From: Vallery Stanishev (vall@physto.se)
Date: Thu Dec 11 2003 - 01:33:28 PST
Hi Chris,
Please find attached a plot of the gound-based spectra of
SuF02-012 & 60 with the ACS/grism spectra overplotted. I would concider
the agreement between them as good.
Cheers,
Vallery
Chris Lidman wrote:
> Hi Vallery,
> Your report is interesting. I'd like to see a couple of extra plots
>
> For SuF02-060, a direct comparison of the FORS2 and ACS spectra, i.e. plot
> one on top of the other. In the current plot it is hard to tell how good the
> agreement is.
>
> Similarly, for SuF02-012, a direct comparison of the FORS2 and ACS spectra.
> The caption to figure 2 says that the former is shown, but I could not see it.
> It is worth noting that Andy's program came up with a type Ia at
> z=1.3 as a possible match for the FORS2 spectrum of SuF02-012, but neither Andy
> nor myself were confident enough to claim it. For your interest, I attach a PostScript
> file which shows the comparison.
>
> Cheers, Chris
>
>
> 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 55378601vall@physto.se vall_1@yahoo.com ********************************************************
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