Dome flats are normally recorded through each filter with the flatfield lamp on and then off. The difference of these two is taken to be the response of the system to illumination through the telescope. Dome flats are recorded with the telescope at the zenith by illuminating the upper windscreen with incandescent lamps on the telescope top-end ring. Move the telescope to the zenith by typing:
ZENITH
into the telescope console terminal.
The upper windscreen is moved over the telescope by typing:
CONFIGURE WINDSCREEN_CONTROL CLOSED
into the telescope console terminal. The primary mirror cover must be open, and the dome lights off. The incandescent lamps are is controlled through the VAX commands:
SWITCH FLATFIELD_ILLUMINATION ON SWITCH FLATFIELD_ILLUMINATION OFF
typed into the telescope console terminal. These can be equated
with the DCL symbols LON and LOFF by typing:
LON :== SWITCH FLATFIELD_ILLUMINATION ON LOFF :== SWITCH FLATFIELD_ILLUMINATION OFF
into the telescope console terminal or including this in your
telescope STARTUP.COM file. Wait until the lamp cools, i.e.,
the mean count level stabilizes, before recording the lamp off frame.
The lamp intensity is set by adjusting the flatfield illumination
control on the telescope console. There is also a switch on the
top-end ring for enabling 4 or 8 of the lamps. Settings of 
20% illumination (4 lamps) for broad band filters and 100%
illumination (4 lamps) for narrowband filters work well for the
0.5''/pixel scale with an integration time of 0.4 sec. Use 40%
illumination (4 lamps) for the 0.25''/pixel scale with broad band
filters.
Return the upper windscreen to normal operation by typing:
CONFIGURE WINDSCREEN_CONTROL VERTICAL_ONLY_TRACKING
into the telescope console terminal.
Dome flats give better photometric accuracy than sky flats because a
significant contribution to sky flats is due to thermal emission from
the telescope at the longer wavelengths which is unrelated to the
relative response of the array to light. A photometric performance
comparison between dome flats and sky flats for the Kn filter is
shown in Fig. 4. Despite their better photometric
performance, there is still a residual sensitivity gradient vertically
on the array of 0.05 mag amplitude. Horizontal gradients are
generally 
0.03 mag, as shown by the scatter in each cluster of
filled circles. Better performance may be obtained by averaging dome
flats taken with the instrument rotated by 180 
.
Figure 4: Sensitivity variation versus detector row number for Kn
data reduced using a dome flat (filled circles) and a sky flat
derived from the data (crosses). The data were obtained by
recording images of a single star placed at different positions on
the array in a 
grid. Each cluster of points corresponds
to measurements with the star at different column positions.