The telescope mirrors were refigured in 2002 because we strongly suspected that the surfaces were not particularly good (which was confirmed by optical testing before repolishing). During repolishing the followin optical prescription was adopted:
The following discussion describes how this prescription was arrived at.
We have essentially no information as to how the as-built optical parameters match these design parameters.
When we built our new guide box, our best measured distance from tertiary to rotator mount was 30 inches (however, see below!). Our old guide box/filter wheel had a total thickness of 9.875 inches, for a total tertiary to focus distance of 39.875 inches.
Our new guide box/filter wheel was designed to increase the tertiary to focus distance by a couple of inches to better match the design prescription (under the assumption that the primary-secondary separation matched the design value, see below). The guide box has a thickness of 9.25 inches, the filter wheel, 1.313 inches, and our best measurement for the optical distance of the Princeton CCD from the front mounting surface is 1.125 inches, for a total tertiary to focus distance of 41.688 inches.
The prescription I was then using for ray tracing in Zemax is:
where the focal plane location and primary-secondary separation were allowed to float to get best image quality.
We later switched to using an Apogee AP7P camera, which has the CCD 1.10 inches behind the front mounting surface (as measured by microscope); we added an adapter plate of width .375 inches (for a reason that is not totally recalled!).
Several of these measurements were made from the edge of the primary to the center of another element (tertiary or secondary). To correct for the distance to the primary vertex requires a correction from the hypotenuse of a triangle to the on-axis distance, plus a correction for the vertical distance between the primary vertex and the edge of the mirror. For the hypotenuse correction, a diameter of the front surface of the mirror was measured (approximately) to be 41 inches; note that using a smaller diameter leads to some inconsistencies in measurements (see JCB 6/02 and JH 6/02). The vertex ``sag'' was computed using a radius of curvature of 200 inches, resulting in a 1 inch sag.
JCB 4/02: measured distance from primary edge to secondary center as 166.2+/-0.3 cm (65.43 inches); correcting for hypotenuse gives (62.14+/- 0.12 inches). Distance from vertex to "edge center" adds another inch based on radius of curvature, to give 63.14. Using geometric center of tertiary, measured secondary-tertiary to be 1227 cm (48.31 inch). Tertiary to rotator mount measured outside of bearing to be 79.4cm (31.25"), rotator mount to filter mount 27.3 cm (10.75")
JH/DS 5/20: measure primary-secondary with tertiary removed. Method 1: mirror to mirror cover lip 2", lip to tertiary mount 5", tertiary mount to secondary center 55.5", total 62.5 + distance from primary vertex to spot on primary out at edge of central hole. Method 2: secondary center to primary edge (as JCB) gives 65", correcting for hypotenuse gives 61.68; add another inch as above to get 62.68. Using plumb line to determine tertiary center, get 47.56 " secondary-tertiary. Tertiary to CCD mount measured through the hole to give 42.185"
JCB 6/02: distance from primary edge to tertiary center 24.75 inches, corrected for hypotenuse gives 13.87, plus 1 for sag gives 14.87 inches. Distance from tertiary to secondary is 47.71. Distance from primary edge to secondary center is 64.81, corrected for hypotenuse is 61.48, plus 1 for sag is 62.48. (Note for consistency check: ) Tertiary to CCD mount is 42.19 inches. CCD mounting plate measured to be .375 inches. Apogee web site gives 1.161 inches for distance from front of camera to CCD.
JH 6/02: distance from primary edge to tertiary center 24.625 inches, corrected for hypotenuse gives 13.64, plus 1 for sag gives 14.64 inches. Distance from tertiary to secondary is 47.875. Distance from primary edge to secondary center is 64.875, corrected for hypotenuse is 61.55, plus 1 for sag is 62.55. (Note for consistency check: )
JH/JCB 6/02: microscope gives distance from front window of Apogee to CCD surface is 0.802 inches. Micrometer give 0.682 inches from front mounting surface to front window. Mounting plate thickness 0.375 inches.
Summary (numbers in parentheses are derived from other measurements) :
|distance||design||Zemax||JCB 4/02||JH/DS 5/02||JCB 6/02||JH 6/02|
|rotator mount-filter mount||10.75||10.563(design)||10.563||10.563|
|CCD mounting plate thickness||0.375||.375||.375|
|front of camera to CCD||1.161||1.109||1.109|
In measured configuration, secondary has range to move approximately 0.125 inch away from primary, and 1 inch towards primary. Primary support could be modified to move primary towards secondary (e.g. increase pad thickness), but not necessarily further away.
For new design, modify distances conservatively given freedom to adjust primary towards secondary, secondary towards tertiary, and CCD away from tertiary:
In early May 2004, we pulled the primary and removed the plastic sleeve. The sleeve was smoothed at the Sunspot machine lab, so the tolerances are now even greater than before. In addition, the sleeve was unable to go all the way to the bottom of the stovepipe because of the weld there, so the inner bottom part of the sleeve was ground away to allow the sleeve to go to the bottom, and hopefully, not get pushed around by thermal expansion at the weld.