Photometry will be performed in the directory into which you sorted your images after reduction. For example, /data/photometry/UGC08201/V/.
Before anything, copy an images.info file from a previously-completed directory to your working directory. Update its list of images to correspond to the field with which you're working.
*The images.info file lists each image file name, the FWHM, and any photometric notes on each image. This file will be used by a program later in the stacking process so make sure you have one in each galaxy/filter directory.
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 images, and best center image, to use when stacking.
IMPORTANT!! BEFORE DOING ANY PHOTOMETRY, MAKE SURE THAT THE IMAGES orientation match that of the dss image! Open up your dss images and flip "YOUR" image to match the dss image.
You need to choose five stars on which you will perform photometry. However, they can't just be any stars; here are a few guidelines to follow:
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 .reg file. This will allow you to quickly identify the five chosen stars in other images. For sanity's sake, use the prevu.cl iraf task on all images with the regions loaded to make sure no star is too close to the edge of any image. It is much better to find a mistake like that now instead of at the end of the stacking procedure.
- The stars should be spread out in the frame. I usually choose one star in each corner and one star in the center. Be careful, however: do not choose stars that are too close to the edge. 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.
- Choose stars that can be photometered in all bands for your galaxy. A very red star may not show up in a U-band image. Note: you can choose different stars for each image, but the steps laid out here will take advantage of using the same stars so that the full procedure does not have to be done on each filter/galaxy combination. This will become clear later.
- Choose unsaturated stars! Important. you can get bad centering if you choose a saturated star.
- Choose stars with counts of at least 100,000.
- Do not choose stars that are too close to other objects (this will interfere with selecting a good aperture).
Display an image and load the regions. Type "imexam" into iraf, and hit "a" once on each of the five stars. 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. Average the five stars' FWHMs, and put the result into images.info. I usually round to one decimal place. Do this for each image.
Real Live Photometry
We will now use the iraf package "phot" to do photometry. The package is located in noao.digiphot.apphot.
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. Three parameters determine the aperture and annulus. The "aperture" parameter is the radius, in pixels, of a circle inside of which counts will be measured. 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. "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. You will use these parameters to measure the counts of your five chosen stars in each image.
1. To begin, display the first image in ds9 (the images should still be xy-flipped) and load the region file. Type phot into iraf, and you will be prompted to enter the image name. Do so, excluding the .fit (this is because phot creates a file called IMAGENAME.mag.1; we do not want it to be called IMAGENAME.fit.mag.1). NOTE: Simply running phot in iraf does not automatically display the desired image. You must display the image on which you want to perform phot separately.
2. You now must determine the aperture and annulus settings to use. First, epar into phot and make sure the parameter "radplot" is set to "yes." Then, find the largest of the five stars and hit "t" in the center of that star. (Get as close to the center as you can without being overly obsessive about it. Most of the time you can get close enough without zooming in, but sometimes it helps to zoom.) A plot will display in a tektronix window (if a Tek window is already open, it will just display there), and you are looking for where the plot begins to flatten out. The curved part of the plot is the star, and the flat part is the sky. To find where the curved part ends, you will have to play around with the aperture, annulus, and dannulus parameters. To do so, with your cursor in ds9, type :ap #, :an #, or :dan #, respectively, and hit enter. NOTE: MAKE SURE THE CURSOR IS IN DS9 WHEN YOU HIT COLON. IF IT'S NOT, THE PARAMETERS WILL NOT CHANGE. You want to put both the aperture and annulus right where the plot flattens out, and I usually use a value of 4 for dannulus (you can use whole numbers; it will be precise enough). This will ensure that the sky you choose will be as close to the true background of the star as possible. Note that you do not need to set the aperture setting until after you've perfected annulus and dannulus. I usually start with an=10 and dan=10, play around with "an" until it's at the correct value, then set "ap" to that value and "dan" to 4. For example, if the annulus is set to 5 and dannulus is set to 10, the plot will display a radius from 5 to 15. NOTE: Sometimes certain images will have rogue bright pixels near the stars you've chosen (cosmic rays, etc.); this is fine, as long as they don't appear in all images -- this would mean the bright spot is a real source, and you'd have to choose a different star. Just be aware of these rogue pixels, and if necessary, put a little space between the aperture and the annulus to avoid the bright pixel.
3. Once you have chosen the three parameters, hit spacebar in the center of each of the stars, once and only once per star. IMPORTANT!! You must do this in the same order in every image. I go clockwise (upper left, upper right, lower right, lower left, center). Each stroke of the spacebar saves information about that star in the newly created .mag.1 file. When all five stars have had their turn with the spacebar, hit "q" twice. The .mag.1 file should now be created.
4. Repeat the above process for every image.
1. To pare the list down, start throwing out less-than-ideal images: those with high FWHMs, those with any streaking or other large-scale structure, etc. To make things easier later, I usually delete the .mag.1 files of the images I'm not going to use, just so there's no confusion. If and when you throw out images, note the reason in images.info. Also, note in the stacking directory, in a notes file, how many images of each field you used, and which images, if any, you rejected (see notes.master.txt in pawnee:/data/stack for an example).
2. Now you need to choose the "center" of all your images. This is exactly what it sounds like: the "center" image is the image around which all the other images are best centered. You need to choose a center from all images in all filters, not just a single filter. To do this, make a list of all the images and prevu the list (prevu @images.list). Put your cursor on a star, leave it there, and hit spacebar to cycle through the images. Notice how the star moves around your cursor, and note an image that seems to be in the center of all the others. Copy that image's .mag.1 file into the other filter directories for that particular galaxy.
Get .mag.1 Files
3. Copy all of your *.mag.1 (including the center.mag.1) files into the corresponding stack directory (e.g., /data/stack/GALAXY/FILTER/.