This Appendix gives a variety of information useful for scheduling observations.

Table shows how the Local Sidereal Time (LST) of the start and end of the night varies throughout the year, and should be used to determine which month is best suited for observing a particular object. Figures to are plots of declination against hour angle for the latitude of La Palma, showing lines of equal zenith distance, and indicating the telescope limits. These plots can be used to determine for how long either side of transit it is possible to observe an object.

Observations should of course be scheduled so that objects are not observed at large values of the zenith distance. This causes problems both with atmospheric extinction (see Appendix ), and with differential refraction, whereby light at different wavelengths is refracted by different amounts. Differential refraction means that:

- If acquisition and guiding is carried out at a very different
wavelength from that of the observation, the object may not be
where you think it is.
- For low dispersion slit spectroscopy, the object being observed can only be simultaneously centred in the slit for all wavelengths if the PA of the slit is aligned with the parallactic angle. If the slit is not aligned with the parallactic angle, there are likely to be wavelength dependent slit losses.

Figure shows the angle of refraction as a funtion of wavelength and airmass. Table gives the airmass as a function of zenith distance. The parallactic angle for a particular observation can be found from Figure , which is a plot of parallactic angle against hour angle, showing lines of constant declination. Note that there is a discontinuity at transit of objects with declination equal to the latitude of La Palma.

**Table:** LST of twilight on La Palma (1989)

[ TIFF ]

**Figure:** Plot of declination against
hour angle for the latitude of
La Palma, showing lines of equal zenith distance. This plot shows the
region within which observations are possible with the WHT.

[ TIFF ]

**Figure:** Plot
of declination against hour angle for the latitude of
La Palma, showing lines of equal zenith distance. This plot shows the
region within which observations are possible with the INT.

[ TIFF ]

**Figure:** Plot
of declination against hour angle for the latitude of
La Palma, showing lines of equal zenith distance. This plot shows the
region within which observations are possible with the JKT, with the
telescope both east of the pier (a) and west of the pier (b).

[ TIFF ]

**Figure:** Plot
showing how the magnitude of atmospheric refraction varies
with wavelength and airmass.

[ TIFF ]

**Figure:** Plot
of parallactic angle against hour angle, showing lines
of constant declination. Note the discontinuity at transit (hour angle
equal to zero) for objects with a declination equal to the latitude
of La Palma.

Tue Aug 15 16:42:46 BST 1995