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During observations with the Long-High (LH) module, the dark current may have
anomalously high values in the first 100-200 seconds of a series of exposures.
Depending on the integration time and the number of cycles selected, data in the
first exposure (or first nod position) may be affected. The "excess flux" is
not evenly distributed over the LH array, being brightest on the blue end
of each echelle order. In Figure 1 we show the difference between LH nod 1 and nod
2 spectra of a faint point source. The faint excess flux is seen as a bright
band stretching across the bottom of the array, both on and between the orders.
This phenomenon manifests itself in extracted spectra of relatively faint
sources as a "scalloping" or order tilting, wherein the slopes of affected
orders are made bluer (flatter), inducing order-to-order discontinuities. The
order-to-order discontinuities are typically 30-50 mJy. The magnitude of the
effect does not seem to be related to source brightness. In Figure 2 we show two extracted LH spectra of a faint point source. The top spectrum has the source
at the nod 1 position, while the bottom spectrum has the source at the nod 2
position. A background sky spectrum has not been subtracted in either case, but
the edges of the (color-coded) orders have been trimmed. The order mismatches
are obvious in the nod 1 data, but vanish in the nod 2 data.
If you have observations of faint point sources with LH, and you are seeing
these order tilts in your spectra, we recommend that you check your LH BCDs in
the first nod position (or during the first few hundred seconds of a series of
exposures), to see if this effect is more apparent. If order discontinuities
such as described above are evident in the first few exposures or the first nod
position, observers should consider not including these data during co-addition.
If your LH data do not show this effect, you can include all the data in your
final co-added spectrum. We are currently exploring ways to remove or reduce
these effects in the way we take and process the data.
Figure 1
Figure 2
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