[vsnet-chat 5092] Re: eta Carinae

[Bish Ishibashi <bish@space.mit.edu>]

Hi Stan and Brian, 

It's definitely prudent for any CCD observers to report their
brightness measurement of an extended source all the time. But
since the message is addressed to me and I knew what aperture 
Harry has been using, I reckoned it's just a Freudian slip. 

For those who are not exactly familiar with the object, eta Car
is roughly about 16" long along the major axis. So the aperture
size of 30" will include most of (if not all) its bright nebular
features. Like Stan says in his message, it probably does not 
matter (as long as one is careful) whether the aperture size is
60" or 30". Take a look at Figure 2 of Davidson et al. 1999, AJ, 
118, pg.1777; it demonstrate that the change in aperture size by 
the Auckland team did not produce any inconsistency (c.f., in the
same figure, notice the inconsistency introduced by Sterken et al. 
after changing the filter system). 

In any case, absolute photometry of this object is very difficult. 
I don't have much better idea what comparison stars should be used
for this target either. But I'd emphasize strongly that the relative
change in brightness, taking in a consistent manner, is what I would
consider as the most crucial measureable quantity. The uncertainty
of measurements are hard to digress, but +/- 2% photometry is fairly
darn good enough to judge the validity of any brightening by >0.2 mag
or so. That's something a CCD system can do very well (though somehow
Sebastian Otero does it well without one). 

Cheers, 

Bish 

PS. Congrats, Stan.

-- 

"Bish" K. Ishibashi, Ph.D. 

Massachusetts Institute of Technology
Center for Space Research                 e  bish@space.mit.edu
77 Massachusetts Ave. NE80-6011           
Cambridge, MA 02139

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