This performs interactive measurements of objects individually selected from the displayed frame. If using aperture extraction the size and shape of the cursor should be set-up in advance using the I or N commands, while the clipping radius should be set (using the 0 command) when using optimal extraction.
There are two basic methods for measurement of background. Either the background is sampled from an annulus around the object aperture, or from a separately chosen area of sky (see command A). The two cases can be distinguished at this stage from the on-screen display.
In the case of the manual sky measurement the middle box is labelled with SKY, while for automatic sky measurement it will remain empty. The left-hand box with either be labelled STAR, in the case of aperture extraction, or PSF when optimal extraction is selected. The remaining right-hand box will be annotated RETURN TO KEYBOARD.
To perform the measurements with the automatic sampling of the sky, the cursor is positioned over the chosen object and the left-hand mouse button (or key 1) is pressed. If optimal extraction is selected the first measurement will define the point spread function (PSF) for the technique. It is important to pick a bright star which is unsaturated for this task. After this measurement has been taken, if you are using a terminal capable of erasable line graphics such as an xoverlay, then the left-hand box label will change to read STAR to denote that further measurements will be photometric.
For further measurements using optimal extraction, or all measurements using the older aperture method an aperture is displayed where the measurement was made. For optimal extraction this ``aperture'' will be the size of the clipping radius (the CLIP parameter). If centroiding is being done (command C), automatic enabled for optimal extraction, then the displayed aperture may not be centered on the cursor position. The results of the measurement are printed on the terminal and recorded in the results file. Measurements can be continued until the third mouse button (key 0) is pressed.
When using manual selection of the background the middle mouse button (key 2) is also used. Selecting this button records the sky estimate in an aperture identical in size and shape to the object aperture, at the position specified by the cursor. On the screen an aperture is displayed at that position. No centroiding is done in this aperture, even if the centroiding option is on. When the measurement of the object is made, the most recent value of the sky is used. This means that the sky has to be sampled BEFORE the measurement of the object. Having a correct background estimate is crucial for optimal extraction to such an extent that PHOTOM will not allow you to make a star or PSF measurement using this method until a background measurement has been provided. If the background needs to be sampled in several places around an object, to minimise the noise or take account of a sloping background, then a sky aperture can be selected a number of times and the mean of these values is used. The calculation of the mean is only cleared when an object is measured, so if a mistake has been made in estimating the mean of the skies then an object measurement has to be made, and a note made that the measurement was in error, before going back to the estimation of the sky. Control stays with the interactive menu until the third mouse button (key 0) is pressed.
The results of the measurements are displayed on the terminal and sent to the file accessed by the RESFILE parameter.
PHOTOM --- A Photometry Package