ever wondered what age the universe is at redshift 6.7?
or how many nanoJanskys there are coming in the R band from a 23rd magnitude star?
well, a couple of web based calculators will tell you, rapidly and precisely
in cosmology, it is easy to get a crude estimate of time lapsed since the Big Bang, given the measured redshift, by assuming a pseudo-Euclidean universe, but often the precise time actually matters.
But, for a ΛCDM cosmology, the redshift-age formula is messy
Fortunately, Ned Wrigh at UCLAt has a handy dandy web Cosmology Calculator, which will give you the answer at the press of a button, for a standard WMAP cosmology, or your alternative cosmology of choice.
Oh, 823 Myr at redshift 6.7, in case you were wondering.
The Spitzer Science Center, in the meantime, has a Magnitude/Flux Density Calculator. And vica verse, of course.
For four different standard photometric systems, including Johnson UBVRI
A 23rd magnitude star, by the way, is 183 μJy in the Johnson R band.
Simple, but very useful.
I am assuming that the R band magnitude up there is a Vega, and not an AB magnitude.
The NICMOS unit conversion tool,
http://www.stsci.edu/hst/nicmos/tools/conversion_form.html
is also really useful for this kind of thing.
Cosmology Calculator is a fine thing, but magnitude calculators will only perpetuate magnitudes. Please consider joining the SCAAM: Society of Concerned Astronomers for the Abolition of Magnitudes.
http://kayhan.googlepages.com/scaam
Never!
Best Way To Obfuscate Physicists From Other Subfield.
Ever!
Look at the calculators under "Tools" at http://ned.ipac.caltech.edu/ Some hosted by NED, some links to other sites. Cosmology, extinction laws, and so on.
Kayhan, magnitude-to-flux conversion calculators are the only thing that will allow people to remain ignorant of magnitudes. Otherwise, they might have to actually learn them.
Converting AB mag to Janskys is so trivial even a theorist can do it, although not a physicist. One has to make these class distinctions somehow.
There's also the Cordes and Lazio Galactic free-electron density model an equatorial to Galactic coordinate converter, and a time format converter, plus various other things of that sort, for observers.
And I have to say, sexagesimal RA and Dec is another of those systems designed to separate the sheep from the lambs...