FITS Output of the Simulator

The simulator produces a FITS event list named acis.evt in the MARX directory and copies the CCD parameter files to the MARX directory. You can supply an alternate name for the event file with the rain_on_acis keyword EVENT_FILE. Some of the interesting FITS keywords found in the binary table header are:

• PARFILE?: the name of a CCD parameter file used by the simulator.
• PHOTONS: the number of photons simulated.
• FLUX: the number of photons simulated divided by the number of exposures simulated.

The FITS binary table used to store the event list always includes the following columns.

• TIME: The MARX timestamp in seconds corresponding to the midpoint of the CCD frame.
• EXPOSURE: The CCD exposure number, starting at zero.
• CCD_ID: The CCD that detected the event, obtained from the CCD_ID keyword in the parameter file.
• AMP_ID: The amplifier, numbered 0..3, associated with the central pixel of the event.
• QUALCODE: An 8-bit quality code computed for each event. Bit 2 is set (value = 4) if the central pixel exceeds 4095 DN.
• CHIPX/CHIPY: The 1-based integer position of the event.
• TDETX/TDETY: The position of the event in the ``tiled coordinate system AXAF-ACIS-2.2'' defined by Jonathan McDowell at the ASC.
• PHAS: A 9 element vector containing the event island pixel values corrected for bias. The relationship between this 9-vector and the 3x3 event island is shown in Figure B.2.


Figure B.2: Ordering of PHAS vector

If the rain_on_acis keyword DITHER is set, then the TDETX & TDETY columns are corrected for the dither, i.e. an aspect correction with optional aspect error is applied. The CHIPX & CHIPY columns however are NOT aspect-corrected - they contain the integer positions of the dithered events. You should examine dithered events in the TDETX/TDETY coordinate system, not the CHIPX/CHIPY system. When DITHER is set, the FITS columns DITHER_X and DITHER_Y are added, specifying the random offsets that were added to the MARX photon positions.

If the rain_on_acis keyword ONE_PHOTON_PER_FRAME is set, then FITS columns RAY_X and RAY_Y are included, specifying the real-valued position (in CHIP coordinates) of the photon which lead to the corresponding event.