Optimal Photometry Toolbox

Making a simple measurement now

In a hurry? Then try the following recipe:

  1. Press the "Select object" button.
  2. Place your cursor on the image over the star.
  3. Press and release mouse button 1.
  4. Now press the "PSF" tab.
  5. Press the "Select PSF object" button.
  6. Place your cursor on the image over the PSF star.
  7. Press the "Calculate results" button.
  8. Inspect the "Current object details" and view the star's instrumental magnitude, or mean count.
In this example the sky background for both stars 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 select as many stars you like by repeating the stages described above (except there is only one PSF object). All stars are re-measured each time the "Calculate results" button is pressed.

Making measurements using sky not in 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 object). Now when you select an object its markers will not have a sky annulus shown. To add sky regions to this object's markers just press the "Define sky aperture" button and drag out a region on the image.

Resizing or moving a marker

To move or resize the marker associated with an object you first need to "select" it. Selecting is done by placing your mouse cursor over the object and then pressing button 1. This should change the marker colour and one or two little boxes will appear on the locus. These little boxes are known as grips.

To move the marker just drag it by pressing down and holding down button 1 over it and then moving the mouse. The markers will follow.

To resize a marker drag a grip or use the sliders and entry windows in the object and PSF details windows ("Clipping radius:", "Annulus inner scale:" and "Annulus outer scale:").

NOTE: all markers are shown as the same size -- the clipping radius of the PSF object, resizing one will resize all.

Viewing all the measurements

Just press the "Results" tab select "View all measurements". This creates a table that lists all the current measurements (except the PSF object). Double click on a line to make that object 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 objects 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) is to just enter the exposure time in seconds in the "Exposure time/qualifier:" entry, 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 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 opening the "Options" menu and selecting the "Set additional parameters" item. You should read SUN/45 to understand these parameters.

SEVERE WARNING: the error estimates in the measurements will be wrong unless you set the image bias and data units to photons conversion factor correctly. For optimal photometry this is particularly important.