From: IN%"offiong@irg.usask.ca" "JJ Offiong" 13-NOV-1996 01:17:14.01
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Subj: RE: Please submit text for proposal sections
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Date: Tue, 12 Nov 1996 19:17:04 -0600
From: JJ Offiong <offiong@irg.usask.ca>
Subject: Re: Please submit text for proposal sections
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Hi David,
Here is my submission for the proposal on absorption lines. It is
basically a slightly re-worked version of the text file I gave to you
previously.
If you need anything else, do not hesitate to ask.
J
p.s. I noticed during your presentation last Wed, that you didn't
have me listed on the ground based data handling team. I am on that
team (just so you know...).
****
Likelihood of Using Stellar Absorption Lines as Wavelength Markers for
Spectra
J Offiong
It is well known that the spectra of many stars, i.e., those of
certain spectral classes, on or not too far off the main sequence,
resembles that of a blackbody. In addition to the Planck curve,
there are a number of spectral features caused by absorption due to
the stellar constituents. The absorption lines are often called
Fraunhofer lines.
In looking at the spectra of stars from various spectral classes over
a large wavelength range "short lists" of absorption lines can be
made. As an example, the spectral features of a solar type star
(spectral class G68V) are given. Clearly evident by inspection in
this spectrum are Calcium II H and K lines around 390 nm, the G-band
which originates from CH absorption around 430nm, Magnesium I at
515nm and Sodium D near 580nm. The exact values of these quantities
can be found in general literature (e.g., Allen: Astrophysical
Quantities).
The biggest difficulty will be the selection of lines. Ideally,
suitable lines will be both strong and isolated. However, the
currently proposed resolution of the instrument (about 5nm/pixel on
the CCD) will limit the number of usable lines. Fortunately, at
least for solar type stars, those listed above should prove strong
and "wide" enough to be detectable on at least two to three pixels of
the CCD.
In order to use an absorption line as a wavelength marker, it will be
necessary to determine the spectral type of the star from which the
light came. This will require the matching of a measured spectrum to
that of a well known spectrum. Traditionally, this has been done
manually, however, recent advances in artificial neural networks has
made automated spectral classification possible. This must be
investigated in the future. Also to be determined is an appropriate
"short lists" of usable absorption lines for different spectral
types.
In conclusion, it is appears that it will be possible to use
absorption lines for wavelength markers. Adequate features exist
that are of appropriate strength and width in the required wavelength
range. Still, more precise work has yet to be done to determine
exactly which lines will be detectable.
Refrences:
[1] Allen, C.W., Astrophysical Quantities, Oxford University Press
Inc., N ew York, 1963.
[2] Lehtinen, R., OSIRIS: A Technical Note on
Level 1 Wavelength Calibration, 1996.
__ "You're not going crazy, Arthur. You're going __
sane in a crazy world." - The Tick
J Offiong
Phone: (306)242-1972
E-Mail: <j@irg.usask.ca>