skyinterp.cl

Name

skyinterp -- interpolate over regions where background is strong

Usage

skyinterp inimage outimage skyimage

Parameters

inimage: Names of existing images to be cleaned
outimage: Names of cleaned images to produce
skyimage: Name of reference sky spectrum
thresh: Threshold sky value. Pixels in the input images which correspond to pixels in the skyimage having sky values above the threshold value will be interpolated over.

Description

This task can be used to effectively clean extracted spectra of regions which are degraded to to imperfect background subtraction in the vicinity of strong emission features. Such regions commonly occur when spectra of faint objects are reduced.

First, inspect a plot of the reference sky spectrum. Determine the flux level which defines, in your judgement, a "strong" emission line which is likely to cause trouble in subtraction. Set the thresh parameter for this task to that value.

For each input image, this task will perform the following operations:

Create a temporary image by passing a 29-point median filter across the image to smooth on long scales.
Create a temporary image whose pixels consist of data from the science image (for regions of low sky background), or data from the median-smoothed image (for regions where the sky flux in the reference image exceeds the threshold).
Pass a 9-point median filter over this image to generate an image smoothed on smaller scales.
Create the output image, consisting of pixels from the input image in areas of low sky background (sky value < threshold) and pixels from the smoothed image created in the previous step in those regions where the sky exceeds the threshold.

Examples

Replace sky-dominated pixels in the image foo.fits based on sky values from the file sky.fits, creating the output image clean.fits:
```
    donspec foo.fits clean.fits sky.fits thresh=15000
```
The plot below shows NIRSPEC data before (white) and after (red) cleaning via this task.