Sky Tweaking and Subtraction

Functions
int	eris_ifu_plot_cube_background (cpl_imagelist *obj)
	Extract background spectrum from cube and plot it.
cpl_imagelist *	eris_ifu_sky_tweak (cpl_imagelist *obj, cpl_imagelist *sky, const cpl_propertylist *header, float min_frac, int tbsub, int skip_last, int stretch, int stretch_degree, int stretch_resampling, int plot, cpl_imagelist **new_sky)
	Main sky tweaking and subtraction function.

Detailed Description

This module provides advanced sky subtraction algorithms for IFU spectroscopy, including OH line emission correction and thermal background removal.

Sky tweaking corrects for temporal variations in sky emission between object and sky observations. The main operations include:

• Sky spectrum stretching to align OH emission lines
• OH vibrational transition band fitting and scaling
• Thermal background modeling using Planck function
• Adaptive scaling per wavelength sub-band

The algorithm separates the sky spectrum into continuum and line regions, fits scaling factors for OH line bands, and models thermal emission.

Function Documentation

eris_ifu_plot_cube_background()

int eris_ifu_plot_cube_background

(

cpl_imagelist *

obj

)

Extract background spectrum from cube and plot it.

Parameters

obj	Input data cube

Returns

0 on success, -1 on error

This function creates an object mask to identify background pixels, extracts the background spectrum, and displays it using gnuplot.

Note

Requires gnuplot for visualization

Uses 30% threshold for mask creation

Possible cpl_error_code set in this function:

• CPL_ERROR_NULL_INPUT if input is NULL

Definition at line 173 of file eris_ifu_sky_tweak.c.

References eris_ifu_lcorr_create_object_mask().

eris_ifu_sky_tweak()

cpl_imagelist * eris_ifu_sky_tweak	(	cpl_imagelist *	obj,
		cpl_imagelist *	sky,
		const cpl_propertylist *	header,
		float	min_frac,
		int	tbsub,
		int	skip_last,
		int	stretch,
		int	stretch_degree,
		int	stretch_resampling,
		int	plot,
		cpl_imagelist **	new_sky
	)

Main sky tweaking and subtraction function.

Parameters

obj	Object cube to be sky-subtracted
sky	Sky observation cube
header	Property list with WCS keywords (NAXIS3/CRVAL3/CDELT3/CRPIX3)
min_frac	Minimum fraction of pixels for background (e.g., 0.3 = 30%)
tbsub	Flag: subtract thermal background from obj (1=yes, 0=no)
skip_last	Flag: skip last wavelength sub-band in scaling (1=yes, 0=no)
stretch	Flag: apply sky stretching before subtraction (1=yes, 0=no)
stretch_degree	Polynomial degree for sky stretching (typically 2-4)
stretch_resampling	Resampling method: 1=linear, 2=spline
plot	Debug plotting flag (>0 enables plots)
new_sky	Output: corrected sky cube (allocated by function)

Returns

Sky-subtracted object cube, or NULL on error

This is the main entry point for advanced sky subtraction. The algorithm:

1. Optionally stretches sky spectrum to align OH lines with object
2. Creates object mask to identify background pixels
3. Extracts background spectra from object and sky cubes
4. Fits OH vibrational band scaling factors (19-20 sub-bands)
5. Models and removes thermal background emission
6. Applies corrections to sky cube and subtracts from object

Note

The new_sky cube is created by this function and must be deallocated by caller

Input cubes must have identical dimensions

Thermal background uses Planck function fit with 3 parameters

If thermal background fit fails, retry with 98% clipping

Possible cpl_error_code set in this function:

• CPL_ERROR_NULL_INPUT if any input pointer is NULL
• CPL_ERROR_ILLEGAL_INPUT if dimensions mismatch or invalid parameters

Definition at line 235 of file eris_ifu_sky_tweak.c.

References eris_check_error_code(), eris_ifu_lcorr_create_lambda_vector(), and eris_ifu_lcorr_create_object_mask().

Referenced by eris_ifu_jitter_process_cubes().