SMART Bias Changes During Extraction

IRS Module Bias Changes

To improve overall performance (at the expense of some sensitivity) bias voltages of some of the IRS modules have been changed at various times during the SIRTF/Spitzer mission. The upshot of this is that different Calibration Files then have to be used to process data depending on the bias voltage in use at the time any gven observation was made. The Calibration Files to be used are designated by the value of the "CAL_SET" FITS header keyword in the relevant BCD file(s). Currently the relevant bias changes are :

NOTE :The 'CAMPAIGN' FITS header keyword lists the campaign number in SSC format.
To translate to IRS Team campaign numbers see : Campaign Numbers.

Long High (LH) data from campaign 25 onwards must be processed in SMART with a different set of Calibration files from those used for campaigns 1 - 24.
Long Low (LL) data from campaign 45 onwards must be processed in SMART with a different set of Calibration files from those used for campaigns 1 - 44.

Although the "CAL_SET" FITS header keyword changes for all data from all modules at both campaigns 25 and 45 only LH and LL modules are effected by the changes.

There was no change to LH at campaign 45, only LL was changed then.

For more information about relevant Calibration Files see : Calibration Files Bias Voltage Change Convention.

SMART Display of Calibration Files

NOTE :
The DEFAULT setting in SMART for a given Calibration file release where there has been a detector bias change is always to use the pre-bias change file version.
During extraction SMART automatically switches to the relevant post bias change file if necessary. (See "SMART Bias Changes During Extraction" below.)

SMART displays lists of Calibration files it uses in a number of places. In these cases SMART will list ALL Calibration files that it potentially could use.
For files where there are two or more versions of a given file because of bias changes BOTH the versions will be displayed.
To check which version of a detector bias change Calibration file was actually used during a given extraction
see the FITS header keywords "SM_LHP25" and "SM_LLP45", described below.

SMART displays a list of Calibration files at :

1. Calibration File Display - Displays a list of ALL Calibration files.
2. Calibration File Selection - Allows switching of Calibration files and/or Calibration release versions.
   This does NOT allow the user to over-ride the automatic setting of pre or post bias change files.
   It does allow the user to choose which version of the pre-bias change Calibration files will be applied to pre-bias change data.
   It does allow the user to choose which version of the post-bias change Calibration files will be applied to post-bias change data.
   Use this facility with care.
3. FITS Header Keywords - The "S_CAL1" - "S_CAL999" FITS header keywords list ALL Calibration files available to SMART during any given extraction.
   As such BOTH versions of any given pre/post bias change Calibration file will be listed.
   To check which version of a detector bias change Calibration file was actually applied during a given extraction
   see the FITS header keywords "SM_LHP25" and "SM_LLP45", described below.

SMART Bias Changes During Extraction

One upshot of the bias voltage changes is that BCD (data) files of observations made with the same module and with different bias voltages cannot be extracted together.
At the beginning of an extraction SMART checks each BCD file header to see which Calibration Files to use.
It then automatically sets the relevant Calibration file.
It cannot use two (or more) different sets of Calibration Files at the same time.
Observations made with different bias voltages MUST be extracted seperately.

1. Long High (LH) Module If two (or more) LH files observed with different bias voltages are selected for extraction at the same time the following message will appear :

   

   Click "OK" to continue.
   This will terminate extraction at once. No data will be extracted.
   Please select the relevant file(s) for extraction seperately.

2. Long Low (LL) Module
If two (or more) LL files observed with different bias voltages are selected for extraction at the same time the following message will appear :



Click "OK" to continue.
This will terminate extraction at once. No data will be extracted.
Please select the relevant file(s) for extraction seperately.

SMART Bias Changes Output FITS Header Keywords

When SMART extracts data for a module that has had a bias voltage change an additional FITS header keyword
is added to the output spectra to document which bias change specific Calibration Files have been used.
This is in addition to the "S_CALxxx" FITS header keywords which list ALL Calibration Files used during a given extraction.

The relevant keywords are :

Comment contains the string :
"Type of campaign 25 LH calibration files used" |smart_proj__define.pro |Reference |

Comment contains the string :
"Type of campaign 45 LL calibration files used" |smart_proj__define.pro |Reference |

For more information see : FITS Header Keywords.