Known Problems and Observing/Data Reduction Tips
Many topics have been transferred to the ISAAC
Data Reduction Guide. Users should consult this guide. As the data
reduction guide becomes more complete, this page will disappear.
Tilted Absorption Lines
There is significant astigmatism in the spectroscopic arm. This can lead to
tilted absorption lines, if the FWHM of the images produced by the telescope
is significantly smaller than the slit width. In this situation, accurate
velocity measurements (eg. for rotation curves) may be difficult to achieve.
Partial correction may be obtained from the atmospheric lines. This effect is
present for data during the period March to September 2000. For data taken
after September 2000, the effect has been minimised by optimising the position
of the collimator of ISAAC
Since February 2000, the spectroscopic arm of ISAAC shows a variable
spectroscopic resolution. In the high resolution mode (the 0.3" slit) the
resolution degrades from 10,000 at low x values to around 6,000 at high x
values. The degradation is probably caused by astigmatism in the spectroscopic
arm. This effect is present for data during the period March to September
2000. For data taken after September 2000, the effect has been minimised by
optimising the position of the collimator of ISAAC.
Imperfections in the optics of ISAAC cause background sensitivity variations
when the background is high and when the paralactic angle is changing rapidly.
The later occurs when the telescope crosses the meridian near Zenith. The
ghosts are generally not a serious limitation if the jitter frequency high
enough (about once a minute) when the object is crossing the meridian near
Zenith. The ghosts are strongest in the Ks band.
Scattering feature in LW spectroscopy
There is a feature in the L2 objective (used in LW spectroscopy only) that
causes light to be scattered. This feature can be avoided by placing the
target in the lower 1 arc minute of the slit and by keeping the chopping throw
to less than 30".