Setting Slit Position Angles (PA)in CGS4
When setting the slit PA in the CGS4 software, it is important to remember:
The array is displayed `upside down' on the movie screen
so that south is to the top, and north to the bottom.
You need to consider in which direction the nod, or slide, should be specified.
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.
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.