Merging Central
Post-Merging Analyses
Using merged data sets.
PSFs
Source Detection on Merged Data: Choosing PSF Maps
One of the primary reasons to combine observations is to detect faint sources and the most commonly used source detection tools in CIAO (wavdetect and celldetect) use the size of the PSF in their algorithms. The Chandra PSF varies significantly across the field-of-view (from a fraction of an arcsecond to serveral arcseonds and the observed orientation varies with spacecraft roll angle). Therefore; there is no single PSF size when arbitrary observations are combined with different aimpoints and roll angles. The Running wavdetect or celldetect on merged data: choosing psffile thread demonstrates three different ways in which users can combine PSF maps from individual observations needed to run source detection and the effects the different approaches have on the source lists returned.
Analysing the PSF in Merged Datasets
PSFs in merged datasets should be an individual item with two possible threads...
- How to combine PSF for source detection Running wavdetect or celldetect on merged data: choosing psffile
- How to analyze the PSF in merged datasets (e.g. to study extended sources) — new thread, extract PSFs from individual observations then combine by weighting with exposure map (see notes and comments).
Calculating Counts and Fluxes
Co-Added Spectra and Responses
In general the merged observations should not be used for spectral analysis, i.e. to extract spectra and responses. The merged event list does not contain sufficient information to generate appropriate response files. The recommended technique for the spectral analysis case is to generate separate PHA, RMF, and ARF files for each observation and to analyze them simultaneously (also referred as "jointly").
The Extracting Spectra from Merged Datasets caveats page, explains in detail the problems related to extracting spectra from merged observations and in which cases "reasonable" results can be obtained.