In this tutorial you will learn how to use AIP to make precise brightness measurements (magnitudes) for objects in a CCD image. AIP provides three distinct photometry tools...
AIP's photometry tools all use aperture photometry. The skybackground is determined in an annulus surrounding the analysis apereture. Tools are available to automatically determine the growth curve and to display the stellar profile. Differential photometry and automated differential photometry are accomplished using the single star photometry tool. The zero point for the magnitude system may be specified by the user.
AIP only eveluates instrumental magnitudes. There is no provision for determining atmospheric extinction or standard magnitude values. If reliable standard magnitudes and colors are required, images must be obtained using filters which match the bandpasses established for the standard photometric systems.
Pick out a CCD image for which you would like to know magnitude values for some of the stars in the image. Then follow along with the steps outlined below.
Select an image for which you would like to determine some instrumental magnitude values.
Step 1: Open the Single Star Photometry Tool
Open an image. Then use...
Measure | Photometry | Single Star...
At the top of the tool window you will see boxes for setting the inner and outer radii for the measurement apertures. The inner radii sets the size of the analysis aperture to measure the light from a star. The outer radii sets the size of an outer annulus used to measure the sky brightness. The sky background is measured in an annulus that begins at the radius of the annalysis aperture and extends outward to the outer radius. The inner radius should be set so it contains all (or as much as possible) of the light from the star you wish to measure.
Step 2: Test the Suitability of a Star for Photometry
Select a star by clicking on it. Then click the Test button at the bottom of the Single Star Photometry tool. A pair of windows will appear. (You may need to adjust the positions of the windows.) Each window will display a graph.
The first window will display a growth curve. It will plot the differential magnitude versus the analysis radius size (the inner radius). You can use this to determine the optimum size for the analysis aperture. Ideally, you want to see a level region on the graph. This should indicate that only light from a single star is being measured, and that the region in the surrounding annulus is free from other stars. Real stellar images actually have faint extended wings, so it is normally not practical to actually measure "all" the light coming from a star. One normally selects an annalysis aperture size that is just near the break point where the growth curve starts to become nearly level. If the analysis aperture is too large one runs the risk of including faint nearby stars. Also, with a larger analysis aperture more sky is included in the measurement and this will increase the noise in the final result. The outer radius is normally set at twice the value of the inner radius. It may be necessary to set the outer radius at a smaller value to avoid including nearby stars.
The second window shows the stellar profile for the selected star. Individual pixel values are plotted versus their distance from the stellar centroid. Ideally, you should see something like a classic Gaussian curve (a bell-shaped curve). There should be a peak at the center and a regular decrease for greater values from the centroid position. If the peak appears flattened, this indicates that more than one star exists within the analysis aperture.
You can use the profile window to estimate the half-width at half-maximum (HWHM). This is the distance in pixels at which the stellar profile reaches one half the maximum pixel value. The full-width at half-maximum (FWHM) is customarily used to describe the seeing for an image. It is this value converted into arcseconds that is customarily reported.
Select an inner and outer radius that will be appropriate for your specific application.
Step 3: Input the Image Data
Below the radii controls in the Photometry Tool window is a frame containing text boxes. These text boxes provide for user specification for the integration time in seconds and the magnitude zero point. AIP may try to "guess" these values or set default values, but the user may always override any default values.
For the integration time, AIP will try to read the exposure time from the image header. For simple, single images this will probably be correct. However, for images that are stacked or added, determining the appropriate integration time may not be so simple. For example, if four 2 minute exposures are tracked and accumulated, the effective integration time will be 8 minutes, or 480 seconds. However, if four 2 minute images are averaged, the effective integration time will be 2 minutes, or 120 seconds.
The magnitude zero point will set the zero point for the instrumental magnitude system. The magnitude zero point will depend on excatly what magnitude system one is intending to use (V, R, etc.). The zero point will depend on the aperture of the telescope, the reflectivity or transmission of all optical components in the light path (especially any filters), the sensitivity of the detector, and the clarity of the atmosphere (atmospheric extinction). For most instrumental systems, reasonable zero points are generally in the range between 15 and 25. The default value specified by AIP is 20.
Adjust the values for the integration time and the magnitude zero point as necessary for your application.
Step 4: Select and Measure a Star
Select (click on) a star you wish to measure. The click the Select Star button in the Photometry Tool window. Results will be written to the Star Data frame and to the Measurement Log.
Remember that these results are instrumental magnitudes only. The data values in the analysis aperture are corrected for sky baclground and converted to magnitudes.
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AIP provides a tool that automatically determines magnitudes for a selected variable star, a comparison star, and a check star. The magnitude difference between the variable and the comparison, and the check and the comparison are computed and displayed. This is accomplished using the Single Image Photometry tool. The Single Image tool uses the Single Star tool to obtain its measurements.
Select an image for which you would like to determine some magnitude differences.
Step 1: Open the Single Image Photometry Tool
Open an image that contains a variable star field, or some other field for which you wish to determine magnitude differences. Then open the Single Image Photometry tool...
Measure | Photometry | Single Image...
The Single Image Photometry tool will appear. Using this tool you can set the inner and outer radius to control the size of the analysis aperture and the size of the region to measure the sky. Select these radii to include all (or most) of the light from the stars of interest and to keep the measurements from interference from nearby companion stars.
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Step 2: Select the Comparison and Check Stars
The comparison star (C) is a star of presumed constant brightness which is used as the reference to determine the magnitude difference for the variable star. The check star (K) is a second comparison of presumed constant brightness which is used to check on the constancy of the comparison star.
