007

Toward 1% Photometry: End-to-End Calibration of Astronomical Telescopes and Detectors
Christopher W. Stubbs and John L. Tonry, 2006