7. Tips and Tricks

7.1. Using the Lamp-off Flat for the K-band Arc

There is continuum flux in the K-band arcs. As part of the standard calibrations for R3K + K-band filter observations, a lamp-off flat with an exposure time equal to the arc’s exposure time is taken right before or after the arc. This lamp-off flat is needed when reducing the arc with Gemini IRAF. It is not needed with DRAGONS.

Nevertheless, DRAGONS can use it if you think it is necessary. Here’s how.

dataselect ../playdata/example3/*.fits --tags ARC -o arc.lis
dataselect ../playdata/example3/*.fits --tags FLAT,LAMPOFF -o lampoff.lis

reduce @arc.lis @lampoff.lis

The lamp-off flat(s), instead of a dark, will be subtracted from the arc.

7.2. Adjusting the Wavelength Zeropoint

Following the wavelength calibration, the default recipe has an optional step to adjust the wavelength zero point using the sky lines. By default, this step will NOT make any adjustment. We found that in general, the adjustment is so small as being in the noise. If you wish to make an adjustment, in pixels, use the shift parameter. A value of 0 is the default and applies no shift. The parameter can be set to a value set my you, eg. -p adjustWavelengthZeroPoint:shift=1.3. Or, you can let the software measure the shift for you by setting shift to None. This will trigger the algorithm that tries to calculate the shift on it’s own.

7.3. Inspect the sensitivity function

The sensitivity function is stored in the processed telluric star file. To inspect the sensitivity function, you can use the following Python code.

import numpy as np
import matplotlib.pyplot as plt

import astrodata
import gemini_instruments

from gempy.library import astromodels as am

ad = astrodata.open('N20210407S0188_telluric.fits')
sensfunc = am.table_to_model(ad[0].SENSFUNC)
w = ad[0].wcs(np.arange(ad[0].data.size))

std_wave_unit = ad[0].SENSFUNC['knots'].unit
std_flux_unit = ad[0].SENSFUNC['coefficients'].unit

plt.xlabel(f'Wavelength ({std_wave_unit})')
plt.ylabel(f'{std_flux_unit}')
plt.plot(w, sensfunc(w))
plt.show()

7.4. Useful parameters

7.4.1. skip_primitive

I might happen that you will want or need to not run a primitive in a recipe. You could copy the recipe over and edit it. Or you could invoke the skip_primitive parameter to tell DRAGONS to completely skip that step.

Let’s say that you want the data aligned but not stacked. You would do:

reduce @sci.lis -p stackFrames:skip_primitive=True

7.4.2. write_outputs

When debugging or when there’s a need to inspect intermediate products, you might want to write the output of a specific primitive to disk. This is done with the write_outputs parameter.

For example, to write the extracted spectrum before it is corrected for telluric features and flux calibrated, you would do:

reduce @sci.lis -p extractSpectra:write_outputs=True