Cookbook for Lick/KAST Reductions

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This page describes the types of calibrations recommended (or required) to use the Low-Redux pipeline to reduce data acquired with the longslit Kast spectrometer.

Calibrations:

AFTERNOON: The following table summarizes the recommended calibration exposures for the grisms+gratings supported within the Low-Redux pipeline.

Blue Side Calibrations

Type	Lamps	Slit	Exp	NExp	Notes
Arc	All	46000	20s	2	IMPORTANT: One should wait ~5min for Cd lamps to warm up.
DomeFlat	Blue	46000	50s	5	Avoid saturation

Red Side Calibrations

Type	Lamps	Slit	Exp	NExp	Notes
Arc	Ne,He,Hg	46000	30s	2	The Red chip QE fluctuates A LOT. Modify your exposure times accordingly.
DomeFlat	Red	46000	5s	5	The Red chip QE fluctuates A LOT. Modify your exposure times accordingly.

TWILIGHT+NIGHTTIME: In addition to these calibrations, if one wishes to flux the spectra it is necessary to observe a spectrophotometric standard with the same instrument configuration. Furthremore, it is best if one chooses from the CALSPEC list of standards which can be found here: $XIDL_DIR/Spec/Longslit/calib/standards/calspec Organizing the Data

The first steps are to organize your data, create a plan file, and edit it as necessary.

• Organize the data
  1. Create a 'Raw' directory -- mkdir Raw
  2. Place all of the Raw frames in the Raw directory
  3. gzip the files (optional)
• Create the plan file
  1. Launch IDL from above the Raw directory
  2. Run long_plan :: This code parses the headers of each raw file and creates an ASCII file which summarizes the data. The attempts a guess at the 'type' of each frame (e.g. arc, domeflat, science).
         Example: IDL> long_plan, '*.ccd*', 'Raw/'
• Edit the plan file (default: plan.par)
  1. Consider copying plan.par to a new file
  2. Edit slitthresh to be 0.10
  3. Remove unwanted images from the .par file
  4. Insure that there are both arc and domeflat files for each grating/grism/slit combination.
  5. Inspect+Edit the image types for the various exposures. For the KAST spectrometer there are 3 types of interest:
     
     

     Image Types

     Type	Description
     arc	Exposure of the arc lamps taken with the instrument configured for spectroscopy (slit + dispersing elements).
     domeflat	Spectrum of a quartz lamp shined onto the dome of the 3m. The instrument is configured for spectroscopy (slit+disperser element).
     science	Spectrum of an astronomical object. This includes 'standard stars'.

     
     
     

Reduce!

All of the reduction steps are run by the code long_reduce :: This routine
    Example: IDL> long_reduce, 'plan.par'

1. Stacks and process the flats
2. Identifies the edges of the longslit
3. Calibrates the various arc images
4. Processes the science frame (flattens)
5. Finds/traces objcets within the slit
6. Extracts using a non-parameteric optimal extraction algorithm

The pipeline outputs the following calibration files:

Reduction Products

Name	Description
pixflat-xxx	FITS file generated from the domeflats which corrects for pixel-to-pixel variations in the CCD.
profile-xxx	Binary FITS table containing the object profile used for optimal extraction. In particular, the tag .profile contains an image of the profile.
slits-xxx	Binary FITS table containing the parameters which describe the slit edges.
wave-xxx.fits	FITS image of the wavelength array created by the code.
wave-xxx.ps	Postscript file showing the results from the 1D arc solution.

Primary Science Product

The primary science products are written a multi-extension FITS file in the Science/ directory. One file is written per exposure. This table describes the various extensions:

Science Products

Extension	Description
0	Processed 2D image of the data.
1	Inverse variance
2	Sky model
3	Model of the object flux
4	Mask of good/bad pixels
5	Structure array (one per object identified in the slit). The key tags are: wave_opt, flux_opt, ivar_opt, wave_box, flux_box, ivar_box.

One can examine the 1 and 2D spectra extracted from each data frame using the following steps:

• cd Science
• UNIX> idl
• IDL> .run long_look
  This will launch a GUI list of all the sci-*.fits.gz files in the current directory.
• Choose the sci-* file that you want from the GUI list
  This will launch a view of the 2D image (using the IDL task atv). After completing the step described below, you can use this Image display GUI to examine the 2D spectrum. A postscript file summarizing the S/N of the various extracted spectra is shown and also a GUI list of the objects in the frame indexed slit-object (e.g. 1-2 for slit 1 and object #2 in the slit).
• Choose an object from the GUI list
  This will launch an x_specplot GUI that the user can use to examine the 1D spectrum. At this point the xatv window will now be active.
• Play with the two GUI's.

Coadd

As you will note, the multi-extension FITS file that contains the spectra are not especially easy to handle. Furthermore, it is common that a user will wish to coadd multiple exposures of a given object. This is accomplished with the task long_coadd :: This routine takes as input a list of sci- files, chooses a wavelength array for registration, and coadds weighting by the square of the median S/N ratio (or exposure time).

    Call: IDL> long_coadd, files, objid, OUTFIL=name
    Example: IDL> long_coadd, ['sci-b263.ccd.gz', 'sci-b265.ccd.gz'], 1, OUTFIL='FSpec/J0905+1014_b.fits'

In the above example, we have set objid=1 to indicate that we wish to coadd the first object identified in the slit. The output file (e.g. FSpec/J0905+1014_b.fits) is a multi-extension FITS array containing three extension: 0=flux array; 1=sigma array; 2=wavelength array.

One can inspect the data using the XIDL routine x_specplot.
    Example: IDL> x_specplot, filename, inflg=2 Flux

Fluxing is relatively straightforward. One includes a calibration standard in the plan.par file and the code will reduce the data in a similar fashion as the science exposures. One does NOT apply the COADD algorithm, but skips to this task long_sensfunc :: This algorithm requires that one use one of the standards included in the Longslit package (contact JH or JXP to add a new file). These files are found here: $XIDL_DIR/Spec/Longslit/calib/standards/calspec.

    Call: IDL> sens = long_sensfunc( sci_file, calib_name, sens_file)
    Example: IDL> bsens = long_sensfunc('Science/sci-b257.ccd.fits', 'feige34_005', 'Flux/feige34_bsens.fits')

With a sensitivity function in hand, the final step is to apply it to the science spectra. This is accomplished with long_fluxcal :: This code simply notes the exposure time and applies the sensitivity function. It also corrects for differences in the airmass of the two exposures assuming an extinction function appropriate for KPNO.

    Call: IDL> long_fluxcal, data_file, sens_file, OUTFIL=fluxed_data
    Example: IDL> long_fluxcal, 'FSpec/J0905+1014_xb.fits', 'Flux/feige34_bsens.fits', OUTFIL='FSpec/J0905+1014b_XF.fits'

One can inspect the data using the XIDL routine x_specplot.
    Example: IDL> x_specplot, filename, inflg=2