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Slit Angles

Setting Slit Position Angles (PA)in CGS4

When setting the slit PA in the CGS4 software, it is important to remember:
  1. The array is displayed `upside down' on the movie screen so that south is to the top, and north to the bottom.
  2. You need to consider in which direction the nod, or slide, should be specified.
  3. 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 of a positive slide (or nod). Remember the telescope moves in the opposite direction to this when you slide along the slit, so be specific when giving instructions to the telescope system specialist. Please be aware that, in order to try and show things as the observer sees them on the array when the slit is north-south, north is shown at the bottom of each plot.

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

In this example, a position angle of 0 is entered into the config. the slit will set to a north-south direction, with the south to the top of the array. When using a positive slide, the offset beam appears to the south of the main beam (or towards the top 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 SE, the bottom NW. A positive slide moves the offset beam on the array towards the SE (i.e., the telescope moves NW)

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 east, the bottom is in the west. A positive slide will move the offset beam to the east, i.e., the telescope moves to the west.

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 NE, while the bottom is in the SW. A positive slide will move the offset beam to the NE, i.e., the telescope moves to the SW.

Some further notes on slit PAs

  • The default slide_slit command in the quad_slide exec is +11.4 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 slide. 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 config demands a position angle east of north, it is only possible to enter such an angle to a maximum of about 10 degrees. 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 0.61 arcseconds. 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 +11.4 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.

And please....

  • Always inform the telescope system specialist when you change the PA. The peakup position will change with PA, and if the TSS is unaware of a change, time will be lost in reaquiring your target.
Contact: Tom Kerr. Updated: Tue Jan 25 11:05:50 HST 2005

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