Chimera 2 Optical Design #2

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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 collimator sits about 1.5 inches past the focus of P200, accepts 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 delivers 0.9 arcsecond images over most of the field and 0.5 arcsecond images over the central half of the field.

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 equal to the volume of the glass in the two cameras (about 650 for the collimator; and the same amount for both lenses).

Blue Camera

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, so the rectangular format is shown in the image below. The color-coded radius is in units of inches so the range of 0.00033 to 0.0008 corresponds to 0.6 (blue) to 1.3 (red) arcsecond. Note that at the very corner the color-code is orange meaning that for the blue channel image diameters are always less than 1.3 arcsecond.

Run 76m

The blue camera (76m) is shown above. The P200 delivers a focal plane on the right hand side. A doublet-singlet-doublet serves as the nearly 230-mm focal-length f/3.3 collimator. The collimator has almost 100-mm of relief to bring the pupil onto the mouth of the camera. As a result, the two cameras are smaller than the collimator.

Run 76m

The blue camera polychromatic (wavelength 0.4 µm to 0.55 µm) RMS image radius as a function of field position is shown in units of inches. The color scale shown includes images from 0.6 to 1.3 arcseconds in RMS diameter.

Red Camera

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.

Run 76m

The red camera (76m) is shown above. Note that the red and blue lenses look similar (by intent). The first doublet is the same, while the remaining elements are slight tweaks of one and other.

Run 76m

The red camera polychromatic (wavelength 0.55 µm to 0.8 µm) RMS image radius as a function of field position is shown in units of inches. The scale here ranges from 0.6 to 1.3 arcseconds in RMS diameter.

Ghost Analysis

A preliminary ghost analysis indicates the following:

  1. A 20-mm-diameter ghost-pupil image will be formed in a bounce from the back side of camera 3 to camera 1. Because of the hole in the primary, the ghost pupil may be non-uniformly illuminated. Further examination of the effect of the ghost pupil is required.
  2. The strongest ghost image is about 8 arcsecond in diameter which is roughly 4.5 magnitudes attenuated (-2.5 log(64)) and displaced from the object. How will this impact our science program?

Sensitivity analysis

Not yet performed.

Thermal analysis

Not yet performed.