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. 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) ‘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 and in SUN/139, 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 8 for its provenance). Proceed as
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.
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.
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 17). This option can usefully be used to estimate how far out from the star useful signal exists.
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 18).
GAIA does not include any facilities for calibrating instrumental magnitudes into standard photometric systems. CURSA (see SUN/190) contains some simple functions for this purpose: see SC/6 for an example of using them and further details.