What does eis_prep do?#

The EIS data presented here were all calibrated from the level-0 FITS files using the EIS PREP routine available in the Solarsoft1 IDL distribution. In addition to converting the measured CCD signal into calibrated intensity units, a key part of EIS PREP is to flag bad data points. These can arise through pixel saturation, cosmic ray hits, or simply defective pixels on the CCD. In this work all such pixels were marked as 'missing' data and not included in the data analysis. The central outputs of EIS PREP are two level-1 FITS files, one containing calibrated intensities at each pixel, and the other containing error bars on these intensities. The steps performed by the current version of EIS PREP at the time of this analysis are described below in detail.

Step 1: flagging saturated data#

The first step of EIS PREP is to flag any saturated data. The EIS CCDs have a 14 bit dynamic range and so satura- tion occurs at 16,383 data numbers (DN). All such pixels are flagged as missing as described above.

Step 2: dark current and pedestal subtraction#

In the raw data, the spectra are found to sit on a background of around 500 DN that arises principally from the CCD bias, and secondly from the CCD dark current. It is not possible to estimate the CCD bias level directly for EIS data, so the bias and dark current levels are estimated directly from the science data as follows. For each 3D data window 2 % of the detector pixels are isolated that have the lowest DN values. The median DN value of these 2 % pixels is then set to be the background level and it is subtracted from the DN values of each pixel.

Step 3: flagging hot pixels#

Anomalously bright pixels are found on the EIS CCD images that arise from 'hot pixels', 'warm pixels' and cosmic rays. Both hot pixels and warm pixels are single pixels that have anomalously high DN values. A hot pixel is de- fined to be one that yields 25,000 electrons pixel−1 s−1 at room temperature (a specification from the CCD manufacturer). Pixels that fall below this threshold but are still clearly identified as being anomalous when inspecting the 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.

Step 4: flagging dust pixels#

Before removing warm pixels and cosmic rays, the next step for EIS PREP is to flag the pixels affected by dust on the CCD. Several small pieces of dust accumulated on the CCD before launch and are found to completely block the solar signal on the CCD at their locations. They are fixed in position and cover less than 0.1 % of the CCD, however two of the pieces do affect the strong lines Fe XI 188.23, 188.30 and Fe XII 193.51 such that the lines can not beThe data " used over 15–300x10x1 spatial ranges in solar-Y." is not legal for a JDOM character content: 0x0001 is not a legal XML character.

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 ≈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. For the 2007 May data-sets analysed in the present work only around 2 % of the CCD pixels are warm pixels and so this is not a significant problem.

Step 6: radiometric calibration#

The final step of EIS PREP is to convert DN values into intensities in units erg cm−2 s−1 sr−1 A−1. The errors on the intensities are computed assuming photon statistics together with an error estimate of the dark current of 2.5 DN.