14 Measuring Instrumental Magnitudes with PHOTOM

This recipe shows how to use PHOTOM (see SUN/45[22]) to measure instrumental magnitudes for objects in a CCD frame. The objects may be either standard stars or programme objects. The techniques for measuring instrumental magnitudes are discussed in Section 10.

The starting point is a CCD frame which has been processed to remove instrumental effects. This process typically includes: removing cosmic-ray events and other blemishes, de-biasing and flat-fielding. It is described in SC/5: The 2-D CCD Data Reduction Cookbook[18] and in SUN/139[19], the manual for the CCDPACK package, and is not considered further here. SC/5 is a good introduction. PHOTOM can be used interactively, or can be supplied with a list of coordinates of stars on which it will perform aperture photometry. It is used interactively in this recipe.

The example CCD frame used in this recipe is available as file:


If you intend to work through the recipe using this file you should make a copy of it in your current directory. Alternatively, you may prefer to use a CCD frame of your own.

First the image containing the stars must be displayed using software which PHOTOM can interact with. Application display in KAPPA (see SUN/95[11]) is ideal11. It is best to create the display window using the xmake utility because in this way you can define the display to have an overlay plane, thus allowing the graphics output by PHOTOM to be cleared without destroying the displayed image. So, start the display with a command like:
  % xmake xwindows -overlay -ovcolour blue
Now display the data with KAPPA display using xwindows as the display device. Briefly, type:
  % kappa

to load the KAPPA package. Then issue the following commands:

  % lutneg
  DEVICE - Name of display device > xwindows
  % display
  IN - NDF to be displayed > ccdframe
  DEVICE - Name of display device > xwindows
  MODE - Method to define the scaling limits /’SCALE’/ > FAINT
  Data will be scaled from 200 to 2666.

lutneg sets up a negative grey-scale colour table12. display displays the image, which should appear as a grey-scale plot. Note that the input file name is (and must be) specified without the ‘.sdf’ file type.

Next, start up PHOTOM by typing photomstart to enable its commands and photom to start. You will be asked for the name of a data frame. Again the file name must be specified without the file type. The default name for the output file written by PHOTOM is photom.dat. If this file exists, an error message will appear and you will be prompted for an alternate name. The sequence of commands and responses should be something like the following:

  % photomstart
     PHOTOM applications are now available -- (Version 1.5-0)
  % photom
  IN - NDF containing input image > ccdframe
  Commands are - Annulus, Centroid, End, File, Help, Ishape, Measure,
                 Nshape, Options, Photons, Sky, Values
  COMMAND - PHOTOM /’Values’/ >

If you hit <RETURN> here you will get a list of the default values that are set for PHOTOM at present. The result will be like:

  COMMAND - PHOTOM /’Values’/ >
   Semim           =   5.0
   Eccen           =   0.00
   Angle           =   0.0
   Centroiding of star in aperture
   Concentric sky aperture
   Inner radius    =   1.3
   Outer radius    =   2.1    times object aperture radius
   Sky estimator   =  Mode
   Sky magnitude   =  50.0
   Photons per ADU =   1.00
   Exposure time   =   1.00
   Saturation level ( data units ) = 0.17000E+39
   Errors from sky variance
  COMMAND - PHOTOM /’Values’/ >

You can use Help to find out what the options are:

  COMMAND - PHOTOM /’Values’/ > help
  Commands are - Annulus, Centroid, End, File, Help, Ishape, Measure,
                 Nshape, Options, Photons, Sky, Values
  Annulus  - Toggle between sky measured in concentric annulus or in selected area
  Centroid - Toggle between measuring around centroid of image or given position
  End      - Exit program
  File     - Supply a file of object positions
  Help     - This help message
  Ishape   - Select aperture shape interactively
  Measure  - Make measurements interactively
  Nshape   - Select aperture shape non-interactively
  Options  - Change values of some parameters
  Photons  - Select error estimate - photon statistics, sky or data variance
  Sky      - Select sky estimator - mean, mean within 2 sigma, mode or user given
  Values   - Output current parameter values
  COMMAND - PHOTOM /’Values’/ >

Some of these choices toggle between values. The way these options work is that when the appropriate command is issued the chosen option is switched from whatever its current state happens to be to its other state. A message is issued indicating the new state. Centroiding, for instance, can be switched on or off. Generally for interactive work it is best to leave centroiding switched on.

The next step is to set some parameters which define the apertures which will be used and various related items. Initially a circular aperture will be used, with the sky background measured in an annulus around it. You should toggle the Annulus command until a concentric aperture is selected.

Now you will need to choose some suitable values for the measuring aperture radii. The background annulus measuring region should be set so that its inner radius is a little outside the central circle, so that it is not unduly contaminated with stray light and its outer radius should not be so big that it includes too many surrounding objects.

How big does the radius of the measuring aperture need to be, and how much bigger should the background annulus around it be? There is no hard and fast answer: it depends on the plate scale of the image, how crowded the field is and whether the programme objects are stars or extended objects. If the aperture is too small then a fraction of the light from the object being measured will fall outside the aperture and not be detected, thus leading to an underestimate of the brightness of the object.

