6.9.5.3 Adding LH background as a fixed cluster offset

You can also add a sky background observation as a fixed cluster offset to your AOR. This is an alternative only if you plan to take your sky observation within 1 degree in radius of your target. The Fixed Cluster mode also results in the observing parameters of the sky being exactly the same as that on the target. In the example given, since SH and LH observations are in the same AOR, selection of fixed cluster offset will result in SH and LH sky background observations being taken at the sky position. The cluster offsets can be either specified by coordinates or by relative offsets. To create a fixed cluster target, click on Observation$->$IRS Staring$->$New Target Then select either ``fixed cluster-positions" or ``fixed cluster" depending on whether you will provide absolute coordinates or relative offsets for the sky position. Then enter the coordinates of the target and the sky in each row of the fixed cluster window. Figure 6.17 shows the target window for fixed cluster-positions with the position of 3C273 and the sky observations entered. Then select the peak-up star exactly as shown earlier and enter the proper motion. Click ok. When you visualize your AOR, you will notice that the sky observation is performed both for SH and LH as expected, for the fixed cluster mode. If you do not want to invest time to take a sky for the SH position, then you will either have to remove the SH observations from this AOR or take the LH background as a separate AOR as illustrated in the previous example.

Figure 6.15: Illustration of a separate AOR for LH sky background observations

Figure 6.16: Figure illustrating the chaining of a LH sky background AOR to the target LH observations.

Figure 6.17: Illustration of a LH sky background observation added as a fixed cluster offset.