Aperture Photometry Toolbox

Making a simple measurement now

In a hurry? Then try the following recipe:

  1. Press the "Define object aperture" button.
  2. Place your cursor on the image over the star.
  3. Press down and hold down button 1.
  4. Move the mouse sideways until the circle contains all the star.
  5. Release the mouse button.
  6. Press the "Calculate results" button.
  7. Inspect the "Object details" and view the stars instrumental magnitude, or mean count.
In this example the sky background is estimated using the region between the outer and inner annuli. If this region contains any contamination then you will need to create sky apertures.

Measuring several stars

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.

Making measurements without an annulus

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).

Resizing or moving an 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.

Making all apertures the same size

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.

Resizing more than one aperture at time

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.

Using elliptical apertures

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.

Viewing all measurements at once

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.

Saving the measurements

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.

Making the same measurements on other images

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.

Changing the frame zero point (magnitudes only)

Just type in the frame zero point into the entry window and press return. All the existing apertures will now be re-measured.

Setting the image exposure time

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.

Controlling the centroiding, noise characteristics, sky estimators...

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.

Understand the results for an aperture

The results shown for an aperture have the following meanings:

Aperture index
just a sequence number to distinguish measurements
X position
before a making a measurement, the position of the
Y position
center of the aperture, after making the measurement, the centroided position -- if you've selected centroiding
the magnitude of the object, this is sky subtracted and scaled using the photons per data unit and exposure factors
Magnitude error
error in the magnitude, the error is derived using either the sky variance, photon statistics, or the data variance, if you are using photon statistics or sky variance then a good knowledge of the photons per data unit is required, photon statistics also requires that you know the bias level, use a gaussian estimate if you don't know any conversion factors (or they don't make sense and you don't have a data variance component)
Sky value
the estimated sky, either derived from the annulus, or detached sky regions, or a constant, scaled by photons per data unit factor
Sum in aperture
sum of object data counts in aperture, scaled by the photon per data unit value and the exposure factors (excludes any sky contribution)
Error code
status of measurement:
measurement has no problems
one or more pixels in aperture is bad
one or more pixels in aperture is saturated
the object aperture intersects the edge of the data array
there are other problems with the measurement