PHT Interactive Analysis User Manual - (V9.0)

2.) Learning Chapters

2.1) Basic Description of the PHT Data Reduction

This chapter gives a survey of the data reduction steps which can be performed by the PHT Interactive Analysis and of how they are implemented in the PIA software. While the Guided Tour presents a step by step sample session, the present chapter aims at giving some insight into the steps which need to be performed.

List of the Data Reduction Steps

The data reduction consists basically of three parts, namely: In general, the data in a PHT observation consist of a measurement on the internal Fine Calibration Source(s) (FCS) and one or more measurements of the astronomical target. Normally, an FCS measurement is reduced in the same manner as the astronomical data up to the signal-per-chopper-plateau level and then used to compute responsivities which are applied to the astronomical data during the power calibration step. The responsivity computation is the final step in the reduction of FCS measurements.

In detail, the data reduction will consist of some of the following steps:

  1. Convert read-out digital data into analog data
  2. Correct for non-linearity
  3. Correct for cosmic particle hits (glitches) on read-out level (single threshold ramp deglitching) OR
  4. Correct for cosmic particle hits (glitches) on read-out level (two threshold voltage differences deglitching)
  5. Determine signal (in V/s) per integration ramp OR
  6. Determine "signal pattern" (in V/s) (for chopped measurements)
  7. Correct for cosmic particle hits on the signal level (signal deglitching)
  8. Analyse/edit data to minimise the effect of drifts in detector responsivity
  9. Determine signal per chopper plateau OR
  10. Calibrate "signal pattern" (for chopped measurements)
  11. Correct for reset interval effect
  12. Perform dark current subtraction
  13. Linearize signals
  14. Subtract straylight (for FCS measurements)
  15. Calculate actual responsivity (for FCS measurements)
  16. Perform vignetting correction
  17. Subtract background within a measurement (for chopped measurements)
  18. Correct for signal losses caused by chopping between different fluxes (chopper frequency dependence)(for chopped measurements)
  19. Correct (graphically) for long term responsivity changes
  20. Convert signal (V/s) into in-band power (W)
  21. Subtract a measurement containing the background (externally) (for staring measurements)
  22. Calculate flux density and surface brightness
  23. Correct for pointing offset by PHT-S observations
  24. Perform AOT-specific processing (e.g., multi-filter, multi-aperture photometry, spectrophotometry, polarimetry, or mapping)
In PIA appropriate steps are combined within the following procedures, often corresponding to a single main display or interface in PIA:
Read Edited Raw Data (ERD) files:
Process ERD:
Includes steps 2-6. Step 5 and 6 are alternatives for reducing the ERD to Signal per Ramp Data (SRD).
Process SRD:
Includes steps 7-11. Step 10 and 11 reduce alternatively the SRD to Signal per Chopper Plateau Data (SCP).
Process SCP:
Includes steps 12-20. SCP data are reduced to the Standard Processed Data (SPD) in step 20.
Process SPD:
Steps 21 and 22. Step 22 reduces SPD to the Astrophysical Applications data (AAP).
Processing AAP:
Step 23.

Description of the data reduction steps

The data reduction steps as realized within PIA are listed and described below. A detailed guide to applying them by using PIA is given in Processing Data with PIA - A Guided Tour. A description of physical effects affecting the data can be found in the IDUM . A detailed discussion of the algorithms used is given in chapter 4, Processing Algorithms.

Reading ERD files

PIA automatically converts all digital data into analog data and stores the data in the internal buffers. A buffer in PIA refers to a data structure in memory which can be loaded and operated on or written out to a file. Parameters from several conversion tables and a calibration file are used to convert the data into an analog form.

Processing ERD

Within this procedure the following steps can be performed: Some editing of the data is generally necessary because some of the read-outs are disturbed. The default read-out selection is to discard the first and last read-out values of each ramp because they may seriously suffer from electronic effects and should usually be discarded. Additionaly, the first n read-outs in every long integration ramp can be disturbed. By default 10-15% of each ramp are discarded in order to minimise the effects of these ramp disturbances without affecting the signal-to-noise ratio more than necessary. For a careful analysis the user should inspect (some of) the ramps and should optimise the selection of read-out values before deriving signals.

Additionally the user may want to have a look at the detector temperature curve which is automatically provided by PIA and which should be reasonably flat - if it is not, the signal values may be influenced by temperature fluctuations.

The user should also look at the dynamic range of the read-out values. The maximum dynamic range lies between about -0.85 and +1.09 [V] depending on the detector. Saturated read-outs will be discarded by PIA, while a poor usage of the dynamic range (e.g. -0.8 to -0.7 [V] leads to higher noise in the results. This cannot be corrected for once the measurement has been performed, but should be noted by the user.

After this processing step the data consist of the signal as a function of time.

Processing SRD

The next step is to combine all the signal values into a representative signal for each chopper plateau (note that a staring observation is considered a single chopper plateau). PIA can display information on which chopper plateaux go with which signals.

Within this procedure the following steps are performed:

A signal selection (editing) is generally necessary because the first ramp of every chopper plateau is disturbed. While the chopper is changing its position, the reset is interrupted. Therefore, such a reset shows a behaviour different from the 'normal' ones, affecting the signal value. By default this first signal is deselected. The PIA user has the option of deselecting any signals to cope with other effects which could affect the signal per chopper plateau, such as initial signal drift, etc.

Processing SCP

The next step is to perform the signal to optical power conversion, in the case of calibration (FCS) measurements.

Since the actual responsivity should usually be used for the power derivation, we describe the processing of FCS calibration measurements first, including the data corrections which can be applied.

For astronomical data at the SCP Level the processing is described below, including the relevant corrections:

Processing SPD

Now we only have to apply the optical corrections (filter transmission, etc.) to obtain flux density and surface brightness from the in-band power. External background subtraction is normally performed on the SPD and internal background subtraction can also be done at this stage. Both are also possible under the AAP level.

Processing AAP

The AAP processing (Astronomical Analysis) is completely AOT dependant, except for the background subtraction (internal or external, which can be applied at this level, but was already explained under the former points) and for the pointing correction in the case of PHT-S observations:
  • Correction for pointing offset by PHT-S observations:

  • Due to the small pixel size of PHT-S (24"x24") and to the sharply peaked footprint shape, already a few arcsecond off-centre positioning may lead to both significant intensity changes as well as to spectral shape alterations. Footprint matrices for every PHT-S pixel can be used for correcting depending on the established offset (see section 3.3.10. Pointing correction for PHT-S).

    It is at this stage that the specific data reduction according to the final observation objectives takes place. PIA offers special data reduction modes for every type of observation, as listed below:

    Chapter history:
    Date  Author  Description 
    13/05/1996  Martin Haas (MPIA) / Carlos Gabriel (ESA-SAI)  First Version 
    19/07/1996 Carlos Gabriel (ESA-SAI) Revised
    26/02/1997 Carlos Gabriel (ESA-SAI) Updated 
    03/06/1997 Carlos Gabriel (ESA-SAI) Update (V6.3)
    09/07/1997 Carlos Gabriel (ESA-SAI) Update (V6.4)
    12/02/1998 Carlos Gabriel (ESA-SAI) Update (V7.0)
    15/08/1999 Carlos Gabriel (ESA-SAI) Update (V8.0)
    02/11/1999 Carlos Gabriel (ESA-SAI) Update (V8.1)