From: Rachel G. (gibbo@panisse.lbl.gov)
Date: Thu Dec 11 2003 - 09:21:57 PST
Hello Vallery,
Could you also send me which ground-based spectra
you used? I'm pretty sure I don't have the best reductions.
Rachel
On Thu, 11 Dec 2003, Vallery Stanishev wrote:
> Hi Rachel,
>
> Please find attached the ascii files I used for the figures.
> I found that 1 pixel shift to the red gives a better match to the
> ground-based spectra. I don't know where this might have come from.
>
> Cheers,
> Vallery
>
>
> Rachel G. wrote:
> > ---------- 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
> >>>>
> >>>
> >
> >
>
>
> --
> ********************************************************
> Dr. Vallery Stanishev
> Department of Physics, Stockholm University
> AlbaNova University Center, 106 91 Stockholm, SWEDEN
> tel: +46 8 55378757 fax: +46 8 55378601
>
> vall@physto.se
> vall_1@yahoo.com
> ********************************************************
>
>
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