POA: POA_CALFOS Overview ST-ECF


What are STPOA and POA_CALFOS? POA_CALFOS - the Post Operational Archive version of the FOS calibration pipeline - replaces CALFOS in the standard IRAF/STSDAS FOS calibration package. In its current version it offers a correction to image motion problems which have led to significant wavelength scale uncertainties in the FOS data archive. V1.2.2 deals with residual effects from the geomagnetic image motion problem (GIMP), with offsets introduced by the electronic setup of the detector (Ybase values) and by variations of the ambient temperature in the FOS optical bench. POA_CALFOS applies POA calibration to datasets which fulfil the criteria below only; for all other data sets it defaults to CALFOS calibration:

Detector: BLUE
Grndmode: SPECTROSCOPY, RAPID-READOUT, IMAGE
FGWA_id: H13, H19, H27, H40, H57, L15, L65
Aper_id: A-1, A-2, A-3, A-4, B-1, B-2, B-3, B-4

The improved calibration items include (in reverse chronological order):

  • NEW TASK FOR OBTAINING DISPERSION COEFFICIENTS (v1.2.1): "pfos_dispfit" will calculate dispersion coefficients for the ccs6 reference file.

  • NEW STPOA TOOL TO ASSIST USERS OF POA_CALFOS (v1.2, update v1.2.1): "pfos_pix2wav" calculates wavelengths from pixels for a table of POA-recalibrated FOS data. This was updated to give sub-pixel corrections for input pixel positions and to work with IMAGE mode data in v1.2.1.
  • NEW DARK CORRECTION SCALING RELATION (v1.2): Dark calibration data covering FOSís lifetime has been used to fit the expected dark flux as a function of L-shell, this new scaling replaces the previous geomagnetic longitude/latitude model.
  • BUG FIX TO CALFOS DARK CORRECTION (v1.2): The dark correction was previously incorrectly applied to ACCUM'd data, this has now been fixed.
  • PRE-COSTAR POA FLATFIELDS(v1.2, update v1.2.1 and v1.2.2): Flatfields rederived to work with the POA_CALFOS pipeline are now also available for pre-costar datasets and paired aperture observations. v1.2 was missing six of these files, they were added in v1.2.1.
  • DISPERSION RELATION (v1.1 onwards, update v1.2.1): A new set of dispersion coefficients have been derived for all the FOS BLUE gratings. A new algorithm for calculating the dispersion relation has been added to the pipeline, which utilizes 10 dispersion coefficients. More accurate H40 coefficients were added in v1.2.1.
  • FLATFIELDS (v1.1 onwards): Flats appropriate to POA shifted data. These are available for all post-COSTAR data. Pre-COSTAR flats are being derived for the final POA_CALFOS release later in 2001.
  • NEW OUTPUT FILE (root.poa): Contains POA related calculations and header keywords for each spectrum (v1.1 onwards).
  • GIMP (v1.0 onwards): Datasets obtained prior to 5th April 1993 (when the onboard GIMP correction was implemented) now have a more rigorous correction applied. Those obtained after this date have the effects of erroneous onboard corrections removed and the new correction applied. (3 sigma amplitude of correction: 0.3 pixel or 0.12 diodes). See here for further discussion regarding the GIMP.
  • YBASE (v1.0 onwards): During operational life of the FOS the location of areas on the photocathode imaged onto the diode area drifted as a result of slow variations in the ambient magnetic loading. This was compensated onboard in Y direction only by application of a small additional magnetic field (YBASE updates), introducing an unwanted and uncompensated motion in X (dispersion direction) as well. (Full amplitude of this effect: 4 pixel or 1 diode in Dec 1996 as compared to Sep 1990).
  • TEMPERATURE (v1.0 onwards): Variation of the ambient temperature in the FOS optical bench over a range of 10 degrees (C) have led to drifts in the dispersion direction by 0.6 pixel or 0.24 diodes. Since the HST aft-shroud temperature steadily increased over time data obtained late into the FOS operational period suffer the most.

The calibration improvements above enhance the scientific value of the data in the FOS archive making it a more homogenous and reliable resource. The better wavelength calibration is demonstrated here and some examples of improved scientific results here.



Do I loose any of the "good" features from standard CALFOS ?:

No - POA_CALFOS is an upgraded version of CALFOS from the standard IRAF/STSDAS distribution and will become the standard pipeline. In V1.2 we have only made changes and additions to the algorithms determining the memory location of data with respect to a reference grid valid for the dispersion relations. In particular, background subtraction, and flux calibration remain untouched. POA_CALFOS will work with most of the same reference data and produce the same output files (plus one extra log file with diagnostics for those interested). The wavelength calibration files and some flatfield calibration files have been updated. The POA FOS pipeline pre-processor 'poa_preproc_fos' will let the user know whether new calibration files will be used in the pipeline processing. All follow-up processing should not notice any difference; the pipeline output products are in the same format and can be used the same as before in data processing and analysis.

HOWEVER - note that the raw data have to be pre-processed in order to prepare the data for POA pipeline processing. Please read the STPOA iraf 'poa_preproc_fos' help file for more details on the pre-processing.



Where can I get it? POA_CALFOS is a standard IRAF STSDAS package. You can download it from here together with test data and installation advice. Moreover, this will soon become the default FOS pipeline used in the on-the-fly recalibration when retrieving data from ST-ECF and CADC.

There is now also a patch available for v1.2.2, here.



How to use STPOA: Once STPOA has been installed as an IRAF Layered Package, you can use it just like any other IRAF package, like stsdas, tables, xray, etc. In order to process FOS data with the new POA pipeline, you must also load the stsdas.hst_calib.fos package, by typing "stsdas" followed by "hst_calib", followed by "fos" in a cl window. To load the STPOA package, type "stpoa" in the CL window. You will see that it contains one sub package called "poa_fos", open that sub-package by typing it's name.

Five tools will now be available: poa_preproc_fos, poa_calfos, pfos_pix2wav, pfos_dispfit and processfos. 'poa_preproc_fos' is a preprocessor for the FOS data which you need to run before you run the pipeline; 'poa_calfos' is the replacement for 'calfos', the calibration pipeline; the task 'processfos' is a tool which will run both the preprocessor and the pipeline back-to-back. Finally 'pfos_pix2wav' is an extra tool which calculates wavelengths from pixels for a table of POA-recalibrated FOS data. For further information on these tasks, you can type 'help taskname' in a CL window. You can also type "help poa_fos opt=sys" to get a more general view of the poa_fos package and it's tools.



Future plans: This release of POA_CALFOS is intended to be the final version. However, we will continue to support queries from users.

Unfortunately it turns out that the FOS/RD detector side is a lot more difficult to understand than was the FOS/BL side. We can see the same effects now incorporated in POA_CALFOS but there are additional almost random errors of the same magnitude. For now we recommend consulting ISR CAL/FOS 149 before making strong statements about redshifts obtained from FOS/RD side data.



See also:


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