NMSU 1 meter telescope : instrumentation mounted on Nasmyth rotator port

Rough sketch of system (dimensions in inches)

Notes on the telescope

• Telescope is a Ritchey-Chretien design and produces an F/6.06 beam.

• Entire unit of instrumentation mounted on Nasmyth port rotates as telescope moves around in the sky. Consequently, weight distribution should not be extremely unbalanced, although some degree of imbalance is acceptable.

Notes on the elements

• CCD/shutter
  • Use existing unit.

  • Location of detector (focal plane) is 1.125 inch behind mounting surface

  • Detector size is about 1 inch square (i.e. 0.7 inch maximum radial extent)

  • Unit mounts into mounting surface with small circular protrusion, of dimensions ... There is a mounting lip around this protrusion, and mounting fixtures should be designed to allow relatively easy removal and installation allowing for repeatable and very stable positioning.

• Filter wheel

  • Design goal is 1 inch thickness for approximately 6 inch diameter around optical axis. Entire filter wheel will probably be significantly larger than this, and outside of this 3 inch radius circle, something (e.g., a motor) could protrude back towards the focal plane.

  • Back plate must accept protrusion from CCD/shutter assembly. Allow for possibility in future of new CCD/shutter and, hence, replacing, the back plate of the filter wheel.

  • Filter wheel itself should allow 6 3x3 inch square filters. Filters not to be more than 8mm thick.

  • Alternate filter wheel should be constructed for 2x2 inch square filters, holding as many as possible. Obviously, centers of 2x2 and 3x3 inch filters must be the same.

  • Preferred design for loading filters: filters fit into separate cells and are held in by small recessed screws pressing against rubber pad mounted against 2 sides of filter. These filter cells would be roughly 0.5 inches larger per side than filters themselves. Filter cells then mount into filter wheel and secured in some fashion.

  • Filter wheel positions must be repeatable to approximately 10 microns (is this reasonable?).

  • Filter wheel motion must be driven by a motor which is computer controllable by a PC.

  • Filter wheel position must be encoded in a fashion which is computer readable.

  • Filter wheel must be light-tight, and there must be no direct path for light to pass through unit except through the filter. Beware of glancing edges.

• Guide box
  • Total width of guide box is 10.75 inches including back and front plates. (This assumes design focal plane location. If this is determined to be in error, width of guide box might change. Consequently, it's best to have design of internal guider unit as thin as possible.)

  • Entire assembly to be as stiff as possible to minimize flexure when rotating on rotator.

  • Mount at rotator is 20 inches diameter. For about 4 inches back from this mounting point, nothing can extend more than approximately 11 inches radially from center. Further back, there are no radial space constraints.

  • Guide assembly to be mounted on the rear of the box, as close as possible to the focal plane. Mounting to rear is preferred to minimize differential flexure between guider and science image planes.

  • Location of guider pickoff mirror as close to on-axis as possible without vignetting beam to science instrument. My calculation suggests center of guider pickoff 1-1.25 inches off axis, depending on exactly how far pickoff is located from the focal plane. Guider pickoff should be aligned parallel to rows/columns of science detector to allow mirror to come as close as possible to optical axis without vignetting.

  • Guider assembly needs to be constructed considering how best to minimize scattered light off the assembly and into the science camera.

  • Spectrasource camera to be used as guider detector is already purchased and must be incorporated into design. Dimensions of Spectrasource camera are ...

  • Relative position of guide camera to CCD camera must be extremely stable (< 10 microns over 90 degree rotation of entire assembly).

  • Spectrasource camera to be mounted into existing focus mechanism, which is computer controllable and encoded. (A smaller unit may be available if current unit dominates current design width.)

  • Possible need to include mounting structure between guider pickoff and guider CCD for off-axis astigmatism corrector.

• Bearing baffle
  • Purpose of baffle is to block light glancing off inside of bearing surface. Baffle should have opening with radius 2.25 inches to achieve this. A hole of this size should comfortably allow the full beam from the science instrument. It may vignette the guider slightly.

• Nasmyth baffle
  • Purpose of baffle is to block light coming from around the tertiary.

  • Baffle location is constrained so as not to fall into main beam to primary mirror.

  • It is not possible to construct a baffle which blocks all light from around tertiary mirror without vignetting beam to science instrument (slightly) and guider camera (significantly). Choose compromise size which will block all light with appropriately constructed tertiary baffle. Estimated size of baffle opening is 2.75 inches radius, but final size TBD (also depends on design and construction of new tertiary baffle).

• Tertiary mirror/baffle
  • Tertiary mirror center is 40.875 inches from focal plane (design distance).

  • Tertiary mirror baffle should be constructed to obstruct any light from around tertiary to have direct path to science focal plane. Specifications ...

  • Tertiary mirror baffle must have obscuration above the mirror to prevent tertiary from seeing around the secondary mirror. Specifications ...