[vsnet-campaign-be 247] Re: [vsnet-spectroscopy 20] Re: [amateur_spectroscopy] FWHM of Del Sco H Alpha Line

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

>I take into account focus into my FWHM calculations.  I first measure the 
>instrumental FWHM using a neon lamp.  Without changing anything, I then record 
>the spectra of the stars.  I can subtract out (using squares) the instrumental 
>FWHM which only leaves the component from the H alpha emission.


Still need to be careful doing that, Doug. 

I presume by saying "using square" you are subtracting the lamp 
line width derived from Neon lamp in quadrature (the lamp cannot 
be at an infinite distance; so that alone raises a "yellow" flag 
to me [of course, in the Department of Homeland Security scale]). 

But you can do that if and only if the blur is effectively described 
in a Gaussian form. So for bad focus, maybe ok. But if you have 
bad tracking that generates an oblique point spread function, you 
won't be adjusting your spectra for instumental effects properly. 
[I am assuming that you are doing a full-aperture prism spectro-
scopy here.]

Hence Michael suggests to measure the equivalent width, which is
constant since the integrated flux will be generally conserved 
even if focus or tracking are moderately bad. Of course this 
method may be insensitive to minute changes in its line profile. 

Bish 

-- 

"Bish" K. Ishibashi, Ph.D. 

Massachusetts Institute of Technology
Center for Space Research                 e  bish@space.mit.edu
77 Massachusetts Ave. NE80-6011           w  617 452 5122
Cambridge, MA 02139                       f  617 253 8084