[vsnet-alert 10904] Re: possible new cataclysmic variable in Ursa Major

[qso at Safe-mail.net]

Brian Skiff lamented :-

"It is unfortunate that the astronomical literature can be misleading, but that is how it is."

There is no literature on NOMAD, just an AAS abstract, the astronomical community has utterly and completely misled itself on this catalogue.  Nor are there any tangible details on how it was compiled either astrometrically or photometrically, or how YB6 was done, and the readme even says it isn't a compiled catalogue (meaning not a critically compiled catalogue, I surmise, as it most evidently is a compiled catalogue, coz it's a bunch o' cats shoved together, a chimera of a thing, or maybe they have some weird definition of compiled, coz they say Tycho2 and UCAC2 are compiled catalogues, which they aren't, their observational catalogues, except within the latter's proper motion bumf maybes compiled, but Tycho2 is essentially, and mostly, data processed from the Tycho experiment).

NOMAD remained moribund on the USNO NOFS server, unused, for several years, until it was bunged on VizieR, since when some quarters scream it's a better astrometric catalogue than its constituent parts (infeasible) and some others think B-V from YB6 is photometrically precise.

http://www.nofs.navy.mil/nomad/nomad_readme.html

And folk will swear blind it's better than anything else and use it in papers and iauc and cbet and atel, up to the most respected of observational professional astronomers, so it's no wonder the rest of the planet falls for it too.

Moral of the story : read the papers behind an astrometric release, note the caveats mentioned therein (there's always some) look around at what others use on relevant lists or groups (not that safe, given the prevalence of its use, coz a lot of folk just like its convenience, there's a lot of that) and do some dummy tests on known objects, constant ones if photometric, ones with known values, and see what the thing you're using gives.  Ideally, do not use any astrometric catalogue for photometry, unless it's CMC14 (so there's no hard and fast rules, you see, everything has to be assessed on its own merits).

Incidentally, Brian don't seem to have included the link to a page worth of simple formulae for 'transforming' the ol' SDSS g' and r' mags to the older still Johnson V :-

http://www.sdss.org/dr6/algorithms/sdssUBVRITransform.html

pick and choose, and maybes if any V is generated via this it's a thought to quote the method used.  That page even has the paper references, which is nice.

Cheers

John

it's a funny ol' universe