Re: isis-1.0.8 is available

>If this is an on-axis source (or close to it), you may want to let the
>psffrac parameter vary between 0.90 and 0.95, assuming that you
>extracted counts in a 4 pixel radius about the source position.

>--John 

That's something to think about.  I'm finding that each additional parameter I 
let float increases the fitting time substantially, so I'm looking for 
parameters I can fix without a great impact on the fits.  In my case I'm 
dealing with pileup of under 10%, and extracted counts in a 4 pixel radius 
around an on-axis point source.  Most of my pileup will be in the 2-photon 
case, due to the low count rates, leading to (to first order) one 
energy-shifted spectrum of piled photons.  Two variable parameters determine 
its normalization--the alpha parameter and the psffrac parameter.  As the 
science doesn't actually depend on either of those values directly, it seems 
to me (correct me if I'm wrong) that allowing only alpha to vary will still 
correctly normalize the first-order pileup.  So even if psffrac is between 0.9 
and 0.95, I shouldn't go too far wrong in my spectral fits in freezing it if I 
let alpha vary.
  Thanks,
Craig