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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