[vsnet-alert 10923] Re: re FWD ATEL 1892

[Michael Linnolt]

Ok, Well thanks a lot for clarifying all the conditions and assumptions under which this transformation should be good. I should add that when doing calibrations, it is important to include a sufficient range of different stars and observers/equipment to make sure you are acounting for many of the important variables, such as: Filter charcteristics, detector responses, optical system characteristics(especially coatings for such a wide range in wavelengths) Source emission characteristics, interstellar and atmospheric absorption differences between 0.5, 1.2 and 2.5u... etc.
 
Mike

----- Original Message -----
From: qso at Safe-mail.net
Date: Monday, January 5, 2009 3:08 pm
Subject: [vsnet-alert 10912] Re: re FWD ATEL 1892
To: vsnet-alert at ooruri.kusastro.kyoto-u.ac.jp

> -------- 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