From: Greg Aldering (aldering@panisse.lbl.gov)
Date: Sat Jan 17 2004 - 13:35:15 PST
Hi Gabriele,
We have noticed that there are wavelength errors in the spectra of 99ac.
In particular, the blue and red spectra a +11 days, and the red spectrum
at +16 days have very significant shifts. This was noticed because the
telluric features didn't line up (and would have been discovered had
a telluric correction been performed).
Below I list the spectra and the locations of the minimum of the
(deredshifted) telluric A-band. Due to resolution effects, the
minima can shift around a little (which is why correcting for
telluric features with standard stars with the same setup is
important), however, all the large shifts are in spectra with good
resolution so the offsets are real (I confirmed them by eye as well).
mw.sn99ac.CTIO4m.990329.r.16.dat 7532 <---- large offset
mw.sn99ac.APO3_5m.990226.r.-15.dat 7534 possibly due to low res
mw.sn99ac.CTIO4m.990315.b.2.dat 7536
mw.sn99ac.CTIO4m.990321.b.8.dat 7537
mw.sn99ac.ESO.990415.r.33.dat 7538
mw.sn99ac.ESO3_6m.990421.r.39.dat 7538
mw.sn99ac.KPNO4m.990406.b.24.dat 7538
mw.sn99ac.MDM2_4m.990304.b.-9.dat 7538
mw.sn99ac.ESO3_6m.990410.r.28.dat 7539
mw.sn99ac.NOT.990315.r.2.dat 7539
mw.sn99ac.CTIO4m.990329.b.16.dat 7540
mw.sn99ac.Lick.990424.r.42.dat 7540
mw.sn99ac.NOT.990315.b.2.dat 7540
mw.sn99ac.LICK3m.990413.r.31.dat 7543
mw.sn99ac.CTIO4m.990324.b.11.dat 7548 <---- large offset
mw.sn99ac.CTIO4m.990324.r.11.dat 7549 <---- large offset
Note that these offsets will affect your line velocity measurements,
as presented in the manuscript. This may also affect some of Gaston's
measurements.
Since the largest offsets all come from CTIO, where I was a observer,
perhaps I can comment. I only take arcs at the start and end of the
night, and so if there is flexure or some other small changes (like
changing a slit or cycling between gratings) this has to be corrected
using the night sky lines and then refined in the process of correcting
the telluric lines. In these cases, in my logs I note that I used
wider slits for 99ac on 990324 and 990329 because of large image motion
(the slit width is in the data header). So, there could be wavelength
shifts due the change of the slit, and also if the SN was not perfectly
centered in these wider slits. Only telluric features can detect the
later situation.
I recommend that a) all the wavelength zeropoints be checked using
night sky lines, and b) that telluric corrections be performed and
the spectra adjusted in wavelength if needed to align the telluric
features. Only then can the velocity measurements be trusted. You
should also check to see whether 99aa is affected.
Cheers,
Greg
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