{"id":754,"date":"2026-05-18T15:00:57","date_gmt":"2026-05-18T15:00:57","guid":{"rendered":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/?page_id=754"},"modified":"2026-05-18T15:00:57","modified_gmt":"2026-05-18T15:00:57","slug":"faq","status":"publish","type":"page","link":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/faq\/","title":{"rendered":"FAQ"},"content":{"rendered":"\n
\n\n

\n Common questions about SIXTE setup, SIMPUT source models, running simulations,\n and analysing simulation products.\n <\/p>\n\n

Setup & Installation<\/h2>\n\n
\n Where does one set XMLDIR<\/code>?<\/summary>\n

Shell variables can be set depending on your shell. In bash, for WFI simulations:<\/p>\n 

export XMLDIR=$SIXTE\/share\/sixte\/instruments\/athena-wfi\/wfi_wo_filter\/<\/code><\/pre>\n    
\n      The examples in this repository usually use SIXTE_INSTRUMENTS<\/code>\n      as the root directory for instrument files. setup.sh<\/code> then sets\n      INSTRUMENT_PATH<\/code> to the relevant instrument subdirectory and\n      XML<\/code> to the specific XML file used for the simulation.\n    <\/p>\n  <\/details>\n\n  
\n    I\u2019m using Docker. How can I connect to a display for fv<\/code>, similar to ssh -Y<\/code>?<\/summary>\n    
\n      You can try to follow the instructions here:\n      \n        https:\/\/heasarc.gsfc.nasa.gov\/docs\/software\/lheasoft\/docker.html\n      <\/a>.\n    <\/p>\n    
\n      For a quick check, use command-line tools such as fstruct<\/code> or\n      fdump<\/code>.\n    <\/p>\n  <\/details>\n\n  
\n    How do I change the instrument setup used for the examples?<\/summary>\n    
\n      Most examples use the SIXTE_INSTRUMENTS<\/code> environment variable as\n      the root directory for instrument files.\n    <\/p>\n    
\n      In setup.sh<\/code>, INSTRUMENT_PATH<\/code> is set to the relevant\n      subdirectory, and XML<\/code> points to the specific XML file used for\n      the simulation.\n    <\/p>\n    
\n      To change the setup, adjust INSTRUMENT_PATH<\/code> if you want to use a\n      different instrument file directory, and adjust XML<\/code> if you only\n      want to switch to another XML file in the same directory.\n    <\/p>\n    
\n      Alternatively, you can pass the desired XML file directly to\n      sixtesim<\/code>:\n    <\/p>\n    
XMLFile=\/path\/to\/instrument.xml<\/code><\/pre>\n    
\n      When changing the XML, also check how many event files the detector setup will\n      produce. For example, switching WFI from the single-chip setup to\n      ld_wfi_ff_all_chips.xml<\/code> uses the full four-chip geometry.\n    <\/p>\n    
\n      In that case, sixtesim<\/code> writes one event file per chip. With\n      Prefix=sim_<\/code> and EvtFile=evt.fits<\/code>, the output files are:\n    <\/p>\n    
sim_chip0_evt.fits\nsim_chip1_evt.fits\nsim_chip2_evt.fits\nsim_chip3_evt.fits<\/code><\/pre>\n    
These files can then be merged for later analysis.<\/p>\n  <\/details>\n\n  
\n    Are there plans to provide XMLs for Einstein Probe?<\/summary>\n    
\n      There are currently no plans to implement Einstein Probe from our side.\n      We are not part of the Einstein Probe consortium.\n    <\/p>\n    
\n      The simulator at\n      https:\/\/epfxt.ihep.ac.cn\/simulation<\/a>\n      uses SIXTE in the background. However, we do not have detailed information\n      about how the setup, XML files, and related configuration are implemented.\n    <\/p>\n  <\/details>\n\n  
SIMPUT Source Models<\/h2>\n\n  
\n    When simulating a target, should we set RA and Dec to zero or use the target position?<\/summary>\n    
\n      For the SIMPUT file, RA and Dec define the source position on the sky.\n      Therefore, RA=Dec=0<\/code> places the source at those coordinates.\n    <\/p>\n    
