Since you are this close, perhaps the rest is in the digitization. Pixels
don't fit exactly into a circle so you get round-off as you compute the area
of a circle with radius 8 pixels.
> Hi Sandy,
>
> Almost, but not quite. BACKSCAL is 2.99679E-06.
>
> Dave.
>
> > Hi Dave,
> >
> > It looks like the total detector area is defined* to be 8192 x 8192
> > pix^2 (2^13=8192). If you look at the *evt2.fits in ds9 and block by 64
> > and then zoom as needed you can convince yourself this is the case.
> >
> > So now we have ...
> >
> > calc '8*8*pi/(8192 * 8192) '
> > = 2.99606e-06
> >
> > calc '2.99606e-06*10.66'
> > 3.1938e-05
> >
> > Is 2.99606e-06 the value you are getting for BACKSCAL?
> > I checked another case and it looks like this is how BASCSCAL is defined
> > if you extract from the raw events list.
> >
> > *I have no idea why, other than to fit the entire I and S
> > array into these dimensions with either I or S aimpoint on center
> >
> > Hope this helps-
> > Sandy Patel.
> >
> >
> >
> > -----------------------------
> > Sandeep K. Patel
> > UAH/NASA-MSFC SD50
> > X-ray Astrophysics Branch
> > W: (256) 544-3965
> > patels@dante.msfc.nasa.gov
> >
> > On Thu, 31 Aug 2000, Dave Smith wrote:
> >
> > > Hi,
> > >
> > > I have extracted spectra using dmextract and a source region file, and
> > > wish to know how the BACKSCAL parameter is derived in the resulting
> > > spectral file. My understanding is that the BACKSCAL parameter is the
> > > area of the region divided by the total area of the detector. So, for a
> > > circle of radius 8 pixels,
> > >
> > > BACKSCAL = 8^2 * pi / ( 6 * 1024^2 ) = 3.19E-5 (ACIS-S configuration).
> > >
> > > This is a factor 10.66 larger than the actual BACKSCAL keyword value. Is
> > > there something I have misuderstood?
> > >
> > > Dave.
> > >
> > >
> >
> >
>
>
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