------ Start of attached email. Subject: [spectro-l] Alnilam Spectrum, 2 nights ------
Earlier this week I took a couple of spectra of Alnilam (Epsilon Orionis).
My intent was to use it as a flux calibration reference, but when I
processed it I realized that this is an emission star.

In comparing the two nights' spectra, I noticed that there was a slight
difference. The H-alpha emission is clearly stronger in the Jan 10 data
compared to Jan. 8, but on closer inspection it seems that the entire
spectrum on Jan 8 is subdued. I'd appreciate any help in understanding what
might cause this.

On the advice of Olivier Thizy I did double-check my sky subtraction (I used
IRIS), it appears to be o.k. (thanks Olivier!)

A couple of possibilities that come to mind are:

1. they were not taken at the same airmass, the Jan 10 data was higher
airmass (avg. 1.42) than the Jan 8 data (avg. 1.2). But this is a narrow
region at the red end of the spectrum. I tried to correct using vSpec's
Extinction function, that didn't solve the differences.

2. The moon was at a brighter phase on Jan 8 (~79% vs. 62% on Jan 10), and
had already risen for the Jan 8 data but was below the horizon for Jan 10.
But I thought sky subtraction would take care of most of that? Plus I was
under the impression that moonlight was not significant at H-alpha.

3. Maybe there was just poor conditions in the sky on Jan 8 and I didn't
realize it. But actually there were clouds in the sky on Jan 10 so I
thought that was the worse night, not the better one.

4. Maybe it's just noise or other error, two nights certainly isn't enough
to show a trend.

Does anybody have any other ideas, or comments on these possibilities?

I'll try to attach an image showing the spectra to this message in jpeg
format, if it does not come through you can see it at this link:

http://www.xanaduobservatory.com/type_b_stars.htm

Clear Skies!

Keith Schlottman

------ End of attached email ------

------ Start of attached email. Subject: [spectro-l] Alnilam Spectrum, 2 nights ------
Earlier this week I took a couple of spectra of Alnilam (Epsilon Orionis).
My intent was to use it as a flux calibration reference, but when I
processed it I realized that this is an emission star.

In comparing the two nights' spectra, I noticed that there was a slight
difference. The H-alpha emission is clearly stronger in the Jan 10 data
compared to Jan. 8, but on closer inspection it seems that the entire
spectrum on Jan 8 is subdued. I'd appreciate any help in understanding what
might cause this.

On the advice of Olivier Thizy I did double-check my sky subtraction (I used
IRIS), it appears to be o.k. (thanks Olivier!)

A couple of possibilities that come to mind are:

1. they were not taken at the same airmass, the Jan 10 data was higher
airmass (avg. 1.42) than the Jan 8 data (avg. 1.2). But this is a narrow
region at the red end of the spectrum. I tried to correct using vSpec's
Extinction function, that didn't solve the differences.

2. The moon was at a brighter phase on Jan 8 (~79% vs. 62% on Jan 10), and
had already risen for the Jan 8 data but was below the horizon for Jan 10.
But I thought sky subtraction would take care of most of that? Plus I was
under the impression that moonlight was not significant at H-alpha.

3. Maybe there was just poor conditions in the sky on Jan 8 and I didn't
realize it. But actually there were clouds in the sky on Jan 10 so I
thought that was the worse night, not the better one.

4. Maybe it's just noise or other error, two nights certainly isn't enough
to show a trend.

Keith Schlottman

In einer eMail vom 13.01.2007 06:55:12 Westeuropäische Normalzeit schreibt
Keith Schlottman:

Earlier this week I took a couple of spectra of Alnilam (Epsilon Orionis).
My intent was to use it as a flux calibration reference, but when I processed
it I realized that this is an emission star.

I-

Komisch. Bei einer kurzen Nachmessung des Sterns heute Abend fand ich keinerlei Ha-Emission im gflatten B-Spektrum des Sterns (der mittlere Gürtelstern von Orion).

Lothar