\n      For the simulation, RA and Dec specify the telescope pointing. If the SIMPUT\n      file does not contain a source in the pointing direction, no flux will be\n      measured.\n    <\/p>\n  <\/details>\n\n  
\n    If I care about extinction and background effects, should I define the coordinates of a real source?<\/summary>\n    
\n      It depends. SIXTE does not simulate extinction and astrophysical background\n      by itself. You need to define these effects as part of your SIMPUT file.\n    <\/p>\n  <\/details>\n\n  
\n    How can I include eROSITA sources or the X-ray background in my SIXTE simulation?<\/summary>\n    
\n      We are working on putting the eROSITA sources in a single SIMPUT file,\n      which you will then be able to add to a simulation. That will take a little\n      while, though.\n    <\/p>\n    
\n      There are large AGN catalogues available at\n      http:\/\/www.bo.astro.it\/~gilli\/mock.html<\/a>.\n      If the X-ray background matters to you, you may want to put your source in\n      the area covered by these catalogues.\n    <\/p>\n  <\/details>\n\n  
\n    Are Emin<\/code> and Emax<\/code> instrument-specific?<\/summary>\n    
\n      No. Emin<\/code> and Emax<\/code> set the energy band in which the\n      flux is defined.\n    <\/p>\n    
\n      Elow<\/code> and Eup<\/code> are somewhat instrument-dependent,\n      because they give the energy range over which the source spectrum is defined.\n      If Elow<\/code> to Eup<\/code> does not completely cover the\n      instrument energy range, SIXTE will throw a warning.\n    <\/p>\n  <\/details>\n\n  
\n    Are output files like mcrab.fits<\/code> or plaw.simput<\/code> the source catalogs?<\/summary>\n    
\n      Yes. More precisely, the SRC_CAT<\/code> extension inside the SIMPUT\n      FITS file is the source catalog.\n    <\/p>\n    
\n      The whole .fits<\/code> or .simput<\/code> file is commonly referred\n      to as the SIMPUT source catalog.\n    <\/p>\n  <\/details>\n\n  
\n    Is the srcFlux<\/code> parameter the total flux in the whole image?<\/summary>\n    
\n      Yes, in the band defined by Emin<\/code> and Emax<\/code>.\n    <\/p>\n    
\n      The total normalization of the XSPEC spectrum is overridden, while the\n      relative normalizations of multiple components are preserved.\n    <\/p>\n    
\n      If you provide an XSPEC file and no srcFlux<\/code>,\n      simputfile<\/code> tries to calculate the source flux for you in the\n      given flux band. However, we recommend always setting srcFlux<\/code>\n      manually to make sure the correct flux is simulated.\n    <\/p>\n  <\/details>\n\n  
\n    Does the simulated observation have to be conducted in the same energy band as the input image?<\/summary>\n    
\n      Not necessarily. The energy band of the simulation is whatever the mission\n      can do.\n    <\/p>\n    
\n      The SIMPUT file should cover this instrument range. It does not hurt to have\n      broader coverage.\n    <\/p>\n  <\/details>\n\n  
\n    Can different extended sources with different model spectra be combined?<\/summary>\n    
\n      Yes. See the\n      extended_sources\/mixing<\/a>\n      example.\n    <\/p>\n    
\n      In particular,\n      02_build_simputs<\/a>\n      creates two extended SIMPUTs with different spectra, and\n      03_run_sim<\/a>\n      passes both files to sixtesim<\/code> as a comma-separated\n      Simput<\/code> list.\n    <\/p>\n  <\/details>\n\n  
\n    Can additional maps such as line-of-sight velocity or turbulence be added?<\/summary>\n    
\n      Yes. For a small number of spatially varying spectral parameters,\n      simputmultispec<\/code> is the relevant approach.\n    <\/p>\n    
\n      See:\n      extended_sources\/multispec<\/a>.\n    <\/p>\n    
