Setting Slit Position Angles (PA)in CGS4

1. You need to consider in which direction the nod, or slide, should be specified.
2. Although you are asked to enter a PA which is east of north in your config, you normally have to enter a negative PA. This is discussed in a little more detail below.

The best way to demonstrate how to specify the slit PA is by showing some examples which is done below. In each example, the top and bottom of the array are marked, as well as north, south, east and west. An arrow indicates the direction the telescope moves during a postive offset (the offset beam will appear to move in the opposite direction).

Example 1: PA = 0 degrees (0 degrees east of north).

In this example, a position angle of 0 is used. The slit will set to a north-south direction, with north at the top of the array. When using a positive offset, the offset (negative) beam appears towards the bottom of the array.

Example 2: PA = -45 degrees (135 degrees east of north).

Here, a PA of -45 degrees has been entered. The top of the array is now in the NW, the bottom SE. A positive slide means that the offset (negative) beam will appear towards the bottom of the array.

Example 3: PA = -90 degrees (90 degrees east of north).

A PA of -90 degrees has been entered. The top of the array is in the west, the bottom is in the east. A positive slide means the offset (negative) beam appears towards the bottom of the array.

Example 4: PA = -135 degrees (45 degrees east of north).

A PA of -135 degrees is entered. The top of the array is now in the SW, while the bottom is in the NE. A positive slide means the offset (negative) beam appears towards the bottom of the array. 

Some further notes on slit PAs

• The default offset in the CGS4 library is -11.74 arcseconds. This positions the offset beam 19 rows up from the main beam. To move in the other direction (towards the bottom of the array), simply enter a negative value for the offset. Remember that the slit is 90 arcseconds long. You normally do not want to specify a nod which places the offset beam off the array.
• Although the CGS4 component demands a position angle east of north, it is only possible to enter such an angle to a maximum of about 10 degrees (40 degrees for the echelle). To set to an angle greater than this, you must specify a negative angle. For example, if you wish to set to a PA of 50 degrees east of north, set to an angle of -130 degrees (simply subtract 180 from your desired PA).
• The pixel size is apprximately 0.61 arcseconds at low resolution. To make sure you nod onto another row, you need to specify a value in arcesonds which is a multiple of this number.
• The system usually does a fairly good job of positioning the crosshead on the slit in the offset position. To increase the accuracy of the offset position beam, you can do one of two things (or both): i) Use a shorter slide distance than the default -11.74 arcseconds, or ask the TSS to peakup in both the main and offset beams and correct the current slide angle.
• The current r.m.s. setting accuracy of the slit PA is 0.15 degrees.