From: Rachel G. (gibbo@panisse.lbl.gov)
Date: Wed Dec 10 2003 - 18:03:34 PST
PS I do have a plot overlaying the HST and ground-based
spectra already, let me find that.
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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