What does eis_prep do?

What does eis_prep do? Dr Peter Young Rutherford Appleton Laboratory, UK Dr Peter Young, Rutherford Appleton Laboratory

eis_prep • eis_prep takes a level-0 EIS FITS file and calibrates it • The output is a level-1 EIS FITS file • Pixel intensities are stored in data number (DN) units in the level-0 file • In the level-1 FITS file they are stored as erg cm-2 s-1 sr-1Å-1 • In addition to the level-1 file, an additional error file is created • The error file contains 1σ errors on the intensities Dr Peter Young, Rutherford Appleton Laboratory

‘Missing’ data • Some pixels in the data should not be used for analysis, e.g., • saturated pixels • defective pixels • cosmic rays • These pixels are flagged by eis_prep by giving them the value -100 in the error FITS file (not the level-1 file) Dr Peter Young, Rutherford Appleton Laboratory

eis_prep: Step 1 • The first thing eis_prep does is flag any saturated pixels as missing data • Saturated pixels have a value of 16,383 DN (214-1) Dr Peter Young, Rutherford Appleton Laboratory

eis_prep: Step 2 • The CCD pedestal and dark current are removed • For most data-sets the method is as follows: • take specified 3D window array • find 2 % of pixels with the lowest DN values • find the median value of these 2 % pixels • subtract this value from every pixel • Around 1 % of pixels will end up with a negative DN value after this process • these pixels will be flagged as missing Dr Peter Young, Rutherford Appleton Laboratory

Warm and hot pixels • The EIS detectors exhibit a number of pixels that yield anomalously high DN values • The brightest of such pixels are termed “hot pixels” • All others are referred to as “warm pixels” • These pixels can have different characteristics: • some are permanent • some last for several weeks and then disappear • some flicker Dr Peter Young, Rutherford Appleton Laboratory

Warm and hot pixels, and cosmic rays • The EIS cosmic ray removal routine (eis_despike) flags most of the warm and hot pixels Cosmic ray Hot pixel Warm pixels Image cleaned with eis_despike Raw data Dr Peter Young, Rutherford Appleton Laboratory

Warm and hot pixels, and cosmic rays • Warm and hot pixels lying within a line profile are often not flagged by eis_despike • For these, it is necessary to flag them by using warm and hot pixel maps • The maps are created using engineering studies and stored in Solarsoft • Hot pixel maps: • $SSW/hinode/eis/data/cal/hp • Warm pixel maps: • not implemented yet Dr Peter Young, Rutherford Appleton Laboratory

Step 3: HPs, WPs and CRs • Step 3a: • use hot pixel map to flag hot pixels • Step 3b (not implemented yet): • use warm pixel map to flag warm pixels • Step 3c: • flag cosmic rays using eis_despike • (note: eis_despike calls the CDS routine new_spike) • All flagged pixels are marked as missing Dr Peter Young, Rutherford Appleton Laboratory

Warm pixels and hot pixels • The numbers of WPs and HPs on the CCDs are increasing with time • There are many more WPs then HPs • Currently (Oct 2007) around 4 % of the CCD pixels are WPs • In two years time predicted to be 12-15 % Louise Bradley, MSSL Dr Peter Young, Rutherford Appleton Laboratory

Step 4: calibrated intensities • DN values are converted first to photons • Photons converted to erg cm-2 s-1 sr-1Å-1 using EIS effective area curves • Curves derived before launch in lab. calibration • Curves available in: • $SSW/hinode/eis/response Dr Peter Young, Rutherford Appleton Laboratory

Step 5: errors on intensity • Fractional errors given by: • 2.5 is an estimate of the CCD read noise [ (Photons) + (2.5)2 ]1/2 / Photons Dr Peter Young, Rutherford Appleton Laboratory

What does eis_prep do?