Select the comparison star by clicking on its position in the image. Then click the C button. Select the check star and click on the K button. Data for the comparison and check stars will be displayed in the Single Image Photometry tool. The magnitude difference K-C will be displayed.
Step 3: Select the Variable Star
Click on any other star in the field. This could be the star that you know or suspect to be variable. Test this atsr for suitability by using the Test button. Examine the growth curve and the profile for abnormalities. After selecting a star click on the V button. The magnitude difference V-C will be displayed. Select any star in the field and click the V button and the magnitude difference with respect to the selected comparison star will be displayed.
The information you obtain from these measurements is logged in the Measurement Log window, but only when you click on the Log button. Click on the Log button for each measurement you wish to save.
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This is multiple-image photometry. If you have a series of images taken over time, this process can automatically extract magnitudes and compute magnitude differences for all images in the series.
Step 1: Setup Image Calibration
In general you will need to subtract darks or apply flat field corrections for all the images in your series of images. Use
Calibrate | Setup | Standard...
to setup the standard calibration protocol. As appropriate, select a master dark frame and/or master flat field frame to be used. These calibrations will automatically be applied to all images in your selected sequence.
Step 2: Open the Multiple Image Photometry Tool
Use
Measure | Photometry | Multiple Image...
Step 3 Select the Image Set to be Measured
Select all the files that you wish to measure. Since images are referenced in terms of a complete path ( device - directory - file name ) images can technically have the same names as long as they exist in different directories.
AIP can open SBIG images, but they must have an extension like .st7. Unfortunately, a CCDOPS auto-grab sequence uses a base file name but with an extension that is the number of the image in the sequence. This number begins with .001 and continues with .002, .003 etc. AIP will not open such images. To use AIP to reduce variable star differential photometry obtained as an auto-grab sequence, the names of all the image files must be changed to have an extension that is recognized by the program.
Step 4: Select the Photometer Radii
After you have selected an image set, the first image will be opened and displayed in an image window. Click anywhere in the image (make the image active, select the image) and a "bullseye" will be drawn in the image that represents the sizes of the analysis and sky apertures (inner and outer). Use the Out and In controls in the Multiple Image Photometry tool to adjust the sizes of these apertures. The inner radius (the analysis aperture) should be set to contain all the light from the stars to be measured. The outer radius (sky annulus) should be set so that contamination from neighboring stars is excluded.
Images which are out of focus or trailed or have poor seeing are not necessarily unuseable for photometry. The photometry radii just need to be adjusted to achieve optimum results.
Step 5: Select the Search Radius
The Multiple Image Photometry tool performs individual measurements on a series of images on the same star field. Once a set of V, C, and K stars has been selected the tool will attempt to track these stars from image to image. Since the field may shift slightly between exposures, it is necessary to specify a search distance in pixels measured from the C star. This is the distance within which the software will search for the comparison star in subsequent images. Specify this parameter by using the Search Radius control. Set this parameter large enough to accomodate any drift from one image to the next, but small enough to exclude the possibility of incorrectly selecting a companion star.
Step 6: Select the C, V, and K stars
Select your comparison star (C), variable star (V), and check star (K). Select each star and then click the appropriate button. A blue bullseye showing the aperture sizes will be drawn around each star. If you decide to change your selection, use the Clear Selection button and then reselect the stars.
Step 7: Select the Star Selection Mode
You can specify the Automatic or the Manual mode by clicking the appropriate button. In automatic mode the software will automatically search for the stars in each subsequent image. The stars will be searched for within the Search Radius of their positions in the previous image. If the image drift is small and relatively consistent you should be able to use the automatic mode. If your telescope tracks reasonably well and you do not move the telescope between exposures in a sequence, you should be able to use the automatic mode.
In the manual mode the user must select the C, V, and K stars manually in each image. If the image drift is large and irregular, you will probably need to use the manual mode. If you move the telescope between exposures in a sequence, you will probably need to use the manual mode. If you have images of the same field from several nights and wish to reduce the data as a sequence, you will probably need to use the manual mode.
Step 8: Set the Tracking Mode
This setting determines how automatic tracking is actually accomplished. The tracking mode determines whether only the comparison star (C only) is used to track the images and compensate for image drift, or whether all three stars (C,V,K) are individually tracked between images. For typical variable star applications, select the C only mode. This presumes that the shift determined for the comparison star will also apply for the variable and check stars. For moving objects such as minor planets and comets, select the C,V,K mode. This will allow the moving object (presumably V) to move with respect to the comparison stars from one image to the next. The Search Radius determines how far from the positions in the previous image the software will search in the current image. Remember that Automatic tracking of any kind will only work in AIP if the image drift is small and consistent from one image to the next.
Step 9: Start the Photometry
Start the Multiple Image Photometry process by clicking the OK button. You will be prompted for a location to save the photometry data that will be generated. Select a convenient location on your computer system. Data for each image will be written to this photomatry file as plain text.
Each successive image will appear on the screen, the C, V, and K stars will be located and their magnitude differences will be written to the photometry file. The last image will remain on the screen with the C, V, and K stars marked with their bullseyes. A graph will be displayed which shows the V-C magnitude and the K-C magnitude. If all was well, the K-C should be a horizontal line indicating no variation. If the variable changed over the series of images, V-C should not look like a horizontal line. (The current release of the software plots the magnitude differences increasing upward. Since larger V-C denotes a decrease in brightness for the variable, this plot may not look right. It is customary for astronomical magnitudes to be plotted increasing downward.)
Step 10: Analyze the Data
Import the photometry file data into a spreadsheet program or other analysis program. The photometry data file, like all data files written by AIP, can easily be read into spreadsheet programs such as Excel.
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