To begin, display the first image in ds9 (the images should still be xy-flipped) and load the region file. You will use these parameters to measure the counts of your five chosen stars in each image.ġ. "Annulus" is the number of pixels from the center of the star where the annulus begins, and "dannulus" is the width, in pixels, of the annulus. The other two parameters, "annulus" and "dannulus," determine the size of the annulus and comprise the area of the image used for the background sky value. The "aperture" parameter is the radius, in pixels, of a circle inside of which counts will be measured. Three parameters determine the aperture and annulus. The package "phot" takes a user-determined aperture (circle around the star) and annulus (donut surrounding the aperture) to measure a star's counts, minus the sky. We will now use the iraf package "phot" to do photometry. Average the five stars' FWHMs, and put the result into. A row of information should appear in the iraf window each time you hit "a." The FWHM (in pixels) is the rightmost of these numbers. Type "imexam" into iraf, and hit "a" once on each of the five stars. It is much better to find a mistake like that now instead of at the end of the stacking procedure.ĭisplay an image and load the regions. For sanity's sake, "prevu" each image with the regions loaded to make sure no star is too close to the edge of any image. This will allow you to quickly identify the five chosen stars in other images. reg file will be in the 8615 of the 8615/8685 pair. reg file, in the lowest angstrom directory of the filter pair. Once five stars that satisfy the above criteria are chosen, put a circular region around each one, and save the region file (containing all five regions) as a. A galaxy will have a higher FWHM than a star and a broad imexam profile. A saturated star will have a black "hole" in its center, and its imexam profile will show a large plateau. Do not choose saturated stars or galaxies.Do not choose stars that are too close to other objects (this will interfere with selecting a good aperture).Choose stars with counts of at least 100,000.NOTE: This does not mean that the stars have to be the same flux, only that they are the same visual size. Choose stars that are roughly the same size (this will help later when selecting apertures).Remember, these stars will be used in ALL images thus, if a star is too close to the edge in one image, in another image it could be completely absent because of dithering. Be careful, however: do not choose stars that are too close to the edge. I usually choose one star in each corner and one star in the center. The stars should be spread out in the frame. However, they can't just be any stars here are a few guidelines to follow: You need to choose five stars on which you will perform photometry. Open the first image, making sure it is xy-flipped. THAT IS, CLICK "ZOOM" AND CHOOSE "XY." DO THIS BEFORE CONTINUING. IMPORTANT!! BEFORE DOING ANY PHOTOMETRY, MAKE SURE THAT THE IMAGES ARE XY-FLIPPED IN DS9. There are four main steps involved in photometry: choosing five good stars calculating the FWHM of each image doing the actual photometry on the five stars in each image and choosing the best 20 (or 12, or 4) images, and best center image, to use when stacking. Update its list of images to correspond to the field with which you're working. More on this later.īefore anything, copy an file from a previously-completed directory to your working directory. Thus, if you have more than the required number of images, you will be able to choose the best 20 (or 12, or 4) images to stack, after you perform photometry. The number of fields needed to stack are as follows: For example, /data/photometry/WYSCH/19/9155.īecause of the relative sensitivities of different filters, for any given field a different number of images must be stacked for different filters. Photometry will be performed in the directory into which you sorted your images after reduction.
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