EIS radiometric calibration#

Lang et al. (2006) described the laboratory calibration of EIS and this is implemented by eis_prep. For the period 2011-2013 EIS users were recommended to use the /correct_sensitivity when calling eis_prep, which implemented an exponential sensitivity decay as a function of time that was uniformly applied to all wavelengths.

Re-assessments of the EIS calibration have been performed by G. Del Zanna and H. Warren, and these are described below along with software routines that implement the calibrations.

The new calibrations are still subject to testing and so eis_prep has not been updated. The recommendation is to run eis_prep without the /correct_sensitivity keyword (thus implementing the original laboratory calibration) and then performing a post facto correction with one of the methods described below.

Users are encouraged to report to the EIS team any problems that result from using the new calibrations.

G. Del Zanna method#

This method is described in Del Zanna (2013). If you derived a line intensity with the original lab calibration, then the corrected intensity is derived by multiplying by the factor

IDL> cal_factor=eis_ltds(date,wvl)

where 'date' is the observation date and 'wvl' is the wavelength.

If you had previously used the /correct_sensitivity option in eis_prep, then the correction factor is given by:

IDL> cal_factor=eis_ltds(date,wvl,/undecay)

Warren, Ugarte-Urra, and Landi method#

We have attempted to extend Giulio's method of using the atomic data to infer the shape of the effective areas by including information from the SDO/EVE instrument. EIS routinely makes full-disk mosaics (HOP130), which can be used to compute irradiances that be compared with those from EVE. Below is a comparison of the EIS and EVE irradiances for 195.

We then use very deep observations taken above the limb to fit a model that includes both an emission measure distribution and potential modifications to the effective areas. The effective area near 195 is fixed to lie on the EIS-EVE curve shown above. Here is an example of this applied to some off-limb spectra taken in 2007.

By repeating this analysis for several periods during the mission we are able to infer the effective areas as a function of wavelength and time. Details are given in a paper recently submitted to astro-ph.

There is software distributed in SSW that computes the new effective areas. Perhaps the most useful routine is eis_recalibrate_intensity.pro which takes an intensity computed with the pre-flight calibration and computes the corresponding intensity for the modified effective areas. One can also use eis_ea_nrl to compute the new effective areas as a function of wavelength and time.

Checking the calibration status of your existing data#

If you have level-1 data on your computer for which you'd like to apply the new calibrations, but you can't remember whether you used the /correct_sensitivity keyword or not, then do:

IDL> d=obj_new("eis_data", eis_file)
IDL> print,(d->getcalstat()).sens

The result will be 1 if /correct_sensitivity was used, and 0 otherwise.

If you use the windata structures, then check the value of windata.hdr.cal_sens.

Applying the new calibrations to the output from EIS_AUTO_FIT #

If you use the Gaussian fitting routine EIS_AUTO_FIT, then the recommendation is to apply the new calibrations at the EIS_GET_FITDATA stage. For example,

IDL> int=eis_get_fitdata(fitdata,/int,calib=1

will apply the Del Zanna (2013) calibration to the output intensity. The options are:

0 - the calibration is not changed
1 - the Del Zanna (2013) calibration is applied
2 - /correct_sensitivity is "undone" and the Del Zanna (2013) calibration is applied
3 - the Warren et al. (2014) calibration is applied
4 - /correct_sensitivity is "undone" and the Warren et al. (2014) calibration is applied

To summarize, the steps to be done are:

• Use EIS_PREP to calibrate the level-0 file with standard options (no /correct_sensitivity).
• Extract the windata structure using EIS_GETWINDATA with the /refill option.
• Perform the Gaussian fit with EIS_AUTO_FIT.
• Extract the intensity array with EIS_GET_FITDATA using the calib option set to 1 or 3 (Del Zanna or Warren).