Combinning arc frames

Quickstart

The command for running the arc frame combination procedure is called by:

pylongslit_combine_arcs PATH_TO_CONFIG_FILE

The arc frames are (optionally) bias-subtracted and then simply summed together. The output is a single arc frame master_arc.fits, which is saved in the output directory specified in the configuration file. The frame is automatically rotated and flipped so it is oriented with x being the spectral axis, with the wavelength increasing from left to right - this is the default orientation for the rest of the software, after the raw data has been processed.

Quality Assesment

The arc combinning procedure will show the final master arc upon exiting. You can press h to normalize the master arc to emphasize the contrast between the arc lines and the background - however, this might exagerate the very faint lines to be more visible than they actually are. The documentation for your instrument should have a map of the expected arc lines and their wavelengths - you can use it to check if the arc lines are visible and the expected range of the detector is covered (a common error is if a lamp from one ion is forgotten to be included in the input directory, meaning to a certain wavelength range is not being covered by the arc lines). The example below is from the example data set GQ1218+0832 from GTC OSIRIS both before and after normalization:

_images/master_arc_non_normalized.png _images/master_arc_normalized.png

Inspecting the arc map from OSIRIS webpage , it is a able to see that overall pattern of the arc-lines is as expected:

_images/R1000B_arc.jpg

Parameter options

The relevant parameters for the arc combinning procedure are (with example values):

"arc" : {
    "arc_dir": "/home/kostas/Documents/PyLongslit_dev/SDSS_J213510+2728/arcs"
},

"combine_arcs" : {
    "skip_bias": false
}

If you set "skip_bias": true, the bias subtraction will be skipped. This is sometimes useful for instruments where the detector gain is different for the regular calibration frames and the arc frames, since this can force the noise in the arc frames to become negative - resulting in numerical issues.

The arc_dir parameter specifies the directory where the raw arc frames are located. No other type of frames should be present in this directory.

For users new to data reduction - short introduction to arc lamp frames

Arc lamp frames are used to wavelength calibrate the detector, i.e. to assign a wavelength to each pixel in the detector. This procedure is described in detail in the wavelength calibration section. Every raw arc lamp frame has a emission spectrum of one or several ions with known wavelengths. In this step the arc frames are combined to create a single master arc frame, so the wavelength calibration can be done on a single frame, instead of on each individual arc frame.