This recipe shows how to use PHOTOM (see SUN/45) 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 and in SUN/139, 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.
displayin KAPPA (see SUN/95) is ideal11. It is best to create the display window using the
xmakeutility 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:
xwindowsas the display device. Briefly, type:
to load the KAPPA package. Then issue the following commands:
lutneg sets up a negative grey-scale colour
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.
photomstartto enable its commands and
photomto 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:
You can use
Help to find out what the options are:
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.
Annuluscommand 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.
Notice a couple of things here:
Now set the other required values:
A few more things to note:
SKYMAGis essentially the arbitrary constant which appears in equations 14, 15 and 16. It is usually sensible to set it to an improbable value, such as 30 (as used here) so that the instrumental magnitudes measured by PHOTOM are not inadvertently confused with calibrated magnitudes. Conversely, if the absolute value of the sky background is known and used then the instrumental magnitudes will approximate to calibrated magnitudes, albeit without atmospheric extinction and colour corrections,
BIASLEwill be specific to the data.
mand 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.
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):
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:
ato toggle the
Annuluschoice. The message ‘
Interactive aperture in use’ should be displayed.
The resulting display will look something like Figure 9.
autophotomwhich allows PHOTOM to be used non-interactively (see SUN/45 for details).
11Strictly speaking you must use display software which accesses the Starlink graphics database (see SUN/48). 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
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.