Initial Ray-tracing Diagnostics

10/11/00
Problems with the SPARO tertiary template.

There appears to be a discrepancy between the BEAM3 configuration file used for the testing of SPARO and the metal template actually used to configure the mirrors at the South Pole. Conceivably this discrepancy could explain some of the problems experience by SPARO in its first run.

The configureation file used in the raytracing of SPARO is the following:

                                  sparo.opt
    Z        X       Y      C         S     Mir/Len   T       P       R 
--------:---------:-----:--------:---------:-------:-----:---------:-----:----:
   1.5  :   0.0   : 0.0 :-.333333:.01020408: mirror: 0.0 :   0.0   : 0.0 :    :
  -0.5  :   0.0   : 0.0 : 1.1429 :.41699219: mirror: 0.0 :   0.0   : 0.0 :    :
-.00273 :-0.07317 : 0.0 :  0.0   :   0.0   : mirror: 0.0 :   0.0   : 0.0 :    :
-.589941:-1.148669: 0.0 :1.023577:-1.30073 : mirror: 0.0 :92.935396: 0.0 :    :
-.60982 :-0.76102 : 0.0 :  0.0   :   0.0   : detect: 0.0 :-36.51063: 0.0 :    :
        :         :     :        :         :       :     :         :     :    :
        :         :     :        :         :       :     :         :     :    ;

This is the original information sent by CMU to Tom Renbarger (Northwestern University) in Summer 1999.

The measurement discrepancies involve tracing the central ray in both BEAM3 and then manually on the stainless steel template that was made in 1998 at CMU using this information. The teritary (the fourth source listed in sparo.opt above) is not the same on the template. This can be detected through both measurements of the optical path (distance in meters the light travels between each surface in BEAM3) and the angles of incidence and reflection.

Angle of Incidence for the Central Ray (degrees)

	Measured:	BEAM3:	Difference:
Secondary:	20.0	19.74	0.26
Chopper:	33.7	33.89	-0.19
Tertiary:	47.6	43.27	4.33

Path difference between surfaces (cm)

	Measured:	BEAM3:	Difference:
Focus of Primary to Incidence on Secondary:	58.2 cm	58.04 cm	0.16 cm
Secondary to Chopper	63.1 cm	63.34 cm	-0.24 cm
Chopper to Tertiary	56.41 cm	57.84 cm	-1.43 cm

Investigations of other files on disk reveal that this is, in fact the setup that bears the closest resembelence to the template. Secondly, we are doubly sure this is the correct optical setup for SPARO in that 0.7 meters of the primary are illuminated by a focused beam, just like the real telescope.

Investigations are underway to check wheter or not the central ray on the template was in some way mismarked, although this is doubtful, given the close agreement between the measureable parameters for the focus of the primary, secondary, and chopper.

Last updated Feb 26, 2001 by C. Greer.