gmosinfoimag -- Description of reduction scripts for GMOS imaging data




The GMOS package contains tasks for processing GMOS imaging, long-slit, multi-object, and integral field spectroscopy data. The specifics of the individual tasks can be found in their help files. This document describes the common features of the imaging tasks.

The tasks are designed to provide a fairly complete and flexible reduction for the purpose of assessing data quality at the time of observation. Real-time reductions may not be optimal for a particular science application. The GMOS package scripts can be optimized for a particular application using the hidden parameters to achieve the best possible results.

The tasks produce logfiles of the performed processing steps. The name of the logfile may be set in each individual task, or at the package level by setting gmos.logfile.

The tasks add header keywords to the output images. These header keywords contain information about the performed processing steps and the values of the critical parameters that were used.

All GMOS images are written as multi-extension FITS (MEF) files. Raw data can have up to six unnamed extensions. Most of the header information is written to the primary header unit [0] (PHU). After being processed with GPREPARE, GMOS data extensions will be named as described in GMOSINFO.

It is recommended to use imtype="fits". This is set automatically when loading the GEMINI package.


GIFLAT - Derive imaging flat field.
GIFLAT is used for deriving normalized flat fields for the GMOS imaging data. The input images are calibration unit (GCAL) flat fields, twilight flats, or dome flats. The output images are the normalized flat field. If the input files are raw, GIFLAT calls GPREPARE to update the headers and GIREDUCE to trim input images, subtract the overscan level, subtract the bias image and create variance and data quality frames. The images are also multiplied with the gains. GIFLAT uses GEMTOOLS.GEMCOMBINE to combine the flat fields.
GIREDUCE - Reduce GMOS imaging data
GIREDUCE is used to trim, subtract overscan level, subtract bias image, and subtract (scaled) dark image, and flat field correct the science data. Any subset of these reductions may be chosen by setting the flags of the task accordingly. After processing with GIREDUCE the images have been multiplied with the gains and the count levels in the output images are in units of electrons. GIREDUCE creates variance and data quality planes if requested. Variance frames based on Poisson statistics are only created if bias subtracting.
GIFRINGE - Derive fringe frame for GMOS images
GIFRINGE is used to create fringe frames for GMOS imaging data. Fringing is significant for i' and z' imaging from both GMOS-N and GMOS-S. GIFRINGE may also be used to create scattered light frames.
GIRMFRINGE - Subtract fringe frame from GMOS images
GIRMFRINGE is used to subtract off a fringe frame created by GIFRINGE. Scaling of thee fringe frame is possible.
GMOSAIC - Mosaic the 3 GMOS CCDs into one image
GMOSAIC will create MEF file with a single pixel extension that contains the a mosaic of the 3 CCDs. GMOSAIC correctly accounts for the gaps between the CCDs.


For typical reductions the user will need appropriate bias, dark, and flat field images, and the science image. A fringe frame (or scattered light frame) may be created from the science data or from observations of a blank field.

1. Use GPREPARE to update the raw data headers for the reductions. The other tasks will call GPREPARE as needed if the data has not been GPREPAREd.

2. Use GBIAS to create a bias image. It is recommended to use fl_over=yes and subtract the overscan level.

3. Use GIFLAT to derive normalized flat fields. For twilight flats, the parameters may need adjusting to correctly clean the images for signal from stars.

4. Use GIREDUCE to trim the images, subtract the overscan level, subtract the bias image, subtract the scaled dark image and divide by the flat field. The dark current for GMOS is low enough that subtraction of the dark image is normally not needed, i.e., it is recommended to use fl_dark=no.

5. If needed use GIFRINGE to create a fringe frame, or scattered light frame. Use GIRMFRINGE to subtract the fringe frame from the science data.

6. Use GMOSAIC to create a mosaic of the 3 GMOS CCDs into one image.


The processed images may be co-added with the task GEMTOOLS.IMCOADD. Photometry may be derived with any suitable photometry package. Images processed to the point of mosaicing with GMOSAIC may be used for mask designs using the GMOS Mask Making Software. Dithered observations may be co-added with GEMTOOLS.IMCOADD and then used for mask design. Further information about the GMOS Mask Making is available at
and by following links on that page.


An example reduction script is available, see GMOSEXAMPLES.


The tasks in the GMOS package have been tested with three and six extension MEF files.

The tasks in the GMOS package are designed to operate on MEF FITS images that have been processed using GPREPARE. GPREPARE will not run on data from instruments other than GMOS. The GMOS tasks will not run on simple FITS files.

Input image names should in general not contain directory paths as not all tasks have been tested in this mode.

Improvements to the (previously semi-functional) variance and data quality propagation in the GMOS package have undergone limited testing; the accuracy of the results should still be verified at each step by the scientist, particularly for spectroscopic data reduction tasks.


gprepare, giflat, gifringe, girmfringe, gireduce, gbias, gmosaic, gmosinfo, gmosexamples, imcoadd