HRC PI Filtering
Why do I need to worry about the HRC gain calibration?
While the HRCs (both -I and -S) have almost no spectral discrimination, they do have some. There is enough to distinguish between hard and soft sources. We have provided an RMF for this based on SAMP (Sum of AMPlifiers) PI . Though it is quite rudimentary and is subject to systematic uncertainties (e.g., gain non-uniformity at small scales, uncorrected gain droop, etc), it is better than the previous version that relied on PHA-based PI, and of course is better than nothing.
The same mild spectral response also allows us to filter out a large fraction of the background without unduly affecting the source intensity (see, e.g. HRC-I Background Spectrum thread).
To improve the signal-to-noise for weak sources and for diffuse extended sources, users can apply a PI filter such as
unix% dmcopy "hrc_evt2.fits[pi=0:300]" hrc_lowbackground_evt2.fits
This filter can be use for both HRC-I and HRC-S data. The same filter is mostly harmless for strong sources, though since background is not a significant factor may be omitted.See also : HRC-S Time-dependent Gain Corrections and PI Filter