SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
5.2.1 Nod
The NOD function of the telescope is an operation in which the target source is periodically moved from one
instrument chop position to the other chop position by re-pointing the satellite. The pointing direction will
change in the direction of the chopper throw - see Figure 5-2.
Nod
position
1
Figure 5-2 - Pointing positions for a telescope NOD function. The circles represent the size of the telescope Airy
pattern projected onto the sky. The two nod positions have been offset left and right for clarity.(A1 and B2 are coaligned).
The purpose of nodding is to subtract out the signal offset that will be present at some level due to the fact
that the detector will receive different amounts of ambient background power in the two chop positions. This
offset is likely to be larger than low-level signals to be detected, and may also vary significantly with time
over the course of an observation if the primary mirror has a time-varying temperature gradient in the
chopping direction (Griffin, 1998). Nodding takes out the offset in the following way:
Let S = Source signal OA = Offset signal for beam A OB = Offset signal for beam B
Then if we subtract the total signals measured in the two nod positions, we get
(A1 - B1) - (A2 - B2) = [(S + OA) - OB] - [OA - (S + OB)] = 2S.
A2
B2
5.2.2 Raster
The RASTER Spacecraft Function is a series of fine pointing operations of separated by slews such that the
pointing of the telescope axis moves in a raster pattern. Figure 5-3 shows how the raster pattern will be
constructed.
116
A1
B1
Offset for clarity
Chop
throw
Source