Wide Field Camera 3
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Posted by Matthew A. Greenhouse on June 17, 1999 at 15:07:23:

I looked over the WFC3 filter list on the web and noticed a note about Pa
Beta that is wrong. It says that Pa Beta is not necessary if Pa Alpha is
included. The value of the Pa Beta filter is that, when combined with the
J-band [Fe II] filter, one can form a Fe/H line ratio that is reddening
independent due to the short wavelength lever arm between these lines.
Since the extinction within the [Fe II] emitting regions (not to be
confused with foreground extinction) is typical poorly constrained, having
access to the above ratio is critical to extracting a good measure of the
amount of gas phase Fe.