The orientation of the satellite determines the orientation of each
individual camera with respect to the sky. The sky coordinate of the
sighting axis (Z-axis) of an instrument is called its pointing.
Use is made of the following right-handed coordinate system:
+Y +X
| |
| |
-X______|_____+X === |_____+Y
/ /|
/ / |
+Z <= towards eye of reader => -Z |
-X
In this system the North-Angle is defined as the the angle to turn
anti-clockwise the -X (== +X of sky-image) onto the plane through the
North Pole and the Z while having the -Z axis (== detector) in your back.
The North-Angle is always related to the sky-image (mirrored around
Y-axis from detector image).
Note, that in this definition the NA will be undefined if +Z points
exactly at any of the two poles.
For the instruments the following definitions are in force:
for each camera:
Z = the heart axis through the instrument with the detector at -Z
and +Z at the mask side pointing to the observed sky
Y = the side of the detector perpendicular to the groundplate with
the groundplate at -Y and the other side of the instrument at +Y
X = the side of the detector parallel at and closest to the
instrument groundplate with +X going right when looking from
outside through the instrument into the detector.
**NOTE: The instrument +X_axis == the sky image -X_axis !!!
WFC-1: X = + SAT-Z = sky image -X axis
Y = - SAT-X
Z = - SAT-Y = instrument pointing
WFC-2: X = - SAT-Z = sky image -X axis
Y = - SAT-X
Z = + SAT-Y = instrument pointing
As each axis is given by its Right Ascension and its Declination
the following pointing relations exist:
WFC1: SAX:
----- -----
ra_x = RA_Z
dec_x = DEC_Z
ra_y = 180 + RA_X
dec_y = - DEC_X
ra_z = 180 + RA_Y = RA_PNT
dec_z = - RAS_Y = DEC_PNT
and
WFC2: SAX:
----- -----
ra_x = 180 + RA_Z
dec_x = - DEC_Z
ra_y = 180 + RA_X
dec_y = - DEC_X
ra_z = RA_Y = RA_PNT
dec_z = DEC_Y = DEC_PNT
It is envisaged that the instrument alignment on the satellite is not
exact. So the difference between the nominal and the actual pointing
of each instrument is defined by the following three (delta) parameters:
DELTA_THETA the angle between the nominal Y-axis and the actual Z-axis
minus 90. (nomally 0 degrees)
DELTA_PHI the angle going from the plane through the nominal Y-axis
and the nominal Z-axis to the one going through the nominal
Y-axis and the actual Z-axis (nomally 0 degrees)
DELTA_ROLL the angle going from the plane through the nominal Z-axis
and the nominal Y-axis to the one going through the nominal
Z-axis and the actual Y-axis (nomally 0 degrees)
These values will be stored and updated in the detector calibration file
(wfc#.det) as part of the data extension named 'align'.
GENERAL DEFINITION NORTH-ANGLE (NA)
Valid for an imaging instrument with its own X and Y axes
---------------------------------------------------------
The angle to turn the +X axis of the SKY-IMAGE anti-clockwise
towards the North Pole, with the observer looking to the sky and
having the detector behind his back.
so:
if Zsat = -Xwfc1 = North then NAwfc1 = 180
if Xsat = -Ywfc1 = North then NAwfc1 = 270
if Ysat = -Zwfc1 = North then NAwfc1 = undefined
if Zsat = Xwfc2 = North then NAwfc2 = 0
if Xsat = -Ywfc2 = North then NAwfc2 = 270
if Ysat = Zwfc2 = North then NAwfc2 = undefined
Always valid: NA_WFC1 + NA_WFC2 = 180 modulo 360
SAX/WFC ORIENTATION from INSTRUMENT DIRECTORY on FOT
----------------------------------------------------
- From FOT number 773 the ROLL as given in the INSTRUMENT directories
(w1instdir_000000_00_0.dat and w2instdir_000000_00_0.dat) of both
cameras are identical to the North Angle as defined for the instruments.
- Prior to FOT number 694 the following anomalous situation exists:
R.A. and Dec. of WFC1 and WFC2 both equal the R.A. and Dec.
of the satellite +Y-axis which is correct for WFC-2 but
The ROLLwfc1 equals 180 - ROLLwfc2
The ROLLwfc2 equals the North Angle of WFC2 + 90 degrees
so: NAwfc1 = ROLLwfc1 + 90 = 270 - ROLLwfc2
NAwfc2 = ROLLwfc2 - 90 = 90 - ROLLwfc1
- From FOT number 694 up to 773 the following improvement is made:
The ROLL situation same as before/
R.A. and Dec. of WFC1 now equals R.A. and Dec. of satellite -Y axis
The ROLL situation is same as before, however.
NOTE:
In a previous definition the X-axis of the instrument was taken to
be the X-axis of the DETECTOR. For orientation of the sky-image,
however, the X-axis of the IMAGE is pointing in the opposite direction.
So, Xwfc(sky) = -Xwfc(det)