SUMMARY
Focus data for the last several months are analysed and the telescope computer files updated accordingly.

This report describes

• the updates to the smu.ifl file that contains the focus offsets,
• the new SCUBA-specific terms, to account for an additional elevation dependence of the SCUBA Z-focus
• a possible explanation of this additional dependence

The need to use one instrument to scout for another requires rationalizing the pointing collimations and focus offsets.

The logged data have therefore been analysed to provide updates for the smu.ifl file. A separate report on the pointing updates will appear soon.

The last analysis of focus data was in December 1999.

For the period Jan-Aug 2000, and on timescales longer than 10 days, the instruments were available (or were used) as follows :

• SCUBA : throughout
• RxA : after 25 July
• RxB : Jan through 20 Feb and after 08 April
• RxW : 8 shifts in the period Apr 08-21, 2 in June, and one E&C shift in August dedicated to RxW work by RRP.

April 08-21

FE  axis  No.    mean   +-      systematics              
--  ----  ---    ----  ----     -----------------------
 S   Z    39     0.06  0.09     Z = 0.34 + (-0.37+-0.10)*sin(el)
                             or Z = 0.26 + (-0.0038 +- 0.0009)*el
     X    19    -0.18  0.26     X = (0.11+-0.02)*Tleg
     Y    19    -0.04  0.23     

 B   Z    17    -0.29  0.13     Z = (0.03+-0.01)*Tleg
     X    16    -0.11  0.38     
     Y    14    -0.40  0.29 

 W_C Z     6     0.17  0.27
     X     8    -0.63  0.55
     Y     7     0.59  1.18

 W_D Z     8    -0.10  0.35     one v.oddball at -1.0
           7     0.03  0.07
     X     6    -0.34  0.31
     Y     8    -0.56  0.46  

July & August

FE  axis  No.    mean   +-      systematics              
--  ----  ---    ----  ----     -----------------------
S    Z    150    0.37  0.10     Z = 0.60 + (-0.30+-0.06)*sin(el)
                             or Z = 0.57 + (-0.0037+-0.0006)*el
     X     63   -0.31  0.22   
     Y     58   -0.01  0.23    

A    Z     63   -0.12  0.14     
     X     34   -0.19  0.26
     Y     31   -0.47  0.20

B    Z     67    0.06  0.15     
     X     41    0.00  0.25
     Y     43   -0.48  0.31

Note the systematic trends with sine(elevation) seen in the Z focus measures with SCUBA : no comparable trends are seen from the other receivers.

Relative focus offsets are then :

     FEs  Axis    Apr    Jul/Aug
     A-S    Z             -0.49
            X              0.12
            Y             -0.46

     B-S    Z    -0.35    -0.31
            X     0.07     0.31
            Y    -0.36    -0.47

    W_C-S   Z     0.11
            X    -0.45
            Y     0.63

    W_D-S   Z    -0.03
            X    -0.16
            Y    -0.52

This enables changes to be made either directly or via the relative measures : eg the X-focus for RxA :

      New_X = Old_X + Delta_X    
            = 0.28  + (-0.19) 
            = 0.09   
or
      New_X = Old_X + Delta_X_SCUBA + relative_X_A-S
            = 0.28  + (-0.31)       + (0.12)
            = 0.09

This has particular application to RxW, as seen below.

Z-focus differences between SCUBA and the FEs in the receiver cabin
The change of Z & X focus with elevation was previously anticipated :

     Z =  + ZB + ZA*sin(el) + ZC*sin**2(el)
     X =  + XB + XA*cos(el)

The recent data suggest that the Z-focus for SCUBA may have a different behaviour to that for the FEs in the cabin, so, on 2000 09 07, Firmin created new CONTROL parameters ZA_SCUBA, ZB_SCUBA, ZC_SCUBA and set them, on the basis of these recent data to values of 0.49, -12.53, 0.845 resp.; the last two numbers remaining unchanged from their previous values applicable to all FEs.

The value ZA (i.e. for FEs other than SCUBA) is left at 0.79.

But what is the source of this difference !?
After Per had vocalized the question thus "What could cause the cabin to move laterally w.r.t. SCUBA" I thought I recollected inclinometry data showing precisely this; that the A-frames flower out (or in) as the elevation changes. Inclinometery measures taken on 951227 (reported here), with an inclinometer placed on top of the left-hand elevation bearing, show a 20" swaying of the A-frame during an antenna slew between the zenith and horizon. This corresponds to 0.8mm of lateral motion at the top end of the A-frame if the bottom of the A-frame is assumed stationary. Of course, what the relative motions of the cabin and Nasmyth platform might be is unclear, but the relationship between Z-focus change and elevation might also then be expected to be linear rather than trigonometric.

The notes in the tables above therefore show the relations between Z-focus and elevation as well as sine(elevation). The experimental change seen in the Z-focus of SCUBA across the elevation range 0-90 is 0.33mm ( = 0.0037*90, with the April and July & August values being essentially identical), which is not too dissimlar from the 0.8mm estimated from the 1995 inclinometry.

The linear and trigonometric relations between Z and elevation are equally significant and for simplicity the additional elevation dependence of the SCUBA Z-focus is incorporated into ZA_SCUBA (which assumes the trigonometric dependence) without loss of accuracy.

Updates effective 2000 09 07

The complete inventory of new FE-specific focus parameters is then

  parameter     old value    new value

  ZA               0.79          0.79

  ZA_SCUBA                       0.49
  ZB_SCUBA                     -12.53
  ZC_SCUBA                       0.85

  SCUBA_X_OFFSET  -0.65         -0.96
  SCUBA_Y_OFFSET  -0.67         -0.68
  SCUBA_Z_OFFSET  -5.4          -4.80

  RXA3I_X_OFFSET   0.28          0.09
  RXA3I_Y_OFFSET  -0.25         -0.72
  RXA3I_Z_OFFSET  -0.88         -1.00

  RXB_X_OFFSET    -0.08         -0.08
  RXB_Y_OFFSET    -0.20         -0.68
  RXB_Z_OFFSET    -1.00         -0.94

  RXW_C_X_OFFSET   2.10          1.34
  RXW_C_Y_OFFSET  -2.55         -1.93
  RXW_C_Z_OFFSET  -1.42         -0.94

  RXW_D_X_OFFSET   0.50          0.03
  RXW_D_Y_OFFSET  -0.10         -0.63
  RXW_D_Z_OFFSET  -1.00         -0.66