image quality problems
WFCAM image quality issues: what to
- When and where was the last focus? Was it reliable (e.g., did the
DR detect plenty of stars, were the E-W or N-S tilts large?). Was it
done too close to the moon?
- Is the ellipticity bad? If so, it could be because the focus is
poor or you are seeing astigmatism.
- Is the DVS open or closed?
- Is the telescope close to a bright moon? If so the guiding may
not be good on faint guide stars.
- Is there any connection with change of position in the sky? You
can check UKIRT_LOG.log but positions are also in the file headers and
HA (degrees), declination and airmass are in the night log.
- Are there large differences between the actual and demand
positions for the hexapod (U, V, X, Y, Z)? These can be looked at in
the channel archiver (e.g., mcu:hex:x0:dem:val and :rbk) or in the file
headers themselves (e.g., M2_X, M2_XD). Differences shouldn't be more
than 5 hundredths although they can build up if the guiding has not
been switched off for a while (e.g., UDS field).
- Are there galaxies in the field that could have a
detrimental effect on measured seeing?
- Is the DR having a problem and/or have unguided frames been
included in the group? Check the DR logs and the file headers include
"GUIDING" which tells you if guiding was on, off or paused.
- Weather problems? Is the telescope pointing into the wind? Bear
in mind that if the wind is from the east the UKIRT readings will be
incorrect for both direction and speed. You should confirm direction
and speed with the CFHT numbers.
- Is the guide rate frequency low? (again available in the headers:
AGFREQ). Remember the lower the frequency the better the image quality.
The ideal rate is 60-Hz.