Hello all,
I have a question that falls in the same general category as the one
discussed earlier today (below).
I have run wavdetect on my ACIS-I field and created a source list. I have
also have computed psf's to determine the 95% encircled energy radii (as a
function of off-axis angle at 1.4967keV) that I will use for count
extraction. To determine appropriate fluxes for my sources I would also
like to correct for the differing effective area of the HRMA/ACIS-I, again
as a function of off-axis angle, using appropriate exposure maps. So,
(1) I am wondering if there is a "standard" effective area that I can use
to normalize my counts after I have adjusted them using the exposure maps.
(2) In part I am interested because I want to use the PIMMS tool to
convert counts/s to x-ray fluxes (erg/cm^2/s) and I imagine some effective
area is assumed by that tool. Does anyone know what effective area PIMMS
uses (at 1.4967keV) or where to find that information?
Thanks for any ideas,
Daryl
******************************
Daryl Haggard
Physics & Astronomy Dpt.
San Francisco State University
email: dhaggard@stars.sfsu.edu
tel: 415-338-6864
On Fri, 14 Jun 2002 kashyap@head-cfa.harvard.edu wrote:
> > Date: Wed, 22 May 2002 17:18:47 -0400 (EDT)
> > From: JunXian Wang <jxw@pha.jhu.edu>
> > To: <chandra-users@head-cfa.harvard.edu>
> > Subject: wavdetect detect source with 0.9 net_photons?
> >
> > Dear Colleagues,
> >
> > I runned wavdetect on a ACIS-I(0123) image using 10^-7 as
> > the sigthresh. It's wierd that I got one source with
> > only 0.9 net_photons. I also got several sources which
> > have less than 3 net_photons. I cann't believe these
> > sources are real. Have any of you met such problem?
> >
> > best
> > Junxian
>
> Hello Junxian,
>
> Peter Freeman and I looked at your dataset, and I am replying
> to your original mail here because others may be interested in
> how this issue panned out.
>
> First, the answer to the main question, from Peter:
>
> Peter> The "0.9 count" source was detected at (4395.5,4424.5)
> Peter> (logical coordinates [1596,1625]). The PSF here is
> Peter> estimated to be 1.26 pixels, so the relevant fluxscale
> Peter> is 1 (sigma = 1.0). The source itself was only detected
> Peter> at scale 8.0, and barely at that (visually the source
> Peter> looks fine, albeit weak).
>
> Peter> The source cell was constructed using a scale of 1 pixel
> Peter> for the smoothing function. Since this scale is too small
> Peter> for analyzing a source only detected at scale 8 pixels,
> Peter> the resulting source cell is too small, containing only
> Peter> a fraction of the source's counts (in this case, 1 count).
> Peter> Ergo net_counts is too small.
>
> So in summary, the source is real, but the flux estimate is not.
>
> Now, this does illustrate the danger of using a complex algorithm
> such as WAVDETECT as a black box. Let me reiterate that WAVDETECT
> is designed to be a source *detection* algorithm, and only secondarily
> as a source flux measurement tool. There are actually 9 counts that
> we can associate with this source if we were looking at it "by eye",
> enough for a reliable detection, but at a much larger scale than
> suggested by the PSF size. The *estimated flux* is however unreliable
> because of the sparse distribution of the counts in this apparently
> extended source.
>
> Note that flux estimation with WAVDETECT is generally quite reliable,
> but there is a small fraction of cases where the algorithm will fail
> to return useful source parameter estimates. This is inevitable.
> There is no such thing as a "perfect source detection algorithm."
>
> Peter also points out that:
>
> Peter> This situation is noted in the WAVDETECT paper. In section
> Peter> 3.2.2, we note "...situations where care must be exercised
> Peter> in interpreting results...the source cell for an [apparently]
> Peter> extended source may be too small if the smoothing scale
> Peter> is ~ [the PSF scale]..."
>
> Peter> The workaround for this is to rerun wavdetect with the
> Peter> parameter fluxscales changed from "1 2 3 4 5" to "4 5" to get
> Peter> a more proper accounting of the source in question.
>
> Regards,
> Vinay
>
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