You can open the photometry window by a right click on a star and select Photometry in the popup menu.
This tool is intended as a quick way to get a magnitude estimate for a star in an image you just take with your camera.
It must not be used for any precision measurement.
The first time you open this window it need to be calibrated for your setup. Select a star in the image with a know magnitude, type this magnitude in the box at the bottom right, and click the button Set magnitude.
You need to repeat this operation every time the image is from another telescope, camera or filter.
The magnitude computation take account for change in the exposure time and airmass. Both of this information must be present in the image FITS header, otherwise only the calibration magnitude is used. The first rows at the top of the window show this status.
The measurement include: