Chimera 2 Optical Design #3: 76q and 76s
Chimera Mk.2 Optical design as described previously (76m) delivered poor-quality images. I made a mistake by not allowing the optimization to settle enough after adjusting lens edge thicknesses for manufacturability. While reoptimizing 76m (which is now called 76q) I reexamined the possibility of using the Wynne corrector. Thus I produced run 76s which uses the Wynne corrector. The advantage of the Wynne is that 76s requires fewer lenses while at the same time delivering better quality images. The disadvantage is that 76s has 8 more surfaces and will thus suffer 8 air-AR coating losses.
This post describes the Chimera II optical design. Because of space constraints, I have designed a reimager system that uses a standard collimator-camera configuration. In order to cover two bands simultaneously, a red and blue camera are used. The design goals were (in order) to achieve f/1.2 focal reduction, have seeing-limited image quality over the Andor SCMOS device, try to keep fabrication costs as low as is reasonable and ensure the design is straightforward to build. To that end I have designed three lenses. The collimator start a fraction of an inch past P200 prime focus (the Wynne corrector is not used).
The three lens prescriptions can be found in zemax archive format below:
- Run 76q - The red, blue, and collimator lenses are in this file
- Run 76s - The red, blue, and collimator lenses are in this file
The main difference between 76q and 76s is the collimator. The 76q design does not use the Wynne; while 76s does use the Wynne. Both lenses start within two inches of the P200 focal plane and accept a full field of view of 13.2 arcminute (diameter) and a wavelength range from 0.4 to 0.9 µm. The collimator accepts the full f/3.3 beam from the 200-inch Hale telescope and with a 8.5 inch focal length delivers a beam diameter of about 2.6 inch. When imaged by a perfect f/1.2 camera the collimated beam deliver seeing-limited images. Detailed description of the full performance is described in the camera sections below.
To control cost, the collimator delivers a pupil on the camera's first element. As a result, the collimator is physically larger than the camera, but there are two cameras required for this system so that the total volume of glass in the collimator is about equal to the volume of the glass in the two cameras (about 650 for the collimator; and the same amount for both lenses).
The blue camera has a beam that is split by a dichroic and operates from 0.4 to 0.55 µm. The blue camera (like the red) operates at f/1.20 and thus yields a 30 µm/arcsecond scale. A rendering of the blue camera is shown below, as well as RMS image diameter for polychromatic light. Note that the format of the Andor SCMOS is rectangular, and the spot diagrams show a variety of fields sprinkled across the format of the detector. The circle at 54 µ represents a 1.8" field of view; but note that RMS spot radii indicate sub-arcsecond native image quality. Zemax reports radii rather than diameters -- recall that one arcsecond is 15 µ radially. The worst images (RMS radius 24 µ) yield roughly 1.8 arcecond (aberration-limited) images while the best images are are 0.7 arcsecond (seeing-limited) images.
The red camera accepts a beam that passed through the dichroic and operates from 0.55 µm to 0.80 µm. The red camera (like the blue) operates at f/1.20. A rendering of the red camera is shown below, as well as RMS image diameters for polychromatic light. The same scale applies as described in the Blue Camera section above.
Path to construction
To take this design to a pre-construction state a ghost, sensitivity, and thermal analysis should be performed.
Not yet performed.
Not yet performed.
Not yet performed.