Gaia is the default display engine for reduced mosaics, and provides a range of methods for performing aperture photometry on frames. Here we describe the essentials; more details can be found in Starlink User Note 214. The Gaia display looks something like this (it has been squashed sideways to save space, and so the zoom window is not displayed):

From the “image analysis” menu, select aperture photometry, results in mags, to get the following window:

In the zeropoint box, set zeropoint to zero or to estimated instrument value (then click ok on resulting error msg about selecting apertures first)

Click on the parameters tab, to get this:

Set exposure time to the correct value

Change sky estimator to clipped mean

Return to the “Aperture” tab

Go to “Options” at top of window and highlight keep apertures same size

Set apertures using the text boxes or sliders:

• semimajor axis should be ~2.5″ (smaller for faint targets)
• translated to pixels for the instrument e.g. 27.5 for ufti
• annulus inner ~1.3 outer ~2.5 (adjust for sources in sky)

Click on define object aperture

Click on the target in the image

Click on calculate result

The aperture can be adjusted by dragging on its control point in the image frame.

The reported magnitude is correct, but the error is not, since the fact that the group is a mosaic of coadded frames is not explicitly taken into account. Use sky variance in estimating your errors.

Finally, note that the sky variance tabulated by NOD_SELF_FLAT_NOMASK_APHOT recipes are currently incorrect; a cure is being worked on.