\n      For many free spectral parameters, simputmultispec<\/code> becomes\n      cumbersome, so photon-list SIMPUTs or 3D grids can be more suitable.\n    <\/p>\n    
\n      See:\n      extended_sources\/photon_lists<\/a>\n      and\n      extended_sources\/data_cube<\/a>.\n    <\/p>\n  <\/details>\n\n  
\n    Can I simulate an extended source without temperature or abundance maps?<\/summary>\n    
\n      Yes. This is demonstrated in\n      extended_sources\/data_cube<\/a>,\n      for example with a cube of RA, Dec, and Energy.\n    <\/p>\n  <\/details>\n\n  
\n    Does SIXTE read idealized 3D photon lists in HDF5 format produced by pyXSIM?<\/summary>\n    
\n      SIXTE does not read pyXSIM HDF5 photon or event-list files directly.\n      The robust interface is a SIMPUT source catalog.\n    <\/p>\n    
\n      The direct pyXSIM workflow is to first project the 3D photon list to the\n      desired line of sight:\n    <\/p>\n    
pyxsim.project_photons(...)<\/code><\/pre>\n    
\n      Then load the projected event list as an EventList<\/code> and export it\n      to SIMPUT:\n    <\/p>\n    
write_to_simput(...)<\/code><\/pre>\n    
\n      Pass the resulting SIMPUT catalog file to SIXTE:\n    <\/p>\n    
Simput=...<\/code><\/pre>\n    
\n      SOXS can also be used to construct photon-list SIMPUT catalogs directly.\n      See\n      extended_sources\/photon_lists<\/a>,\n      especially\n      create_photlist_simput.py<\/a>\n      and\n      02_run_xifu<\/a>.\n    <\/p>\n  <\/details>\n\n  
\n    How can we introduce the cosmic X-ray background in simulations?<\/summary>\n    
\n      By adding many background AGN, clusters, and similar sources to your SIMPUT\n      file.\n    <\/p>\n    
\n      A set of background SIMPUTs has been created by Marchesi et al. here:\n      http:\/\/www.bo.astro.it\/~gilli\/mock.html<\/a>.\n    <\/p>\n  <\/details>\n\n  
\n    Is the length parameter connected to the example_lightcurve.dat<\/code> time range?<\/summary>\n    
\n      It is the length of the generated light curves in seconds. A shorter value\n      covers only part of the values in example_lightcurve.dat<\/code>.\n    <\/p>\n  <\/details>\n\n  
\n    Are the flux units in example_lightcurve.dat<\/code> calibrated by the spectrum file?<\/summary>\n    
\n      This is the flux as a fraction of the value given in the FLUX<\/code>\n      column of the source.\n    <\/p>\n  <\/details>\n\n  
Running SIXTE<\/h2>\n\n  
\n    Do we expect the warning that the spectrum does not cover the full energy range of the ARF?<\/summary>\n    
\n      The warning is given if the spectrum in the SIMPUT file has an energy\n      range, given by Elow<\/code> and Eup<\/code>, that does not contain\n      the nominal energy range of the instrument.\n    <\/p>\n    
*** The spectrum 'mcrab.fits[SPECTRUM,1]' does not cover the full energy range of the ARF! ***<\/code><\/pre>\n  <\/details>\n\n  
\n    Why are some WFI simulation results 18×18 arcmin instead of the nominal 40×40 arcmin?<\/summary>\n    
\n      For some examples, the single-chip setup is used. In this setup, one chip is\n      centered on the optical axis.\n    <\/p>\n    
\n      This is useful for quick tests or compact-source simulations where the full\n      four-chip geometry and chip gaps are not relevant for the analysis.\n    <\/p>\n  <\/details>\n\n  
\n    Are the PSFs for the simulation at mirror level or detector level? Can I provide my own PSF?<\/summary>\n    
\n      They are based on the mirror PSFs, but sensor effects are taken into account\n      in the simulation.\n    <\/p>\n    
\n      The simulation is done with the full PSF resolution. One can output an impact\n      file containing the locations where photons hit the chip. When reconstructing\n      images, the position is spread over the pixel where the charge was registered.\n    <\/p>\n    
\n      Providing a custom PSF is possible. More information is given in Section 4.1\n      on PSF randomization and Appendix C.1 on telescope XML configuration in the\n      manual.\n    <\/p>\n    
\n      The provided PSFs are used to trace photons from the sky and map them onto\n      the detector. Afterwards, detector effects are applied.\n    <\/p>\n  <\/details>\n\n  
\n    Are grading or crosstalk effects included in XMLs other than X-IFU, for example WFI?<\/summary>\n    
\n      The physics of bright-source events in WFI is different from that in X-IFU.\n    <\/p>\n    
\n      In WFI, bright sources are mainly affected by pile-up, as shown in:\n      bright_sources\/pileup<\/a>.\n    <\/p>\n    
\n      Crosstalk is not really an issue for WFI.\n    <\/p>\n  <\/details>\n\n  
\n    What is the difference between sixtesim<\/code> and xifupipeline<\/code>?<\/summary>\n    
\n      xifupipeline<\/code> is deprecated. It was used before\n      sixtesim<\/code> was created.\n    <\/p>\n    
\n      sixtesim<\/code> can now simulate all instruments supported by SIXTE.\n      The same applies to tools such as erosim<\/code> and\n      athenawfisim<\/code>.\n    <\/p>\n  <\/details>\n\n  
Analysis<\/h2>\n\n  
\n    In an event file row with PILEUP=1<\/code>, are multiple events already merged?<\/summary>\n    
\n      Yes. They are already merged.\n    <\/p>\n    
\n      Have a look at the PH_ID<\/code> column, which should have two entries\n      for this event. This column contains the unique IDs of the photons that were\n      merged into the event.\n    <\/p>\n  <\/details>\n\n  
\n    What are use cases where CRVAL1<\/code> or CRVAL2<\/code> will be non-zero?<\/summary>\n    
\n      Whenever you are not pointing at RA=Dec=0<\/code>.\n    <\/p>\n    
\n      Essentially, CRVAL1<\/code> defines the sky coordinate corresponding\n      to the pixel value CRPIX1<\/code>. The same applies to\n      CRVAL2<\/code> and CRPIX2<\/code>.\n    <\/p>\n  <\/details>\n\n  
\n    What exactly does EventFilter<\/code> mean?<\/summary>\n    
\n      EventFilter<\/code> is a filtering expression using the FITS extended\n      filename syntax.\n    <\/p>\n    
\n      It is described in more detail in the manual and by:\n    <\/p>\n    
fhelp rowfilter\nfhelp colfilter<\/code><\/pre>\n  <\/details>\n\n  
\n    Can a DS9 region file be used as a filter?<\/summary>\n    
\n      Yes. There is a regfile<\/code> option that takes the path to a region\n      file.\n    <\/p>\n    
\n      To use region files, you first need to define a WCS in the event file.\n      This can be done with the radec2xy<\/code> tool.\n    <\/p>\n    
\n      An example is given in\n      extended_sources\/simple_extsource\/02_run_sim<\/a>,\n      which adds WCS coordinates with radec2xy<\/code>.\n    <\/p>\n    
\n      Another example is\n      extended_sources\/simple_extsource\/04_extract_spec<\/a>,\n      which uses:\n    <\/p>\n    
regfile=input\/${reg}.reg<\/code><\/pre>\n  <\/details>\n\n  
\n    Does the extracted light curve differ from the input SIMPUT light curve only because of instrumental noise?<\/summary>\n    
\n      No. The SIMPUT light curve changes the source photon rate itself.\n    <\/p>\n    
\n      The light curve extracted with makelc<\/code> contains the simulated\n      source variability plus counting statistics, detector effects, optional\n      background, and any other sources included in the selected event region.\n    <\/p>\n  <\/details>\n\n  
\n    If a variable source is in a crowded field, does the light curve simulation account for nearby sources?<\/summary>\n    
\n      All variable sources in your catalog should be simulated at the same time.\n    <\/p>\n    
