[vsnet-alert 10912] Re: re FWD ATEL 1892

[qso at Safe-mail.net]

-------- Original Message --------
From: Michael Linnolt <linnolt at hawaii.edu>

> Yikes, reliable extrapolation from r' to V is going to be quite poor using IR J/K color differences, and  I would not trust these conversions to any great extent, given all the variables involved with such a major color band shift.  Its tough enough to adjust to Johnson-V from slightly-off photographic visual band color differences even!<

Then feel free to not trust them.  Go used some half baked stuff instead because everyone else does.

Oops, wait a minute, maybes you're already using 'em, afer all AAVSO VSP includes kicking out V from CMC14 r' and 2MASS J-Ks based on a conversion formula from John Greaves, apparently.  Then again, it uses TASS Mk IV V too, so maybes not necessarily a recommendation.

Incidentally, to qualify the conversion, and follow on from what Brian responded to you, J-Ks has to be 0.0 to 1.0, r' twixt 9 and 15 ish, and the source Johnson V magnitudes I calibrated against were lots and lots, thousands, of Loneos.phot V mags from Brian's calibrated file thereof.  Calculated V minus loneos.phot V for the relation has a standard deviation of 0.06, ie calculated is within 0.06 of true 67%.  Two sigma it, 0.12 95% of the time.

Although Brian's final point is more salient, if you've an SDSS r' filter you can use, then substantial areas of sky can be calibrated quite nicely and tightly using CMC14 and SDSS r'.

There is no inherent reason extrapolating from r' to J and Ks is inherently poor just because you think there is, unless the object has NIR excess, which isn't that common away from the Galactic Plane.  Brian's point re interstellar extinction is generally applicable, as well as also applicable here.  Multiple object "ensemble" magnitude determination alleviates things further.

Cheers

John