If your programme objects are stars and all your CCD frames have the same point-spread function (that is, the seeing remained the same whilst all the frames were acquired) then the choice of aperture is not too critical. All the objects measured, both programme stars and standard stars, have the same profile and hence they all lose the same fraction of their light. This systematic underestimation of the brightness is simply calibrated out when the instrumental magnitudes are converted to magnitudes in a standard system. In this case quite a small aperture can be used in order to minimise statistical errors in the background and contamination by faint stars.

The situation is rather different if the programme objects are extended objects. Here the programme objects will have a different intensity profile to the standard stars and hence for a given aperture size a different fraction of the total light will be lost. Thus it is important to determine the total magnitudes for both standard stars and programme objects and a larger aperture is appropriate.

An aperture radius of about twenty seconds of arc is often a reasonable starting point.

The background can be sampled using various algorithms. A simple mean will obviously be sensitive to any contaminating source, such as faint stars, within the annulus, but a mode will tend to be less affected by aberrant, outlying values.

Next set the size of the measurement aperture:
  COMMAND - PHOTOM /’Values’/ > n
  SEMIM - Semi-major axis /5/ > 8
  ECCEN - Eccentricity /0/ >
  ANGLE - Orientation /0/ >
  COMMAND - PHOTOM /’Values’/ >

Notice a couple of things here:

Now set the other required values:

  COMMAND - PHOTOM /’Values’/ > o
  INNER - Inner annular radius /1.4/ > 1.3
  OUTER - Outer annular radius /2/ > 2.1
  PADU - Photons per ADU /1/ >
  SKYMAG - Magnitude of sky /50/ > 30
  BIASLE - Bias level ( data units ) /0/ >
  SATURE - Saturation level ( data units ) /1.7E38/ >
  COMMAND - PHOTOM /’Values’/ >

A few more things to note:

PHOTOM is now set up ready to measure stars and sky background. Type m and when prompted for the display device use xoverlay. The text boxes that appear towards the bottom of the display refer to the corresponding mouse buttons. Proceed as follows.
Position the cursor over the object to be measured and click the left mouse button or enter 1 from the keyboard.
Repeat the procedure for all the objects which you wish to measure.
To finish, click on the right mouse button or enter 0 from the keyboard and you will return to the PHOTOM ‘COMMAND’ prompt.

The resulting display will look something like Figure 8. As each star is measured the terminal or workstation will output the results (and echo them to the output file specified when starting PHOTOM):

  COMMAND - PHOTOM /’Values’/ > m
  DEVICE - Display device /@xwindows/ > xoverlay
  Select operation according to screen menu
  Left hand box  - Press left hand mouse button
  Centre box     - Press centre mouse button
  Right hand box - Press right hand mouse button
           nx       ny        a        e       theta
          384      256       8.00     0.000      0.0
             x        y      mag     magerr      sky       signal code
      1    57.70   157.74   19.322    0.011    489.965  18675.065
      2    58.90   232.74   18.571    0.007    493.119  37287.484
      3    66.94   250.43   20.447    0.025    493.768   6622.757 E
      4    81.58    65.64   17.059    0.003    489.962 150087.483
      5   362.25    66.61   18.209    0.005    491.481  52030.981
  COMMAND - PHOTOM /’Values’/ >

If you are working through the recipe the actual values you obtain will probably be slightly different because you will have positioned the apertures differently. The meaning of each of the columns is described in SUN/45. Notice the following:


Figure 8: Display during measurement using a concentric annulus

It is also possible to use an interactive aperture to sample the background. Here representative areas of sky are sampled independently of the measured object. The procedure is as follows.
Type a to toggle the Annulus choice. The message ‘Interactive aperture in use’ should be displayed.
Type m
Move the cursor to a blank patch of sky. Usually the patch chosen will be close to the object to be measured. Click on the middle mouse button. An aperture corresponding to the patch of sky measured will be shown. You can repeat this procedure for several patches of sky if you wish. Note that the sky must be measured before measuring any objects.
Move the cursor over the object to be measured and click on the left mouse button (or enter 1 from the keyboard).
You can make further measurements of the objects and the sky background as you wish.
To finish click on the right mouse button (or enter 0 from the keyboard).

The resulting display will look something like Figure 9.


Figure 9: Display during measurement using an independent aperture

You should measure all the stars which you are interested in in the current frame. Their instrumental magnitudes will be included in the output file written by PHOTOM. Alternatively, if you prefer, you can make a note of the instrumental magnitudes as they are displayed (though this approach is more prone to mistakes).

This recipe has shown the interactive use of PHOTOM. PHOTOM also contains an application called autophotom which allows PHOTOM to be used non-interactively (see SUN/45[22] for details).

11Strictly speaking you must use display software which accesses the Starlink graphics database (see SUN/48[21]). However, you will not normally be aware of the graphics database and certainly do not need to know anything about it. It is simply a mechanism which allows different applications to co-operate in using the same plot.

12An image displayed with the lutneg colour table mimics the appearance of a conventional astronomical photographic plate: stars appear as dark spots on a light background. Various other colour tables are available in KAPPA. For example, lutgrey sets up a positive grey-scale (light stars against a dark background) and lutheat sets up a pseudo-heat sequence.

13The intensity profiles of the images of extended objects usually fall off more slowly with increasing radius than those of stars and hence when working with extended objects it is necessary to be careful to choose an aperture sufficiently large to include the required fraction of the total light from the object.