\n      If your extraction region contains flux from any variable or non-variable\n      source, you would see it in your light curve.\n    <\/p>\n    
\n      SIXTE traces the origin of each photon, and this is collected in the\n      SRC_ID<\/code> column in the event list. This means you can check the\n      contribution from other sources by selecting all photons in the source region\n      and counting how many come from a source with a SRC_ID<\/code> different\n      from yours.\n    <\/p>\n  <\/details>\n\n  
\n    Can I load the region file while generating the spectrum, as in NuSTAR, instead of typing it manually?<\/summary>\n    
\n      Yes. See the regfile<\/code> option, which exists in\n      makespec<\/code>, makelc<\/code>, and other tools.\n    <\/p>\n    
\n      You also need a WCS system defined in the event file. You can use the\n      radec2xy<\/code> tool for this.\n    <\/p>\n    
\n      An example is given in\n      extended_sources\/simple_extsource\/02_run_sim<\/a>,\n      which adds WCS coordinates with radec2xy<\/code>.\n    <\/p>\n    
\n      Another example is\n      extended_sources\/simple_extsource\/04_extract_spec<\/a>,\n      which uses:\n    <\/p>\n    
regfile=input\/${reg}.reg<\/code><\/pre>\n  <\/details>\n\n  
\n    For realistic X-IFU observations of bright sources, is throughput deterioration modeled in ARF generation?<\/summary>\n    
\n      No. The ARF generation does not consider this effect. This is believed to\n      also be the case for real XRISM data, where the general approach is very\n      similar.\n    <\/p>\n    
\n      You can estimate the recovered flux using quantities such as the\n      GRADING<\/code> columns or the NGRAD1<\/code> and related header\n      keywords.\n    <\/p>\n  <\/details>\n\n  
\n    For realistic X-IFU\/XRISM observations, are crosstalk broadening effects considered when generating the RMF?<\/summary>\n    
\n      No. We currently do not perform RMF generation.\n    <\/p>\n    
\n      To account for grading effects, you could in principle extract one spectrum\n      per grade and fit them simultaneously, although you would need to take care\n      of normalizations.\n    <\/p>\n    
\n      Crosstalk is best handled through the choice of observation strategy and\n      filtering.\n    <\/p>\n  <\/details>\n\n<\/div>\n","protected":false},"excerpt":{"rendered":"
Common questions about SIXTE setup, SIMPUT source models, running simulations, and analysing simulation products. Setup & Installation Where does one set XMLDIR? Shell variables can be set depending on your shell. In bash, for WFI simulations: export XMLDIR=$SIXTE\/share\/sixte\/instruments\/athena-wfi\/wfi_wo_filter\/ The examples in this repository usually use SIXTE_INSTRUMENTS as the root directory…<\/p>\n","protected":false},"author":1008,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":[],"categories":[],"tags":[],"_links":{"self":[{"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/pages\/754"}],"collection":[{"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/users\/1008"}],"replies":[{"embeddable":true,"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/comments?post=754"}],"version-history":[{"count":5,"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/pages\/754\/revisions"}],"predecessor-version":[{"id":759,"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/pages\/754\/revisions\/759"}],"wp:attachment":[{"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/media?parent=754"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/categories?post=754"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sternwarte.uni-erlangen.de\/sixte\/wp-json\/wp\/v2\/tags?post=754"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}