Why and What: It's obviously useful to be able to see the spectrum you just took -- you can see if you have the right object, if the signal-to-noise looks adequate, and so on. Maybe there's an unexpected feature -- better to confirm it right away than be wondering weeks later whether it's real or not.

The program qccds.py does the following for a CCDS stellar spectrum:

• Copies the data to a local directory
• Subtracts the bias
• Produces a sky-subtracted, optimally-extracted spectrum.
• Optionally applies a wavelength calibration (if available);
• If your spectral range includes a suitable night-sky line, shifts the spectrum so as to zero out the night sky line's apparent velocity.

Note added 2012 September: These instructions are updated for the servers installed in 2012 August (mdm24ws1, etc.). I was able to test the basic functionality, but not the wavelength calibration. My guess is the wavelength calibration will work, but I think there's a good likelihood that the zeropoint shifting based on a night sky line will take some more debugging.

Quick Instructions: You should be logged on as obs24m or obs13m, as appropriate.

• Open a terminal window (the little terminal icon in the top bar).
• Make a scratch directory for your quicklook data, and descend into that directory.

  
  		mkdir myquick
  		cd myquick
  

• Grab your own copy of the qccds script:

  
  		cp /usr/local/pkg/thorsoft/scripts/qccds.py .
  

  (Don't overlook the final dot).
• Type pyraf to start pyraf. (This is an iraf wrapper written at STScI, which has many advantages over the classic IRAF cl.)
• In pyraf, type

  
  	noao
  	imred
  	specred
  

• Import the qccds task by typing

  
  	pyexecute('qccds.py')
  

  This takes the script and magically converts it into an iraf task!
• Now, type epar qccds to set it up. (If you're not familiar with pyraf, you'll note that this pops a simple GUI form that is easy to use, unlike the usual IRAF epar).
• [Note: If you don't get a gui and the program complains, it may need you to copy the qccds.par file from the same directory in /usr/local/pkg over to the /lhome/obs24m/iraf/uparm directory.]
• By default, rawdir points to the directory on hiltner (or mcgraw) where CCDS data are usually written. Modify this, and also the file prefix, as needed so it points to your data. When you specify the file prefix, don't forget to include a dot if there is one.
• At first you'll want to de-select the wavelength calibration and night-sky shifting, which are discussed further below.
• You should be set up! If you have a spectrum, e.g. number 5, test the script by typing

  
  	qccds 5
  

  and you should get some results. There's more documentation below about how the aptly-named apall task is used, and so on.
• The script will copy the data into an image called (e.g.) x0005.fits; the extracted spectrum will be called x0005.ms.fits. This is an iraf multispec format spectrum, which has four different "bands"; they are (1) the optimally extracted, sky-subtracted spectrum; (2) same, but without optimal extraction; (3) the sky background spectrum; and (4) an estimate of the standard deviation of the first spectrum. In splot you can step through these with left and right parentheses to decrement and increment.
• Note that if something goes wrong and you want to re-do the quick look, you can simply repeat it without erasing the previous results, which will be automatically overwritten.

Hints on extracting spectra If you're in an uncrowded region, you should be able to run the task with handap ("find aperture by hand?") flag set to NO -- it should find the object on the slit and extract it without further intervention. But this often goes wrong, and you have to set the flag to YES so you can intervene.

The spectra are extracted by the IRAF task apall, and if you're an expert on that you should be all set. In case you're not, here's what's happening. You'll be asked lots of questions, for which the default answer ("yes") is usually what you want. For those questions you just hit enter, sometimes in the original window, other times in the graphics window that pops up.

The first thing you'll do is specify an aperture to extract, that is, the range of CCD rows in which your object falls. Here are the relevant commands:

• d - Delete the aperture nearest the cursor. Needed if the program locked onto the wrong object.
• m - Mark this object, i.e., put an aperture on the object nearest the cursor.
• z - Automatically adjust the width of the aperture.
• l and u - mark lower and upper limits of the object aperture respectively
• r - Replot to see your changes.
• b - Edit the background regions (they're indicated by the horizontal bars at the bottom of the display). Useful if there's a bright object in the background region. This has its own subcommands:
  • wa - "window all", expand the viewport to the whole picture. Also useful are wt (set top of viewport to location of the horizontal cursor, wj and wk (for left and right limits respectively).
  • t - delete the current background limits
  • s (4 times) - mark the new background limits; use twice to the left, twice to the right, to establish new windows.
  • f - Fit the background so you can see if it's reasonable
  • q - At this level, Quits from the background customization.
  Note that the background subtraction is iterated a couple times to clip discrepant points; a weak source in the background is not a cause for concern. In the f command you may see some points marked as rejected.
• q - quits aperture editing.

As you go along the dispersion, the location of the object's spectrum does not remain exactly constant, so after you've specified the aperture, the program will ask if you want to trace the aperture to follow these deviations. The trace should be pretty straightforward, but there are some options if you want to use them:

• d - Delete the point nearest the cursor.
• f - Fit the trace again.
• :f - Change the type of function that's fit; options are
  • leg - legendre polynomial
  • spline3 - cubic spline
• :o 3 - Change the order of the fit function (to 3 in this example). The interpetation depends on the type of function you're fitting:
  • For a legendre, it's (weirdly!) the number of coefficients you have, so 1 is a constant, 2 is a straight line, 3 is a parabola, etc. -- the iraf "order" is larger by 1 than the conventional meaning of a polynomial order.
  • For a spline3, it's the number of sections of cubic you're fitting.
  If you have a very faint source, you'll want to go to a legendre 2 or even a legendre 1 to avoid following noise.
• q - as usual, quit the interactive fitting.

General remark on spectrum extraction: As noted earlier, qccds extracts the spectrum by driving the iraf task apall. Apall has a huge number of parameters; qccds sets these to reasonable defaults, basically putting ease-of-use ahead of flexibility. Remember, this is only for quick-looks.

If you have strong emission-line sources, you may do better in the apedit if you find what column the line occurs in, and use that as your displine.

Wavelength Calibration: You can apply a pre-existing wavelength calibration to your quick-look spectrum. Here's how:

• Take some lamp spectra. If you run qccds on it as best as you can, it will at least copy it over and bias subtract it. You will want to strip out a one-d spectrum using apall, with the background to subtract set to "none".
• Fit the lamp spectrum using the iraf task identify. This is a big subject and often is somewhat tricky.
• If you've fit the comp spectrum in some directory other than your quicklook directory, you'll need to copy the following into the quicklook directory:
  • The calibration spectrum you've used, e.g. x0123.0001
  • A subdirectory called database containing the calibration fit. This will be in a file which has the same name as the calib image, but with id prepended; in our example it would be idx0123.0001.
• The extracted spectrum will be called, e.g. dx0234.fits, where the d stands for "dispersion corrected."