In a hurry? Then try the following recipe:
You can create as many apertures as you like by repeating the stages described above. All apertures are re-measured each time the "Calculate results" button is pressed.
Open the menu "Options" and select the "Use annular sky regions" item. This now switches off the creation of the annulus region (note this can also toggle the current aperture). Now create an object aperture and then press the "Define sky aperture" button to create sky apertures. These sky regions are just associated with this aperture (note the colour change in sky aperture associated with an object aperture).
To move or resize an aperture you first need to "select" it. Selecting is done by placing your mouse cursor over the aperture and then pressing button 1. This should change the aperture colour and one or two little boxes will appear on the locus. These little boxes are known as grips.
To move the aperture just drag the aperture by pressing down and holding down button 1 over the aperture and then moving the mouse. The aperture will follow.
To resize the aperture drag a grip or use the sliders and entry windows ("Semimajor axis:", "Eccentricity:", "Position angle:", "Annulus inner scale:" and "Annulus outer scale:"). Note the sliders and entry windows only work on the selected aperture. To activate a value you type into a entry window press the <Return> key.
To create apertures with a fixed size, or which are constrained to be the same size, choose the "Options" menu and select the "Keep apertures same size" option. Apertures will now be created with the values shown in the "Semimajor axis:", "Eccentricity:", "Position angle:", "Annulus inner scale:" and "Annulus outer scale:" windows.
If you select one of these apertures and resize (or reorient) it, in some way, then all the other apertures will track these changes.
To do this you must select all the apertures you want to modify together. To do this select an aperture, then hold down the <Control> button and select the next aperture etc. You can now say change all the apertures to the same radius by typing a value into the "Semimajor axis:" entry window and then pressing return. You can also get the same effect by moving the slider.
Open the "Options" menu and select the "Use circular apertures" item. Any new apertures that you now create will be elliptical. Use the grips to reorient and resize the aperture.
Just select the "Results" tab and press "View all measurements". This creates a new window that lists all the current apertures and their related values. Double click on a line to make that aperture current.
Just press the "Save" button. This will write all the current measurements to a file "GaiaPhotomLog.Dat" by default. You can write to a different file using the "Results" entry field.
The "Append" button appends the current measurements to the end of the results file (preceded by the name of the current image in a comment). Note the "Save" option overwrites the results file without prompting.
To do this display your new image (either in a clone or by superseding the existing image) and then open the "File" menu and select the "Read measurements..." item. This allows you to select a file with your existing measurements in. These apertures will now be redrawn. Press "Calculate results" and then save the new measurements to a new file.
You can also use the results files together with the AUTOPHOTOM program (see SUN/45) to redo the same measurements non-interactively.
Just type in the frame zero point into the entry window and press return. All the existing apertures will now be re-measured.
The easiest way to set the exposure time (so you can normalize all your observations to unit flux) is to just enter the exposure time in seconds in the "Exposure time/qualifier:" entry, (found under the "Parameters" tab) after making sure that the "Exposure time source:" menu is set to "simple constant".
Note that the exposure time is just used as a ratio to scale the intensities, so using seconds is only a convenience.
If your data have their exposure time set in the image headers (e.g. FITS-like headers), then you can use this instead. Just select "FITS keyword" and enter the header item name in the "Exposure time/qualifier" entry.
The aperture photometry is actually performed by the PHOTOM program AUTOPHOTOM. This is described in SUN/45. All the additional parameters that define the conversion factor from data units to photons, any bias in the data and modify the way that the sky background is estimated can be set by selecting the "Parameters" tab. You should read SUN/45 (use the command showme sun45 to view the on-line documentation) to understand these parameters.
WARNING: the error estimates in the measurements will be wrong unless you set the image bias and data units to photons conversion factor correctly, this includes when measuring errors using sky variance. When you have no information about these values select "gaussian sky", which should give an upper error limit based on the variation in sky values.
The results shown for an aperture have the following meanings: