3.4  Cosmic ray removal

See CDS software Note 46: Missing Pixels and Cosmic Rays for details.

An effect that strongly constrains the observations is the presence of cosmic ray hits. Exposure times cannot be too long, otherwise cosmic rays literally wipe out the use of large fractions of the spectra. Various software routines are available to automatically `flag' cosmic ray hits with `missing' values, but an interactive check is highly recommended, although this takes a very long time if the number of exposures and extraction windows is large.

An useful GUI is XCDS_COSMIC which allows to use e.g. CDS_CLEAN_EXP (OK for pre-recovery not Y-binned data) or CDS_CLEAN_SPIKE

Important NOTE: you will need to have defined $CDS_EXTERNAL to point to a compiled version of external.f if you are going to use CDS_CLEAN_EXP.

xcds_cosmic,qlds

Figure 3.6.1 shows some cosmic ray hits flagged as missing with this software. Often, the number of cosmic ray hits is such that a line profile in one exposure is lost and no line intensity can be deduced. Therefore, it is highly recommended to raster small regions on the sun a few times, clean the single spectra from the cosmic rays, and then average the spectra. In this way, the effect of the cosmic rays is removed (unless cosmic rays hit the same emission line at the same spatial position along the slit). Another way of reducing the effect of the cosmic rays is to spatially average the line profiles.

Important note: the standard CDS_CLEAN_IMAGE does not work properly for Y-binned data, nor when strong brightenings (or small flares) are present in the data.

Data must be visually inspected.