User Manual
Doc. Number: ESO-323064
Doc. Version: 2
Released on:
Page:
63 of 82
Document Classification: Public
of the target in the two nodding positions do not overlap. The nodding positions are located
symmetrically (at
SEQ.NODTHROW
/2) around the centring location. Because of the extended
wings of the PSF a nodding throw greater than 4
’’ is recommended, preferably
6
’’.
·
Jitter width
(TSF:
SEQ.JITTER.WIDTH
): refers to the small offset added to each
nodding offset (see §5.2.5.2). It helps correct for bad detector pixels.
SEQ.JITTER.WIDTH
gives the width of the jitter box in arcseconds. The maximum offset from the nodding
position is therefore given by half of this value. The successive values of the jitter offset in
a given template are drawn from a set of 100 numbers determined from a Poisson random
number generator. Jittering is currently not possible if
SEQ.NABCYCLES
= 0 (staring mode).
Upper limit for jitter size is half the nod throw, but preferably much smaller so that the PSF
from the star never crosses the middle of the slit.
·
List of position angles
(TSF:
INS.DROT.POSANG.LIST
) is the list of position
angles at which the nodding sequence is repeated. This parameter is relevant only for
spectro-astrometric observations (i.e., for template CRIRES spec_obs_SpectroAstrometry).
Note that the values of the position angles are absolute.
·
Reset jitter for each DROT posang
(TSF:
SEQ.JITTER.RESET
). Only relevant
for spectro-astrometric observations and when
SEQ.JITTER.WIDTH
(Jitter width) is not
zero. If this parameter is set to
yes
(i.e.,
SEQ.JITTER.RESET
=T) the list of jitter positions
calculated by the observation software is repeated identically, in detector coordinates, for
each value of the slit position angle
Examples for Nodding
:
1. If
SEQ.NABCYCLES
= 1, the telescope first points to the A position: it is located to
the North (if
INS.DROT.POSANG
= 0) at a distance equal to
SEQ.NODTHROW
/2 (and
assuming
SEQ.JITTER.WITDH
= 0) from the centring position. After
SEQ.NEXPO
exposures, the telescope nods to the B position: it moves to the South by
SEQ.NODTHROW
, and takes
SEQ.NEXPO
additional exposures. Then the telescope
moves back to its original position. The telescope has then executed one AB
nodding cycle.
2. If
SEQ.NABCYCLES
= 3, the telescope first points to the A position, as defined in
the previous paragraph, and takes
SEQ.NEXPO
exposures. It then moves to the B
position, takes twice
SEQ.NEXPO
additional exposures. It then moves back to the
A position, when it takes twice
SEQ.NEXPO
exposures. Then, it moves a last time to
the B position for a last sequence of
SEQ.NEXPO
exposures before returning to the
original position. The telescope had therefore executed 3 nodding cycles: AB BA
AB.
When the nodding is used, the total exposures time is given by:
2 x
SEQ.NABCYCLES x DET1.NDIT x DET1.DIT x SEQ.NEXPO
In the spectro-astrometry case, the total exposures time is given by:
2 x
SEQ.NABCYCLES x DET1.NDIT x DET1.DIT x SEQ.NEXPO
x n
posangs