IRS: Data Calibration

Calibration Overview

Approximately once a month, a new IRS campaign is performed and a set of standard stars with known spectra is observed. The observations are sky-subtracted and extracted using SPICE, with exactly the same parameters as in the extraction pipeline. These spectra are used to monitor the general photometric stability of the observatory. Approximately twice a year, these observations are used to update the absolute photometric calibration in the general pipeline. The design specifications of the IRS call for a repeatability of 5% and an absolute photometric accuracy of 10%.

The photometric calibration is done by matching the observed spectra to model spectra, using a low-order polynomial. This method has the advantage that it is does not depend on the details of the spectral features, but only on the general shape of the continuum and the photometric scaling of the models. The talk from the first Data Analysis Workshop entitled Basic Steps in Photometric Calibration summarizes the status of the calibration up to S11, using data available up to campaign 18.

At this point in time, the SSC provides spectrophotometric calibration for isolated point sources only. For extended sources or to use non-standard extraction apertures, the user will have to develop his/her own calibration coefficients, using observations of standard stars. It should be noted that the pointing scheme of the telescope was substantially improved after campaign 17 (3-15 Jan 05). Some observations performed before campaign 17, but calibrated with later pipeline versions, will show significant order mismatches.

Doing your own calibration

1. From Leopard, download postBCD products for staring observations of standard stars (from Campaign 17 onward). The primary calibrator for the low-resolution modules is HR 7341, while that for the high-resolution modules is HR 6688 (ksi Draconis). For every high-resolution observation of HR 6688 AOR there will be another AOR observing the nearby sky. You will need to download that too.

   Be aware that the bias voltage for the LL module changed in Campaign 45, while the bias for the LH module changed in Campaign 25. To do your own calibration, use only stars that have been observed with the same bias as your target. For example, if you are interested in LH observations performed in Campaign 30, you will need to download all the observations of HR 6688 from Campaign 25 onwards. The Spitzer BIC provides a translation between date of observations and campaign number. A list of the actual AORs used for calibration is given here.

2. Subtract the sky from the standard star observations. For the low-resolution modules, the pipeline provides ready-made sky-subtracted products. The files are called bksub.fits, bmask.fits, and bkunc.fits, and are the result of subtracting nod2 images from nod1 images and viceversa. (For an explanation of these names see the Data Handbook). To do something different, start with coa2d.fits, c2mask.fits, and c2unc.fits. For the high-resolution observations, you will always need to subtract the sky by yourself. For each high-res AOR, subtract the sky products called coa2d.fits from the standard star products of the same name. Add the uncertainty images (c2unc.fits) of the sky to those of the standard in quadrature. Perform a logical 'OR' operation between the c2msk.fits from the sky and that from the star.

3. Extract you spectra. Start SPICE and run it in batch mode over the sky-subtracted images of your standard. The setup should be exactly the same as in you intend to use for your science target. The SPICE products you will need for the calibration are called *_extract.tbl.

4. Combine all the individual *_extract.tbl files. These files contain the instrumental spectra, in units of e/sec/micron. The SSC uses a clipped median to combine all the spectra, with the uncertainties propagated accordingly. The bit flag values are OR-ed. There are other possibilities: a clipped average has better noise characteristics, for example. Note that, alternatively, one may combine all 2D images for each nod, and then extract a single image. This is not advisable, because pointing errors will put the 2D trace in slightly different parts of the detector for different observations.

5. Download a model for the standard star.

6. Obtain the polynomial correction. To do so, divide the averaged *_extract.tbl by the model, and fit the result with the lowest-order polynomial possible. The SSC uses polynomial fits of orders less than 3 for each individual order in each module. These are the 'Fluxcons').

7. Run SPICE on your science data, with the same setup as the calibrations. Obtain extract.tbl products.

8. Divide the extract.tbl products by the fluxcons. You can make SPICE use your calibrations to tune the spectra, in which case SPICE will take care of this step. Or you can do it in a separate script. And you are done!

Go back to IRS page