Receiver-dependent offsets also apply and are as follows (July 2003, in
mm) :
| FE |
X |
Y |
Z |
| SCUBA |
-0.09 |
0.15 |
-5.27 |
| RXA3I |
0.33 |
-0.26 |
-1.01 |
| RXB |
0.16 |
-0.12 |
-0.96 |
| RXW_C |
0.74 |
-0.44 |
-1.00 |
| RXW_D |
0.18 |
0.14 |
-0.77 |
| FTS |
0 |
0 |
0 |
NB: Focus offsets are now (2006) kept in Rx/FE-specific files like
/jac_sw/itsroot/src/wrappers/rxa/initialise/instrument_rxa.ent
The Z focus is also a function of temperature :
DZT = DZT_CONST + ( DZT_COEFF * MEAN_TEMPERATURE )
where
- DZT_CONST = -0.24,
- DZT_COEFF = 0.058,
- the temperature is the mean A-frame leg temperature in degC,
from the WS.
(NB DZT=0 at about 4degC), and
- DZT is in mm.
(DX, DY, DZ) may be seen in the lower left of the 's0'
screen (as dX, dY, dZ), while X, Y, Z (before the addition of the
zero-points) are shown in the top few lines of the 's3' screen.
X_encoder, Y_encoder, Z_encoder lie a few lines below this.
Potentially the following range of values may occur
for SCUBA at a temperature of 4oC :
| Axis |
Elevation = 0 |
Elevation = 90 |
Elevation = 0 |
Elevation = 90 |
soft
lower
limit |
soft
upper
limit |
| (mm) |
(encoder units) |
| X |
-0.14 |
-5.93 |
7700 |
9286 |
400 |
13300 |
| Y |
+3.88 |
+3.88 |
6878 |
6878 |
275 |
13250 |
| Z |
-17.80 |
-16.41 |
1996 |
2375 |
300 |
12900 |
The last 2 columns show the 'soft' limits for these travels, and the
expected demand values are seen to be well away from these limits.
Even a +5.9mm change (as suggested by obsn. #0028 on UT20030706 that
preceded the reported SMU tables failure) would only change the demand
by -1615 units : still not enough to reach these soft limits.
In the event of an SMU fault that effectively disables one axis,
it is obvious that the axis to best cope with this is Y, since
it is unaffected by changes in elevation.
If it can be set to one particular value before being disabled
then set it to
-
DY = 0.0 mm, or an
encoder value of 6878 for SCUBA.
If you must fix X or Z to one value, then
you must effectively optimize the setting for one elevation,
and accept focus errors at other elevations. Optimal values
for SCUBA are :
| Elevation |
X |
Z |
| mm |
encoder
units |
mm |
encoder
units |
| 15 |
-0.33 |
7779 |
7.49 |
2049 |
| 30 |
-0.91 |
7912 |
7.77 |
2128 |
| 40 |
-1.49 |
8071 |
7.99 |
2187 |
| 50 |
-2.21 |
8268 |
8.20 |
2245 |
| 60 |
-3.03 |
8493 |
8.39 |
2298 |
| 70 |
-3.95 |
8744 |
8.54 |
2339 |
| 80 |
-4.92 |
9010 |
8.64 |
2366 |
| 90 |
-5.93 |
9287 |
8.68 |
2375 |
These values may be seen in appropriate parts of the 's3' screen.
For other receivers add the following amounts :
| Receiver |
Axis |
mm |
enc.units |
| RxA3i |
X |
0.44 |
-120 |
| Z |
4.26 |
1166 |
| RxB |
X |
0.25 |
-68 |
| Z |
4.31 |
1180 |
| RxW_C |
X |
0.83 |
-227 |
| Z |
4.27 |
1169 |
| RxW_D |
X |
0.27 |
-74 |
| Z |
4.50 |
1232 |
| FTS |
X |
0.09 |
-25 |
| Z |
5.27 |
1443 |
Default increments (SX,SY,SZ) during focus measurements are 0.3mm in Z and
1.0mm in X & Y. Fluxes drop to about 70% of peak flux at 2 increments
away in all axes. So having the X or Z axis optimized for eg 50 degrees
elevation and observing at 70 degrees elevation incurs a focussing error
of less than 2 increments (0.6mm in Z, 2.0mm in X). For most observing
therefore you may wish to adopt the el=50 or el=60 values above as most
useful - or tailor the optimization to match your specific future
observing needs, i.e. the likely elevations.
