Functional Checkout of the Secondary Mirror

This is a draft plan for a generalized SMU FCO, based on the successful FCO of Jul 2003. It assumes a successful electro-mechanical checkout by ETS, and a partial (daytime, remote ?) checkout of its capability to move (tables) and oscillate (chopper).

The Goal
To maximize the performance (read 'efficiency') of the SMU.

Potential Problems
The expected problems are essentially those caused by changes in

• the orientation of the entire SMU structure due to the removal and replacement, (although I'm not sure that there is a parameter that one can adjust (CHOP_ANGLE_ZERO ?) to rotate the chopping coordinates even if you wanted to !?) and/or
• the relative positions of the tables w.r.t. their hard and soft limits, and/or
• the relative tilt of the subreflector w.r.t. the optical axis of the telescope (primary reflector).

The Goal - II
The goal is indeed to "maximize the performance (read 'efficiency') of the SMU", and this should be seen also as the guiding principle for the process below.

Preliminaries
Settings prior to the overhaul are as described in smu.ifl (in mt_smudir), and here. While it should be anticipated that nominal settings (zero-points) and limits might all change, it may be sensible to assume that some quantities, like relative positions, remain the same. Some examples may suffice:

• the default Z focus position for SCUBA may change, but the relative offsets between SCUBA and other receivers will remain the same,
• the values assigned to the soft limits may change, but (given the same screw pitch) the difference between the upper and lower limits should remain the same.
• Following repairs to the tables (as in Jul 2003) it may be asssumed that Chopper functionality is unchanged; eg. that the subreflector and Chopping hardware have not been separated. A cautious FCO would nonetheless include a perfunctory check.

Tables
Determine hard limits on X,Y and Z tables (with ETS oversight) and set new soft limits if necessary:

• A May 2000 document ( /JACdocs/JCMT/a/003/17) describes the limits, ranges and hard stops of the X, Y and Z axes as follows :

  Axis	limits (mm)	range (mm) encoder units	hard stops (mm)	range (mm) encoder units	current soft limits range
  X	+23.5	47.0 12866	+24.0	48.0 13140	12900
  Y	+23.5	47.0 12080	+24.0	48.0 12337	12975
  Z	+22.8	45.6 12486	+23.3	46.6 12760	12600

• Set X soft limits (X_SLIMN & X_SLIMP) such that range again = 12900 units.
• Set Y soft limits (Y_SLIMN & Y_SLIMP) such that range again = 12975 units.
• Set Z soft limits (Z_SLIMN & Z_SLIMP) such that range again = 12600 units.

  Set zero-points using the similarity arguments above :

• Set new X-zero-point (X_ZERO) = (X_SLIMN + 7262) / X_SCALE
• Set new Y-zero-point (Y_ZERO) = (Y_SLIMN + 5585) / Y_SCALE
• Set new Z-zero-point (Z_ZERO) = (Z_SLIMN + 6570) / Z_ZERO
  
  where
                  X_SLIMN = X axis lower (N-egative) soft limit, etc., and
                  X_SCALE = -273.756 encoder units/mm.
                  Y_SCALE = +257.02
                  Z_SCALE = +273.817

The case of a replaced or repaired encoder may cause concern about the rollover of the encoder zero-point, but it is not a show-stopper, and was well handled in July 2003 - see the 'Reality' sections of that document.

(Chopping : Q ? : Is there a similar set of limits to be (re-)assessed for chopping ?)

Nighttime Plan

1. Start optimistically : the expected errors in placement of the SMU lander upon its legs (1mm ?) translates to errors on the sky of only a few arcsecs.
   • Configure telescope for SCUBA observing.
   • Set (dX,dY,dZ) = (0,0,0), (uaz,uel) = (0,0) - assuming that prior to the problem these were the default offsets for SCUBA.
   • Set (or leave) other parameters, such as POSOFF_EW & _NS at their previous (working, useful) values.
   • Take a jiggle map of a bright source
   • If source is seen on the array : skip to 5.

2. If the source does not fall on the SCUBA array pointing errors exceed 60". SMU frame replacement errors are estimated (NIK) at 1mm implying expected pointing errors of only a few arcsec, so this larger problem may be presumed to be due to a severe re-orientation of the SMU w.r.t. the main optical axis. Finding a source by spiralling outward may be possible, but will be just as time consuming as taking a much larger HOLO map.

   Configure RxH3 as front end and point at HOLO, with offsets = (0,0) and a Z-focus setting of +31mm. (dZ should show on screen s0 as +31.00) (Ensure that the source at UKIRT is switched ON).

   • slogin to mamo as OPERATOR and run
     mamo> rxh3_eng         (see instructions here (old?) or here (Oct 2007)).
   
   The entry to watch is     Lo Sig Pwr, the signal strength.
   It may be possible to 'peak-up' on this signal using appropriate setoff commands, but if in doubt take a small (1000x1000 point) holography map : this should only take a few minutes, and will cover enough space to catch the signal. Jan will reduce offline (thank you) and provide 'pointing' offsets. The technique is insensitive to the focus offset.

3. However, such pointing offsets are not due to pointing/collimation changes (for there have been none, one may assume) so they must be due to a change in the configuration of the SMU tables - in particular, the relative alignment of the optical axes of the SMU w.r.t. the primary mirror. Again, it is not anticipated that the optical-table-quality tables will be restored to a configuration too different from that prior to the shutdown, so these 'pointing errors' ought to be small and steps 3 & 4 ought to be unnecessary ! . . .

   The offsets must be due to an error in the orientation of the tables w.r.t. the optical axis, either in translation or tilt. The Nov 2003 repair generated new SMU tilts which appear in smu.ifl as parameters POSOFF_EW and POSOFF_NS, with values -2825 and -2991 encoder units, resp.. The determination of new tilts or translations (which are essentially focus changes, i.e. changes to X_ZERO, Y_ZERO, Z_ZERO) is accomplished by making adjustments and measuring the gain, i.e. the signal strength of a source at one position relative to another. In the case of translations, this is precisely a ALIGN_X or ALIGN_Y observation.

   A translation error in X or Y of 1mm generates a displacement of the best image by 7mm at the SCUBA focal plane, or 8" (calculations by IMC). Adjust the X & Y zero points (X_ZERO, Y_ZERO) accordingly to move the peak holography signal to the centre of the map.

4. Repeat pointing on a more point-like source if necessary.

5. Repeat steps 2-4 until convergence

6. Update pointing model or collimations, and update the zero-points and/or tilts in smu.ifl and then do 'mt_compifl smu' (see FJO). Repeat step 2-4 to ensure signs are OK; i.e. (uaz,uel) and (dX,dY,dZ) should now be (0,0) and (0,0,0) and the gain should have been maximized.