[Aavso-photometry] ASAS182612 Campaign & CIs of Comparisons

[arne]

Michael Sallman wrote:
> Arne,
> 
> Is there a preferred way to deal with these "ghost images"? Maybe take a
> couple of darks after a saturated image?
> 
> Thanks,
> Mike SVI
> 
> arne wrote:
> 
>>Most
>>front-illuminated CCDs will also show ghost images (called residual
>>bulk image) if you saturate a star while observing unfiltered or with
>>a red filter, and the ghost image will hang around for several subsequent
>>images.  If you changed fields, or did a GEM flip, the residual image
>>might affect an important star.  So be careful, think ahead, but don't
>>be afraid of saturating a field star.

Residual Bulk Image (RBI) is a common artifact with front-illuminated
CCDs, but the amount of ghosting depends on the detector.  The basic
problem is that long-wavelength photons penetrate deeper into the
silicon and create hole-pairs outside of the pixel electric field.
The electrons then drift into the nearest pixel through time.  It seems
to only occur on saturated images, and is more noticible the colder
you run your chip.  If such a ghost image is objectionable, then
you can (a) wait until the charge migrates back into the pixel, which
can take an hour or so; (b) warm the chip up and then cool it back
down; (c) do an IR "flood" and readout the chip several times to
fill the epitaxial traps that cause the problem.  My preferred solution
is to never saturate the target star (which almost always is in the
center of the field), which at least keeps any ghost image away from
the field center.  Usually the effect is only a percent or two of
the peak pixel value, and only is seen for Ic or unfiltered images.
Arne