From: Rachel G. (gibbo@panisse.lbl.gov)
Date: Wed Dec 10 2003 - 18:03:15 PST
---------- Forwarded message ----------
Date: Wed, 10 Dec 2003 16:49:24 -0800 (PST)
From: Rachel G. <gibbo@jimbean.lbl.gov>
Reply-To: Rachel G. <ragibbons@lbl.gov>
To: Chris Lidman <clidman@eso.org>
Cc: Vallery Stanishev <vall@physto.se>, Rachel G. <ragibbons@lbl.gov>,
scpexec@lbl.gov, Andy Howell <howell@astro.utoronto.ca>
Subject: Re: Vallery's analysis of grism spectra
Hi Chris and Vallery,
One thing Vallery and I really should also do is
compare our reductions with each other -- especially since
we reduced the data somewhat differently. Vallery, if you
could send your spectra to me, that would be good. I should
find time over the next few days to look at this once
another version of our observing plan for the HST search
goes out (should happen Friday).
Thanks,
Rachel
On Wed, 10 Dec 2003, Chris Lidman wrote:
> Opps, I forgot the attachment.
>
> C.
>
> Vallery Stanishev wrote:
>
> > Dear All,
> >
> > The document I wrote was not meant to be a complete description
> > of the reduction or complete analysis. And apparently there
> > are some points that are not clear.
> >
> > About Rachel's comment #2. Yes, I first flat-field the images,
> > and then subtract the background and extract the spectra.
> >
> > About #3. Before combining Riess' images I shifted them
> > according to what is given in the fits headers. Here by
> > 'shift' I do mean sub-pixel shift. I did this with IRAF's
> > IMSHIFT, which allows sub-pixel shifts.
> > Perhaps this is not the best way to handle this, but since
> > the software to deal ACS/grism dithered images is not
> > out yet I used IRAF.
> >
> > About the dithering, sub-pixels, etc.. Riess' images were
> > dithered by a few pixels only. On all 16 images the SN was
> > within a square of 20x20 pixels. I also don't know if this
> > is large enough to start seeing differences in the wavelength
> > calibration (but this may be tracked down form the field dependence
> > of the wavelength solution given in the aXe's conf. file).
> > My guess would be that this 20-pixels shift is
> > fairly small to affect the wavelength calibration and hence
> > the flux calibration of Riess' spectrum.
> >
> > I agree that the point of doing sub-pixel dithering is
> > to improve the resolution. If the software to handle such
> > data is about to be released we can use sub-pixel dithering.
> > However, I'm not quite sure that the spectral resolution is
> > of big concern for us. Given the broad SN features, especially
> > at z>1, a spectral resolution of ~90A seems to be enough to type the
> > object and to get the redshift with reasonable accuracy.
> > Moreover, for these very faint high-redshift SNe the S/N
> > will not be high and we shouldn't hope to get something
> > more than that. I would be more concerned about bad pixels
> > and in this aspect I think that integer pixels dithering
> > should be enough.
> >
> > Regards,
> > Vallery
> >
> > Rachel G. wrote:
> > >>As Vallery points out, there is a discrepancy with Adam's spectrum,
> > >>especially on the red end. Please send comments to Vallery and cc to
> > >>this mail list - hstsearch@lbl.gov.
> > >
> > >
> > > Two immediate comments on Vallery's conclusions
> > > regarding Riess' data:
> > >
> > > 1. We should be able to handle sub-pixel dithered data. It
> > > isn't an easy problem, but there is existing software that
> > > can properly handle such data. I can look into this.
> > >
> > > 2. The background subtraction is an important step.
> > > Remember flat-fielding is wavelength dependent, so the
> > > background you subtract has to be flattened properly as
> > > well. This could very well be the source of some of the
> > > differences with Riess' reduction (along with #1).
> > >
> > > 3. The correlated noise you see in your shift and add
> > > of Riess' data cannot be directly compared to a straight
> > > sum. If you simply sum Riess' data ignoring image shifts,
> > > you should find the data have better SNR than ours (although
> > > they could have been taken when background levels were
> > > higher). But even in the present sum, one can see Riess'
> > > spectrum has more interesting structure (features) than do
> > > ours.
> > >
> > > Definitely summing sub-pixel dithered data has to be
> > > handled more carefully.
> > >
> > > In addition, Given there looks to be a bit of a lull
> > > in the search planning (for a few days perhaps) I will
> > > finish my write-up of the reductions and analysis I did.
> > > The details of how this stuff is done is important and I
> > > suggest we compare results before moving on.
> > >
> > > Rachel
> > >
>
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