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[vsnet-campaign-be 251] Re: [amateur_spectroscopy] FWHM of Del Sco H Alpha Line

  Focus is a concern in FWHM measurements because "W" and "M" are very
focus-sensitive.  Equivalent width measures - like any kind of 
spectoscopic measurements - are also affected by defocusing.  The S/N
is degraded and the instrumental profile becomes more and more affected
by defects in the optical train the worse the focus.  It is extremely
difficult - perhaps impossible - to take into account all defocusing 
effects on the instrumental line profiles.

   I couldn't agree at all that defocus is a non-issue.  It's sloppy 
lab technique to be careless about focus - or any other aspects of
one's instruments.  That kind of carelessness leads to bad - or, at
least, untrustworthy, science.

   The EWs of H-alpha in Delta Sco that Doug is measuring are pretty
close to what little other info I have, and the two values he gives
agree well within the stated errors.  As Michael suggests, It would
be an interesting experiment, though, to see how Doug's EWs are
affected by deliberate defocusing.


Regards,
Thom Gandet

Michael Koppelman wrote:
> 
> How about focus? The group I work with at the UofM measures what they call equivalent width, which is the width of the feature if it was squared into a rectangle. This makes focus a non-issue, or so I'm told.
> 
> This would be an interesting result.
> 
> Cheers,
> Michael Koppelman
> 
> On Saturday, July 12, 2003, at 08:32 AM, DWest61506@aol.com wrote:
> 
>      I measured the Full Width at Half Maximum (FWHM) of the H Alpha line on a spectrum of mine taken on 07/22/02 UT of del Sco and got 9.5 +/- 1.5 Angstrom.  I measured my most recent spectrum http://members.aol.com/dwest61506/page78.html taken on 07/12/03 and obtained a FWHM of 10.1 +/- 1.5 Angstrom.  Both measurements have been corrected for instrumental effects.  Based on the values and the error estimates I would conclude that there hasn't been any significant change in the FWHM of the H Alpha line in del Sco in the last year.