Yes, I have seen it. The bright source is dithering across a CCD node
boundary or gap. The differences in the gain or event threshold
will be sufficient to change the count rate of the source on the
dither period in the Y direction, which is 1000 s, exactly. The
dither period in the other direction is 707 s, which wouldn't be
noticed unless the source falls off of the detector or hits a bad
row periodically.
If it's dithering across a node boundary, limiting to a specific
energy range that avoids the soft end should help reduce the
variation. If it is dithering across a chip boundary, you will
find it very difficult to eliminate the periodicity because of
the QE differences between BI and FI chips.
Herman Marshall
Chandra HETGS Calibration Scientist
CXC, MIT
--- > From owner-chandra-users@head-cfa.harvard.edu Tue Jul 9 15:24:59 2002 > Date: Tue, 9 Jul 2002 15:23:50 -0400 (EDT) > From: Jifeng Liu <jfliu@astro.lsa.umich.edu> > To: chandra-users@head-cfa.harvard.edu > Subject: ACIS-S3: a spurious period of exact 1000 seconds > MIME-Version: 1.0 > Sender: owner-chandra-users@head-cfa.harvard.edu > Precedence: bulk > > Dear Colleagues, > i am examining a ACIS-S3 50ksec exposure, and find a period of exact 1000 > seconds. I use wavdetect to get all source regions with default ellipsigma > setting, then i pick out some bright sources with Nphoton > 2000, and use > XRONOS to carry out timing analysis, and find that a source with 43000+ x-ray > photons shows a period of exactly 1000 seconds, the folded light curve shows a > sine curve, and varying amplitude is about 20%. the other bright sources, and > the background events do not show such periods. This period is so EXACTly > 1000.00 seconds, that i cannot help thinking it's some kind of artifacts. > Have any of you met such problems before? > > cheers, > -- > jifeng > >
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