From: Rachel G. (gibbo@panisse.lbl.gov)
Date: Thu Apr 08 2004 - 17:55:20 PDT
Hi Andy,
Properly calibrating will probably involve some
iterative process, perhaps maximizing the SNR along the SN
spectrum in addition to those of nearby brighter objects
(luckily, there are a few). I'm hoping bright objects can
help us determine how well we've minimized this error if we
can determine what their SEDs are. This, together with the
fact that if the SN features themselves are at all
recognizable in the ratty spectrum we expext, they are
hundreds of Angstroms wide, so even a less careful alignment
and calibration will result in something we can use.
Unfortunately, it'll be a few days before I can start to
think hard about this. I have a few ideas, but for the next
couple of days will probably focus on simpler tasks like an
IAU circular. In fact, I'm ready for a nap now.
Rachel
On Thu, 8 Apr 2004, Andrew Fruchter wrote:
> Adam told me he had a direct image in every visit.
>
> Your Phase II relies on a sole pointing for the offset between the
> grism and the direct image. We can probably align the grism spectra
> using something like tweakshifts. The only problem then may be a
> small error in the spectral zeropoint (of order a fraction of an ACS
> pixel -- perhaps 30A if we are not particularly lucky).
>
> Andy
>
> On Apr 7, 2004, at 3:26 PM, Rachel G. wrote:
>
> >
> > Hi Andy,
> >
> > I realized this would mean we'd have to think more, as
> > opposed to using the Pirzkal et al. software in the
> > straightforward way with the direct image and so forth. We
> > do have some of Adam's data from last year, and I'd planned
> > to start figuring out how to deal with it as soon as I'm
> > back in Berkeley. Vallery Stanishev in Stockholm will also
> > be working on this.
> >
> > Have you heard or do you have a sense for how large the
> > calibration errors that will creep in might get? Because, I
> > also thought Adam worked in this same mode. He could not
> > have done all his observations in one Visit. And, as you
> > know, his results look super.
> >
> > Ciao,
> >
> > Rachel
> >
> > On Wed, 7 Apr 2004, Andrew Fruchter wrote:
> >
> >> Hi Searchers,
> >>
> >> I have just approved visits F0-F5 for observation. However, you
> >> should
> >> know that you are using a very non-standard approach to grism
> >> observations. You are not taking a direct image in each visit (indeed
> >> the standard mode takes one associated with each dither position).
> >> This opens you up to the possibility of sub-pixel shifts between the
> >> direct image and the grism spectra.
> >>
> >> In principle one generally should not need a direct image. One should
> >> be able to use the zeroth order image (and this could be a more
> >> accurate way to align spectra in many cases). However, the
> >> calibration
> >> work necessary for this approach to be used has not been done, as far
> >> as I know.
> >>
> >> Cheers,
> >> Andy
> >>
> >
>
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