A quick comparison of photometry values from the USNO B1.0 and that available on Supernovae.net

Given the following image:

http://www.supernovae.net/snimages/reference/n5194.jpg

If you take: the 14.9/17.5 star at about 1:30 from the core of M51

Now look at the USNO B1.0 mags for star ID=

:

B1=18.07/B2=16.70/R1=15.17/R2=15.22

'average' the B1.0 R and B mags:

B=17.385 R=15.195 then the differences are B~=0.1 and R~=0.3mag

which is in reasonable agreement with the 'accuracy' statement here:

http://xxx.lanl.gov/pdf/astro-ph/0210694

see: Section 5. Photometric Calibration

for the B1.0 and plates containing 'faint photometry standards' and the target star.

The kicker with this star is it's I mag 13.87 which kinda makes sense if you look at how much brighter in R than B the star is.

See this long url for the full B1.0 data on the star::

http://cdsweb.u-strasbg.fr/cgi-bin/VizieR-5?-out.add=.&-out.add=_RAJ2000,_DEJ2000&-source=I/284/out&USNO-B1.0===1372-0290320

The star ID is the last field in the url.

The BIC/RIC on the page is identified to be a '1' meaning the standard star and the target star are on the same plate.

Here is a small Excel 97 spreadsheet that uses the Blue/Red/Infared mags from the USNO B1.0 and

calculates the 'approximate' Vmagnitude:

It uses calculated flux for the magnitudes in B1.0 and interpolates to the Vmag based upon an exponential function fit.

I have found quite a few statements of filter vs wavelenght versus Jansky's. The spreadsheet above allows

you to change them and your changes will be used in the calculations if you do not like the ones from the referenced url below

Reference for math and constants below:

http://ircamera.as.arizona.edu/astr_250/Lectures/Lec13_sml.htm

Flter Wavelength Flux(Jy)

U 360 1880

B 440 4400

V 550 3880

R 700 3010

I 880 2430

Magnitudes cannot be added to get the total flux from two or more objects. First you must convert to flux then add the flux then convert back to magnitude.

Fluxes:

B=-2.5log(fB / 4400Jy) = -2.5log(fB ) + 2.5 log(4400Jy)

V=-2.5log(fV / 3880Jy) = -2.5log(fV ) + 2.5 log(3880Jy)

R=-2.5log(fR / 3010Jy) = -2.5log(fR ) + 2.5 log(3010Jy)

I= -2.5log(fI / 2430Jy) = -2.5log(fI ) + 2.5 log(2430Jy)

Or Flux = Jy*10**(m/(-2.5))