From: Gabriele Garavini (gabri@physto.se)
Date: Mon Jan 19 2004 - 09:54:21 PST
Hi
thank you very much for notice that. I'll go back to the reduction files
and check out which is the origin of the problem. I was immediately
concerned about the expansion velocities of course (that appear peculiar
in 99ac), but the shift we are speaking about may not be the only
responsible for their peculiarity. I think the observed shift is at most
10 A in the minimum of SiII this propagates to 500 km/s. The velocities of
99ac are lower than expected by more than 2000 km/s. But definitely I have
to correct for that. I'll report to you on the origin of the problem
asap.
Thank you again
Cheers
Gabriele
On Sat, 17 Jan 2004, Greg Aldering wrote:
> 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
>
>
-- _____________________________________________________________________________ Gabriele Garavini (gabri@physto.se) FYSIKUM-Stockholm University Roslagstullsbacken 21 S-106 91 Stockholm SWEDEN Phone:+46 8 55378661 Fax:+46 8 553 78 601
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