Central Ray Tests

8/22/01 Information about the SPARO central ray.

On the metal template used to align SPARO on the Viper telescope, there are several rays traced out in magic marker. Since these rays can potentially contain valuable information about the validity of the template, it is important to devise a way to relate rays on the template to the rays in the Beam4 ray tracing package.

At the beginning of the analysis of the tertiary (detailed here), the following Beam4 ray trace file was used:

6 rays      sparo.ray
  Y0      X0     Z0   @wave
 0.000:1.300000:-1.0:       B
 0.000:1.000000:-1.0:       :
 0.000:1.600000:-1.0:       :
-0.350:1.300000:-1.0:       :
-0.350:1.300000:-1.0:       :
Additional columns were used to glean output data from the ray trace, however these four columns were the only input data. This was the file used by Tom Renbarger to do the preliminary investigations into problems with SPARO's alignment. As one can see, the first ray, at Y=0, X=1.3 is the center ray. For all intents and purposes, the X value is the amount that the ray enters the system off the optical axis.

In my initial analysis, I naievely took the first ray in the ray table to be the center ray of the system, and revealed the inital discrepancy in my first report. As my analysis became more sophisticated, however, it soon became clear that the ray marked 'CR' was not the same as the ray in the center of the star pattern in the Beam4 file. The were several clues:

  1. I initially used the CR on the template as the basis of a coordinate system on the template, so I could, via a rotation, measure parameters on the template and relate them to the system in Beam4. This seemed to work well. However when I measured the incidence points on each surface, and tried to relate them to the incidence points of the 1.3 m OA ray in Beam4, there was absolutely no correlation. To this end, I used a different source to relate my coordinate systems. (Read about this here.)
  2. Talking with Jeff Peterson, he made note that the center ray for Viper is really 1.35 m OA, not 1.3 m OA. Since Tom Renbarger traced the CR on the template, and garnered his information from the Viper secondary, it stands to reason that this ray would be the 'official' 1.35 m OA ray.
  3. I measured the angle that the template CR makes with the chopper as it moves from the prime focus area to the secondary. It makes an angle of 8 +/- .1 degrees from vertical. The 1.3 m OA ray makes an angle of 5.2 degrees, while the 1.35 m OA ray makes an angle of 7.8 degrees.
  4. Ray-tracing the 1.35 m OA ray and comparing it to the template causes the discrapancies mentioned in the first report to disappear. (Read about this here.)
  5. Ray-tracing the 1.35 m OA and 1.3 m OA ray and then comparing them to the incidence points of the template CR and measuring the differences yields the following: for the secondary and the chopper, the 1.35 m OA ray is much closer. For the condensing mirror, the 1.3 m OA ray is closer, but this is still under investigation.

For now, it appears safe to say that it is more likely that the template CR is the 1.35 m OA ray, barring the last point, which is still being looked at. Moreover, it looks like we have ruled out any possibility that it is the 1.3 m OA ray that we initially used for comparisons with the template.

Last updated August 22, 2001 by C. Greer.