IRAC Channel 4 Preflash Suggestions
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Purpose of a preflash: To remove the increase in observed flux
as a function of stare time in 8 micron long-term photometric
monitoring observations. The ramp can be up to several percent
in magnitude. The flux asympotically increases and can take
several hours before it is below the threshold of
detectability/importance to the science data.

Basic idea: Stare at a bright extended (>10 arcsecond) target
for a long time (30 minutes) in channel 4. By staring the pixels
observing the bright source are conditioned so that the ramp
disappears and will stay so until the next anneal cycle (presumably).

Methodology:

1) Find a sufficiently bright (>2000 MJy/sr, 4000-5000 MJy/sr)
extended source like an UCHII or massive star forming region.
The region should have a minimal extent of 10 arcseconds as
that is the typical size of the source plus annulus (or PRF
fitting region) used in exoplanet observations. The less
structure in the source the better, but structureless
astrophysical sources are impossible to find. The source should
be extended and without a strong point source contribution. Two
sources that have been identified that fit the bill are M17 and
G75.3 UCHII.

2) Observe the source at the same position as the science target.
For sources at the center of the 4.5/8 micron array or 8.0 micron
subarray, this is easy. If the science target has been offset,
then make sure the preflash target is offset the same way (for
fixed cluster targets this is straightforward).

3) Use the maximum frametime that will not saturate in either
the subarray (which should be used if the target AOR is subarray
or the observer uses a full array AOR with the target offset to
the ch4 subarray position) or full array.

Why it works: One conjecture is that the 8 um ramp is caused by
very long time constant traps in the detector. There is a large
population of these traps which remove photons before they can
be detected. The preflash populates all of these traps allowing
all of the photons to be detected. This trap population may also
be responsible for the long term residual images observed in the
8 micron array. While plausible, this explanation does not explain
the ramp at 5.8 microns which is in the opposite direction (sources
get fainter with time), although the two arrays are very similar.
Effects of this type may vary significantly from array to array.

Known good preflash targets (use 0.4 subarray - preferred or
2 second full array). Longer full array frametimes may be more
effective in trap saturation, but are very likely to produce
signifcant residual images. Targets are ordered in goodness of
preflash (brightness, size, uniformity)

M17 preflash spot   18h20m28.01s, -16d12m20.1s
UCHII G75.3         20h21m39.45s, +37d31m04.3s