Description
- Class:
- Alias:
picture_frame
Overview
The picture_frame step removes thermal artifacts (the “picture frame”
effect)
from calibrated ramp images, after the jump step
and prior to performing the clean_flicker_noise
or the ramp_fitting step.
The picture frame artifacts are corrected by scaling and subtracting a reference rate image, containing only the thermal background, stored in a PICTUREFRAME reference file.
The correction is available for NIRSpec FULL frame exposures only. It is incorporated into the calwebb_detector1 pipeline for ramp data, but it may also be called as a standalone step on a ramp or rate exposure. The only difference in processing if the input is a rate image or rateints cube instead of a ramp is that a draft rate file is not necessary: the scene mask is derived from the input and correction is directly applied to the rate image(s).
Correction Algorithm
The picture_frame step proceeds similarly to the
clean_flicker_noise step: it generates a draft
rate image from the input ramp, creates a background mask from the rate image,
fits the correction to the background data in each group in the ramp, and subtracts the
correction from the group image.
To create the background mask, the step uses the process outlined in Creation of a scene mask, with the difference that science regions are masked by default for picture frame processing.
After background data is identified, median values are computed over the edges (\(m_{eref}\)) and center (\(m_{cref}\)) of the picture frame reference image, at the valid background locations only. Then, for each group image in each integration, the cleaning process is:
Make a difference image (current group – first group) to correct. The first group is left uncorrected.
Calculate the median values at the edges (\(m_e\)) and center (\(m_c\)) of the difference image at valid background locations.
Use the median values for the difference image to scale and offset the picture frame reference image (\(p\)):
\[ \begin{align}\begin{aligned}scale &= (m_c - m_e) / (m_{cref} - m_{eref})\\correction &= scale * (p - m_{eref}) + m_e\end{aligned}\end{align} \]Subtract the correction image from the group image.
References
The picture frame correction algorithm is based on work by M. Regan and E. Bergeron (in prep).