![]() Unlike alignment and stacking, you only need to do this set of steps once (i.e., once for each galaxy).ġ. This final task adds WCS coordinates to the headers of the images. Repeat the above alignment and stacking steps for the other filters.imcombine stack.fit combine=average reject=avsigclip lthresh=INDEF hthresh=INDEF hsigma=3.2.Here's the imcombine command to enter into iraf (leave out the hsigma parameter at first): Also, once you reach a good hsigma value the image will not change significatly. This should rarely, if ever, happen, but if higher and higher hsigmas don't seem to be working, perhaps consider trying to tweak lsigma. NOTE: There was one case I ran into where, instead of increasing hsigma, I had to increase lsigma. You can be pretty liberal with this continue going higher until you're absolutely sure all streaking and graininess are gone. Keep going up until the streaking/graininess disappears. If this is the case, remake the stack with increasingly higher values of hsigma, beginning at 3 and going up in increments of 0.1. However, every once in awhile you'll get a field whose stack looks very streaky or grainy. To stack the images, all you need to do is combine them using imcombine, using certain parameters. See stack.txt at the beginning of this page. If the star moves around, the alignment failed. If you place a region circle on a star, you will be able to check if the images are truly aligned. Press spacebar to cycle through the images. Enter into iraf: geotran Coordinates.list Go through the newly aligned images by entering "prevu (prevu.cl can be found in my photometry section) into iraf. All names should have an "S" at the end of them this stands for scaled and "A" is for aligned.Ħ. Open up Aligned.list and add "A" at the end of all file names. Obviously, update this line for whatever field/filter with which you're working.ĥ. Enter: ls /data/photometry/GALAXY/TELESCOPE/FILTER/*A.fit > ToAlign.list. Go back and redo those photometry steps.Ĥ. It usually means that you accidentally hit spacebar on the wrong star during phot, or you spacebar'd the stars in the wrong order. If the grid becomes askew (you'll know), or if the plus signs suddenly move around, something's wrong. The small plus signs should stay in relatively the same place, and the grid of lines should remain basically horizontal and vertical. ![]() IMPORTANT!! Watch carefully as the Textronic window cycles through the images. Hit "q" in the Textronic window to cycle through all the images. Enter into iraf: geomap Coordinates.list 1 xpix 1 ypix where x/ypix are the size of your image. Enter into a terminal: ls *.txt > ToCoordinates.listģ. Change to the galaxies/GALAXY/TELESCOPE/FILTER/ direcotry.Ģ. The following steps will align your images around the center image you chose.ġ. These pixel values will be used by an IRAF task to shift each image to that of the center image. This program also create *.txt files in the "galaxies/GALAXY/TELESCOPE/FILTER/ directory with the center and image pixel values of all 5 stars from the photometry steps. Copy and paste into an IRAF terminal for the directory /data/stack/. This program creates a file called "scale.cl" which has IRAF scripts to scale the images for the appropriate airmass. For each galaxy/filter combination the location of the center.mag.1 file will indicate the beginning of a new galaxy/filter/ set of individual *.mag.1 files: Add the new image's *.mag.1 to the file called image.list (or generate a brand new one). I will explain here, in greater detail, the procedures.Ĭorrect for Airmass and Setup Files for Aligning the imagesġ. The file stack.txt contains an overview of the steps involved in stacking, for quick reference later. Stacking, in short, consists of aligning and literally stacking images of like fields on top of one another to increase flux and depth. After reducing, calibrating, and performing photometry on data, we are finally able to stack.
0 Comments
Leave a Reply. |