Quick Reference Tables
LWS Orientations:
PA(LWS) =ROTPOSN - 64° or =SKYPA+206°
| Desired LWS Orientation |
SKYPA |
ROTPOSN |
| North UP |
154° |
64° |
| North RIGHT |
244° |
154° |
| North DOWN |
334° |
244° |
| North LEFT |
64° |
334° |
TV Guider Orientations:
PA(TV)=SKYPA+90°
| Desired TV Orientation |
SKYPA |
ROTPOSN |
| North UP |
270° |
180° |
| North RIGHT |
180° |
90° |
| North DOWN |
90° |
0° |
| North LEFT |
0° |
270° |
Notes:
- SKYPA is the angle used by SKY and FACSUM. It
is the angle to give the OA when specifying the desired
orientation.
- ROTPOSN is the angle recorded in the image
header. From this angle you can reconstruct the LWS
position angle, as described below.
LWS Position Angle FAQs
- What is the meaning of the SKYPA value displayed on
FACSUM?
- The SKYPA value displayed in FACSUM represents the position angle of
the guider FOV relative to the center of the LWS FOV in the
focal plane.
- What is the position angle of an image displayed on the
LWS detector?
- The position angle of both the LWS slit and LWS detector
columns with respect to celestial North is given by
SKYPA+206°.
- What is the position angle of the LWS slit?
- See previous answer; the LWS slit is aligned with the
detector columns. Also see next question.
- Can I use any position angle with LWS?
- Usually the choice of position angle is constrained by
the need to place a guide star on the TV guider. Since the
guider and LWS rotate together in the Forward Cassegrain
Module, the LWS position angle cannot be set independently
of the guider.
- What is the position angle of the TV guider display?
- The position angle of guider columns with respect to
celestial North is given by SKYPA+90°.
- How do I orient LWS so that North is "up" on my images?
- Set SKYPA to 154°.
- How do I orient the guider so that North is "up" on my
guider display?
- Set SKYPA to 270°.
- My image header only lists the ROTPOSN keyword. How
can I tell to which position angle this corresponds
on my LWS image?
- Given ROTPOSN, the following relationship gives the LWS
position angle: PA(LWS) = ROTPOSN + 296°.
LWS Position Angle Diagram
The following diagram demonstrates the relationships between the
various LWS position angles for an arbitrary SKYPA value.
- Vector N represents celestial North as projected
onto the focal plane.
- Vector YIM represents the cardinal direction of
the image rotator coordinate system. ROTPOSN is the angle
between YIM and N.
- Vector G represents the direction from center of the LWS
FOV to center of the guider FOV. SKYPA is the angle
between G and N.
- Vector TVY represents the direction of increasing row
number on the guider detector (i.e., "up" on the guider
display).
- Vector INSTY represents the direction of increasing row
number on the LWS detector (i.e., "up" on the
instrument display).
Go to:
LWS Home Page -
Instruments Home Page -
Keck Home Page
Last modified: Tue Dec 21 22:13:34 HST 1999
