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At line 1 removed 2 lines
[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
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For full CCD spectra, a different approach is used. Sections of the CCD have been identified where there are no (or very weak) emission lines and so it can be assumed that the background here represents a measure of the CCD background. The regions are then averaged to yield the background level which is subtracted from the spectra.
!! Step 3: flagging hot pixels
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It is to be noted that both of these methods will yield some pixels with negative DN values. For window data it will only be 1 % of pixels because of the method used, but for full CCD spectra it can be up to 50 % of pixels if the raster contains very weak emission (e.g., coronal holes or off-limb spectra). ''The default mode for eis_prep sets pixels with zero or negative DN values to be missing data.'' This is because it is not possible to assign a photon noise error to the data points. (Note that negative DN values are expected since, if there are zero solar counts, then the uncertainty in the CCD counts will be 0 +/- read noise.)
By setting the /RETAIN keyword in eis_prep, pixels with zero or negative DN values will not be flagged as missing. The errors for the pixels will be set to the dark current error estimate (see Step 6).
!! Step 3: cosmic rays removal
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rays. The cosmic ray removal is performed by EIS DESPIKE,
a wrapper routine that calls NEW SPIKE, a routine developed
for removing cosmic rays from SOHO/Coronal Diagnostic
Spectrometer (CDS) data-sets ([Thompson et al., 1998|http://solar.bnsc.rl.ac.uk/swnotes/cds_swnote_46.pdf]; [Pike
& Harrison, 2000|http://adsabs.harvard.edu/abs/2000A%26A...362L..21P]). For CDS data processing it was typical
for not only the identified CCD pixels to be flagged,
but also the nearest-neighbour pixels on the CCD. This
is because there is often residual signal from the cosmic
ray next to the brightest pixels. EIS sees significantly less
cosmic rays than CDS apart from during the approximately 5 minute
passes through the South Atlantic Anomaly. As the most
useful function of EIS DESPIKE was actually to flag warm
pixels, and since warm pixels are only single pixel events, then
the nearest-neighbour option is usually switched off for EIS. It is to
be noted that the NEW SPIKE routine was designed to be
cautious when removing cosmic rays from line profiles, thus
possibly artificially enhancing the emission line intensities
at these locations.
!! Step 4: flagging hot and warm pixels
Both hot pixels and warm pixels are single pixels
that have anomalously high DN values. A hot pixel is defined to be one
that yields 25,000 electrons pixel-1 s-1 at
rays. Both hot pixels and warm pixels are single pixels
that have anomalously high DN values. A hot pixel is defined to be one that yields 25,000 electrons pixel-1 s-1 at
At line 72 changed 7 lines
data are referred to as 'warm' pixels. Separate maps of the locations
of hot and warm pixels are generated by the EIS team every 2-4 weeks
following inspection of 100 s dark exposures and they are
stored in Solarsoft. The pixel maps that are closest in
time to the science observation are used by EIS PREP to
mark the hot and warm pixels as missing data. \\
Please also check the post [Top and bottom of hot/warm pixels maps|TopBottomHotWarmMaps] for more details.
data are referred to as 'warm' pixels. Maps of the locations
of hot pixels are generated by the EIS team every 2-4 weeks following inspection of 100 s dark exposures and they are
stored in Solarsoft. The hot pixel map that is closest in
time to the science observation is used by EIS PREP to
mark the hot pixels as missing data.
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!! Step 5: flagging dusty pixels
!! Step 4: flagging dust pixels
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The next step for EIS PREP is to flag the pixels affected by dust on
Before removing warm pixels and cosmic rays, the next
step for EIS PREP is to flag the pixels affected by dust on
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!! Step 5: flagging warm pixels and cosmic rays
At the time of performing the present analysis,
EIS PREP did not specifically remove warm pixels, but
many of these were removed naturally by the cosmic ray
removal routine, EIS DESPIKE. This latter is a wrapper
routine that calls NEW SPIKE, a routine developed
for removing cosmic rays from SOHO/Coronal Diagnostic
Spectrometer (CDS) data-sets (Thompson et al., 1998; Pike
& Harrison, 2000). For CDS data processing it was typical
for not only the identified CCD pixels to be flagged,
but also the nearest-neighbour pixels on the CCD. This
is because there is often residual signal from the cosmic
ray next to the brightest pixels. EIS sees significantly less
cosmic rays than CDS apart from during the approximately 5 minute
passes through the South Atlantic Anomaly, and the most
useful function of EIS DESPIKE is actually to flag warm
pixels. Since warm pixels are only single pixel events, then
the nearest-neighbour option is switched off for EIS. It is to
be noted that the NEW SPIKE routine was designed to be
cautious when removing cosmic rays from line profiles thus
many weak warm pixels found within spectral lines are not
removed, artificially enhancing the emission line intensities
at these locations.
At line 96 changed one line
with an error estimate for the dark current. This varies from 2.24 to 2.37 DN for the four CCD quadrants.
with an error estimate of the dark current of 2.5 DN.
This page (revision-7) was last changed on 07-Dec-2016 14:14 by PeterYoungTop