Measuring Instrumental Magnitudes

This recipe shows how to use GAIA to measure instrumental magnitudes for objects in an image. The technique used here is `aperture photometry', which involves positioning a circular cursor over the object to be measured and comparing the total intensity within the aperture with the intensity in a similar area of blank sky. There are various techniques for measuring instrumental magnitudes, and they are discussed, along with related matters, in SC/6: The CCD Photometric Calibration Cookbook[16]. If you are not familiar with photometric techniques then SC/6 is a suitable place to start. The aperture photometry facilities in GAIA are provided by invoking the PHOTOM package (see SUN/45[10]) `behind the scenes', though you will just see seamless interaction via the GAIA windows.

Before you attempt to measure instrumental magnitudes from a CCD frame any instrumental effects, cosmic-ray events and other blemishes should already have been removed. This process is described in SC/5: The 2-D CCD Data Reduction Cookbook[7] and in SUN/139[9], the manual for the CCD data reduction package CCDPACK, and is not considered further here. SC/5 is a good introduction.

The image used in this recipe is ngc1275jkt.sdf, a reduced V band CCD image of NGC 1275 obtained with the JKT (see the recipe in Section  for its provenance). Proceed as follows.

1. Start GAIA and load image ngc1275jkt.sdf.

2. You may wish to adjust the appearance of the displayed image. The most likely items to change are the colour table (click on the Color Map: button in the lower right of the control panel).and the magnification (click on the Scale: button in the bottom left of the control panel in the centre top of the window)

3. You can now proceed to measure instrumental magnitudes. Click on the Image Analysis menu on the menu-bar along the top of the main window. Choose the Aperture photometry item. Two further items will be presented: Results in magnitudes... and Results in data counts.... Choose the former, Results in magnitudes.... (Both these options allow you to perform aperture photometry. The former displays the results in magnitudes, the second in counts.) An Aperture photometry - magnitudes dialogue box (see Figure ) will be displayed. You can drag this dialogue box off the display panel if necessary.

   You should set the Frame zero point (mags) to an improbable value, typically 30, so that the instrumental magnitudes are not inadvertently confused with calibrated ones.

   Clicking the Help menu in the bar at the top of the photometry dialogue box, followed by On Window... will bring up a window with a pretty comprehensive description of how to use the aperture-photometry facilities.

   The measurement process is straightforward. Following the instructions in the on-line help, proceed as follows.

   1. Click the Define object aperture button.

   2. Place your cursor on the image over the star that you wish to measure.

   3. Press down and hold down mouse button 1.

   4. Move the mouse sideways until the circle contains all the star.

   5. Release the mouse button.

   6. Click the Calculate results button.

   7. Inspect the Current object details (displayed in the upper-middle portion of the dialogue box) and view the instrumental magnitude of the star.

   You can change things like the inner and outer radii of the annulus for measuring the sky background by moving the sliders in the Aperture Photometry - magnitudes dialogue box. Apertures are drawn around stars as they are measured.

   Figure: The GAIA Aperture photometry - magnitudes dialogue box

   

4. Clicking on the Options menu in the bar at the top of the Aperture photometry - magnitudes dialogue box will allow you to alter settings by using `push buttons' that are labelled:
   • Use annular sky regions

   • Use circular apertures

   • Keep apertures same size

   By de-selecting the first button here (Use annular sky regions), you can use interactive apertures to measure the sky background, and by de-selecting the second you can use ellipses instead of circles. Because GAIA is acting as a `front-end' to PHOTOM most of the parameters which can be set in PHOTOM can also be set in GAIA.

   There is a nice feature in GAIA that is of use when deciding how big to make the aperture radius. By Clicking on the View menu in the main window and selecting the Slice... option it is possible to obtain a `cut' or `slice' across any star image on-the-fly (see Figure ). This option can usefully be used to estimate how far out from the star useful signal exists.

   Figure: A GAIA `Slice' display panel showing a slice through an object

   

5. If you click on the Parameters button in the Aperture photometry - magnitudes dialogue box the appearance of the box changes to resemble Figure . You can now set various parameters, such as the Photon data per unit, image bias level, default sky level etc.

   By default the statistic used to estimate the sky background is the mean. Usually it is preferable to use the mode because it is less affected by contamination due to faint stars. To select the mode click on the Sky estimator: button and select the mode (see Figure ).

   Figure: The GAIA Aperture photometry - magnitudes dialogue box with the options to set parameters selected

   

6. You should measure all the stars that you are interested in the current frame. (Click on the Aperture button prior to resuming measuring, if necessary.) When the job is done, click on the File menu in the menu-bar along the top of the Aperture photometry - magnitudes dialogue box (not the one in the main GAIA window) and select Save measurements... to save the results in a file of your choice. The instrumental magnitudes are listed in this file.

   GAIA does not include any facilities for calibrating instrumental magnitudes into standard photometric systems. CURSA (see SUN/190[5]) contains some simple functions for this purpose: see SC/6[16] for an example of using them and further details.