Getting Quick-Look Spectra with qccds

John Thorstensen, Dartmouth College

2009 March 14 - Revised 2012 September

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 does the following for a CCDS stellar spectrum:

Note that the script leaves the original data untouched -- you needn't be afraid of it screwing anything up.

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.

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:

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:

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:

Adjusting to zero using a night-sky line: This should work provided your spectral range has a strong night-sky line like [OI] 5577. It uses my own emission line centroiding task to find the center of the night sky line you specify, in the third band of the multispec (see above). It then shifts the solution to zero out the night sky line's velocity. The task can fail if the night-sky line is overwhelmed by background (e.g. in twilight), or if the night-sky line is many pixels away from where the wavelength solution says it should be (e.g. if the grating slipped, or something).