11.2.1 The Focused Images

Aperture photometry using the task phot (in digiphotx.apphotx) was done on the 5 standard stars in the PG1633+099 images and the 8 standard stars in the SA110-503 images (cf. Table ). As previously mentioned, these two fields were observed with the telescope focused. The apertures used were $2,3,\ldots,30$ pixels (1 pixel = 0.5073''). The crucial sky fitting algorithm parameters (fitskypars) were set as follows: salgorithm=gauss, annulus=30.0 pixels, and dannulus=10.0 pixels, giving a background region of 30-40 pixels, or 15.22-20.29''. The length of the box within which the star is centered, centerpars.cbox, was set to 10 pixels.

mkapfile was used to determine the aperture correction from a number of apertures, $7,8,\ldots,14$ pixels to ``infinity'' (taken as the largest aperture, 30 pixels). The resulting aperture corrections versus the seeing is shown in Figure .

The figure shows, as expected, that the size of the aperture correction decreases with increasing aperture size. It also shows, that the aperture correction (for most of the apertures used) is significantly seeing dependent.

It was decided, that an aperture of 9 pixels (4.57'') was optimal. The aperture correction versus seeing for this aperture only is shown in Figure .

 

As can be seen, the dependence of aperture correction on seeing is very smooth. This is not totally surprising, since the seeing is calculated by mkapfile itself, and is one of the parameters it uses in the stellar profile model.

To further reduce the scatter, a 3rd order (order=4) Legendre polynomial was fitted to the points using gfit1d. The fit was very good, with residual no larger than 0.0008 mag, and typically 0.0002 mag. The final aperture corrections for the focused stars were calculated from the fit. The errors from mkapfile were used.

 

Seeing versus night is plotted in Figure , right panel. The corresponding aperture correction variation with night is shown in Figure , left panel.

 

Finally, phot was run again, this time with only one aperture: 9 pixels.