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Wavelength Calibration

JACH | JCMT | UKIRT | Computer Services | USG

Wavelength Calibration

The first step that needs to be done for wavelength calibration is the extraction of the arc spectrum. This is done using the same parameters and data from the extraction of the object spectrum.

     ap> apall rodir/ro970409_154 out=spect/970409_154 ref=rgdir/rg970409_156 
          recen- trace- back- intera-
     Searching aperture database ...
     Aug  6 11:51: DATABASE  - 1 apertures read for rgdir/rg970409_156 from
          database
     Resizing apertures ...
     Aug  6 11:51: APRESIZE  - 1 apertures resized for rodir/ro970409_154 
          (-0.97, 0.76)
     Aug  6 11:51: DATABASE - 1 apertures for rodir/ro970409_154 written to 
          database
     Extracting apertures ...
     Aug  6 11:51: EXTRACT - Aperture 1 from rodir/ro970409_154 --> 
          spect/970409_154
This extraction is done non-interactively, and uses the data from the rgdir/rg970409_156 extraction as a reference. This is why the data from the rgdir/rg970409_156 extraction was written to the database. The arc spectrum is named according to the UT date and observation number of the arc frame.

Now, we must identify which spectral lines have what laboratory wavelengths. The reference spectrum to use in this case is the 2.18 micron Ar spectrum, found on the UKIRT web pages. The onedspec package was first loaded, causing the prompt to change to on>.

     ap> cd spect
     ap> onedspec
     on> identify 970409_154

  
Figure 15: Spectral line identification windows

Upon running the identify task, the identification window (shown in Figure 15, top left) will appear. This window presents a wide selection of commands; press q to see a full list. Commands used in this example are:

  • m -- This is used to mark the spectral lines. Place the cursor over a line, press m, and the centering algorithm will find the centre of the line, prompting the user for the wavelength. The window will then be updated to show a mark over the line, as in Figure 15, top right. In this example ten spectral lines have been identified.
  • f -- Once a number of lines have been identified, the dispersion relation can be found. Pressing f will bring the user to the residuals of the function fitting window. Press h to see the fitted function. In this example, the fitted function is shown in Figure 15, bottom left, and the residuals are shown in Figure 15, bottom right.
    • :func cheb -- Do this while in the function fitting window to change the function to chebyshev.
    • :order 2 -- Do this while in the function fitting window to change the order to 2 (straight line).
    Press f again to fit the curve to the new parameters. Press q while in the function fitting window to return to the identification window.
Once the dispersion relation has been found, press q to save the relation to the database. If a prompt appears, answer ``yes''. In this example, a straight line was used, and the RMS was 2.22x10-4.

It should be noted that for recent versions of the identify task, a parameter called units has been added. If this more recent version is being run, then use the following line when running the identify task:

     on> identify 970409_154 units=microns
Doing so will eliminate a step to come later in the wavelength calibration sequence. All commands that are to be used with identify remain the same.

The reftable file is now edited to tell IRAF which dispersion relation goes with which file.

     on> edit reftable
This command brings up the user's default editor. For this example, the file looks like this:
     970409_156 970409_154
     970409_164 970409_154
which applies the dispersion relation from the file 970409_154 (the arc spectrum) to 970409_156 and 970409_164 (the object spectra)

Next, the header of the object files is modified to point to the arc spectrum file.

     on> refspect 970409_156,970409_164 refe=reftable verb+
     [970409_156] refspec1='970409_154'   Accept assignment? (no|yes|YES) (yes):
     [970409_164] refspec1='970409_154'   Accept assignment? (no|yes|YES) (yes):
The default was accepted for both prompts.

Now, the dispersion relation can be applied.

     on> dispcor 970409_156 d970409_156
     970409_156: REFSPEC1 = '970409_154 1.'
     d970409_156: ap = 1, w1 = 1.857515, w2 = 2.529492, dw = 0.002635, nw = 256

     on> dispcor 970409_164 d970409_164
     970409_164: REFSPEC1 = '970409_154 1.'
     d970409_164: ap = 1, w1 = 1.857515, w2 = 2.529492, dw = 0.002635, nw = 256
Notice that the output spectrum begins with a 'd'. The wavelength calibration is not yet complete, however.gif Upon examination of the image header, or graphing the spectrum, the units of wavelength are angstroms, and not microns. The previous value is found, and ``microns'' is substituted for ``angstroms''.
     on> imhead d970409_156 l+
     d970409_156[256][real]: bs5167
         No bad pixels, no histogram, min=unknown, max=unknown
         Line storage mode, physdim [256], length of user area 969 s.u.
         Created Tue 13:46:21 24-Jun-97, Last modified Tue 13:46:21 24-Jun-97
         Pixel file "HDR$d970409_156.pix" [ok]
     [...]
         WAT1_001= 'wtype=linear label=Wavelength units=Angstroms'
     [...]
     
     on> hedit
     images to be edited: d970409_156
     fields to be edited: WAT1_001
     value expression: wtype=linear label=Wavelength units=microns
     d970409_156,WAT1_001 ("wtype=linear label=Wavelength units=Angstroms" -> 
       "wtype=linear label=Wavelength units=microns"): y
     d970409_156,WAT1_001: "wtype=linear label=Wavelength units=Angstroms" ->
       "wtype=linear label=Wavelength units=microns"
     update d970409_156 ? (yes): y
     d970409_156 updated
The same was done for the file d970409_164.

At this point the wavelength calibration is completed.


JACH | JCMT | UKIRT | Computer Services | USG


Last Modification Date: 1997/08/06 - Last Modification Author: cavanagh
Brad Cavanagh (cavanagh)
Contact: Tom Kerr. Updated: Wed Oct 6 12:07:40 HST 2004

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