Creating Mask Files

Once all object priorities and slit angles have been set, you are ready to create mask file(s). This step will allow you to create a minimal ODF for each mask required for the current FITS image. The software has the ability to select objects based on their priority to optimally place the slits in the mask. The following steps outline how to create the mask:

1. Click on the "Design Mask" button in the catalog window. This will bring up a pop-up window similar to the window shown below.
2. Enter the "Central Wavelength" in nanometers and select the "Grating&Order" and "Filter" using the pull-down menus. Not all filters are available at both GMOS-N and GMOS-S, refer to the filter lists linked from the GMOS components page for details.
3. The default slit width is 1 arcsecond if the slitsize_x column is not included in the Object Table. This value may be changed in the window below - as it has been done in the example.
4. To simultaneously create more than one mask for the field, the "Number of Masks" parameter may be changed. Adjust the slider position adjacent to this parameter to increase/decrease the number of masks requested.
5. When happy with the parameter values, click on "Make Mask Files".
6. If you have already attempted to create a mask for the current image, the program will warn you that the mask name already exists and that it wishes to overwrite this file. If you do not wish to overwrite the old mask name, either click on "Enter New Filename" or "Browse for New File Name". Clicking on the "Browse..." button also allows you to enter a new filename.
7. When the masks are designed, the pop-up window shown below will not contain information about how the objects are sorted into the masks. If you find that you have too few high priority objects for the number of masks you have, you can go back to the catalog window and change the priorities, however be sure to close all mask making windows first.
8. If the masks look like they have an appropriate number of objects, then click on "Make Mask Files".
9. Once the mask has been created, click on "Close" on the mask design window.

Some points to note:

• The information in the above window tells you the number of objects selected for each mask sorted by priority. In the example, two masks were requested and thus two ODF FITS files were created: mask_OTODF1.fits and mask_OTODF2.fits for Mask 1 and Mask 2 respectively. These files will contain the information about which objects were selected for each mask. In the window shown above, the files are referred to by the matching *cat names.
• If the OT catalog did not have a slitsize_y column then the slit length will be set to 5 arcseconds. Otherwise the slit length is taken from the values of slitsize_y for each object in the table.
• The position of the slit is taken from the values listed in the catalog columns and converted to pixel position using x_ccd/y_ccd + slitpos_x/slitpos_y.
• The length of the spectra for each slit is determined by the selection of filter, grating and order and bandwidth combination. Pre-defined calculations determine the expected first order spectrum length. The spectrum length is stated at the bottom of the window (visible after scrolling down).
• If multiple masks are created, each mask will preferentially include the maximum possible number of Priority 1 objects with lower priority objects filling any spare spaces. The following mask will not include any objects used in the prior masks, except for acquisition objects, which will be included in all masks.
• If any priority changes are required, close all mask making windows first, otherwise any changes made in the catalog window are not recognized by the already-open mask making windows.

Back to GMOS Mask Making Software Instructions

Last update April 30, 2004; Inger Jørgensen
In original form February 10, 2003; Dione Scheltus