d J. Alan Schier Co , 3130 Foothill Blvd., Unit #1, La Crescenta, CA 91214

The Echellette Spectrograph and Imager (ESI), currently being delivered for use at the Cassegrain focus of the Keck II telescope employs an all-spherical, 308 mm focal length f/1.07 (underfilled) Epps camera. The camera consists of 10 lens elements in 5 groups: an oil-coupled doublet; a singlet, an oil-coupled triplet; a grease-coupled triplet; and a field flattener, which also serves as the vacuum-dewar window. A sensitivity analysis suggested that mechanical manufacturing tolerances of order +/- 25 microns were appropriate. In this paper we discuss the sensitivity analysis, the assembly and the testing of this camera.

Keywords: spectrograph, imager, echellette, Keck, camera, astronomy, prism.

The Echellette Spectrograph and Imager (ESI) is a multipurpose instrument currently being delivered by the Instrument Development Laboratory of Lick Observatory for use at the Cassegrain focus of the Keck II telescope. The P.I. for the instrument is Joe Miller, director of Lick Observatory. ESI is scheduled for first light on August 29, 1999. ESI is a medium resolution spectrograph, with a fixed 175.6 line/mm echellette grating from Milton Roy (now Spectronics, Rochester, NY 14625) and two large (25 kg) cross-dispersion prisms made of BSL7Y glass from Ohara Corp., (San Clemente, CA). The prisms were polished by Zygo (Middlefield, CT) and coated with multi-layer dielectric anti-reflection coatings (AR) by Coherent Inc., (Auburn, CA). ESI also contains a reflective collimator which is an off-axis segment of an on-axis ellipsoid, figured by TORC, (Tucson, AZ) and coated by Newport Thin Films Laboratory, (Chico, CA). ESI currently has three scientific modes available: a medium resolution Echellete mode; a low resolution prismatic mode; and an imaging mode . ESI is described and referenced in detail in several previous papers ^1,2,3,4,5.

The spectrograph and camera are designed to accommodate the (0.39 to 1.10)-micron passband without refocus. The spectrum is detected by a single flat (2048 by 4096 by 15-micron) thinned, back-illuminated, MIT-Lincoln Labs CCD. The entire spectrum is recorded in a single exposure for both the echellette mode and the low dispersion prismatic (LDP) mode. The echellette mode accommodates a 20 arcsec-long slit and has an average resolution (l /D l ) of 26,087 per pixel. The LDP mode takes multiple spectra of as many as 50 objects or along a single 8 arcmin long slit at a per-pixel resolution (l /D l ) of 4839 (at 0.39 microns) to 1052 (at 0.80 microns). The imaging mode covers a (2 by 8)-arcmin field of view at 0.153 arcsec per pixel.

The operating temperature range for both the spectrograph and the camera is û4— C to +6— C. The survivability range is û20— C to +30— C.

The camera optical design, shown in Figure 1, was conjured by one of the authors (H.E.). It consists of ten lens elements in five lens groups. The camera has an effective focal length of 308 mm. It has an entrance aperture diameter of 287 mm and a final plate scale of 97.7 microns/arcsec on the sky. The collimated beam diameter is approximately 160 mm. The cameraÆs effective f/ratio in imaging mode is thus f/1.93 and slightly faster in spectroscopic modes due to anamorphism.

The camera design is all-spherical and it includes two large CaF₂ lenses. Group #1 is a doublet, group #2 is a CaF₂ singlet, groups #3 and #4 are triplets while group #5 is the field flattener/dewar window. The elements in groups #1 and #3 are optically coupled with a fluid (Cargille laser liquid Type 5610 n(D)=1.5000) to minimize internal reflections. The elements in group #4 are greased together with Dow Corning Q2-3067 optical couplant. The six larger elements were fabricated by TORC and the 4 smaller elements were fabricated by Cosmo Optics Inc., (Middletown, NY). Broad passband AR coatings were applied by Coherent.

The ESI optical design is complicated by the fact that a wide variety of pupil anamorphic factors and effective entrance pupil distances are presented to the cameraÆs entrance aperture by reason of the three operating modes. In practice, the camera design was slightly compromised in the imaging and LDP modes so as to favor the echellette mode. Nevertheless, the echellette mode remains the most severe test of system image quality. The system performance is described in considerable detail by Sutin⁸, in which he used the pre-construction ESI camera Run No. 2990 (02/27/96) by Epps⁹.

The system performance was recalculated using the final as-built camera Run No. 102297AC. Orders 6 in the infrared through 15 in the ultraviolet were raytraced at five wavelengths uniformly spaced over each free-spectral range without refocus. The rms spot-size diameter (Drms) was calculated for each image. Averaging these image diameters over all wavelengths and all orders the Ave(rms) = 19.0 +/- 3.3 microns. The corresponding 80% encircled ray diameter average is Ave(80%) = 22.5 +/- 4.2 microns, while Ave(90%) = 28.4 +/- 5.6 microns.

A detailed computer-based sensitivity analysis was divined by one of the authors (B.S.). Individual lens elements were perturbed axially, radially, and in tip/tilt. The perturbations were 25 microns for axial (Z) and radial (Y) movements. The tip/tilt (R) perturbation angles about each lens element center were the Arctan(25 microns / element clear-diameter (cd)). The perturbed camera model was ray-traced within the ESI spectrograph numerical model in echellette mode at 50 different wavelengths, covering all 10 orders, with 1000 rays in each monochromatic spot diagram. The perturbed system was then compared to the unperturbed system and the worst centroid location change and worst increase in the rms spot-size diameter (Drms) were noted. For the first set of results, no re-focus or active centroiding was done. These calculations correspond to the worst-case scenario. For the second set of results, the median lateral image centroid shifts were subtracted, and a focus correction was estimated and applied. This case corresponds approximately to an active collimator scenario.

In addition, a similar analysis was performed for the lens groups. Sensitivities were determined by moving entire groups and calculating the resulting effects on centroid location and Drms. The following Table summarizes this analysis:

The columns are:

1. lens element number, starting at the first element
2. Degree of Freedom

3) perturbation size (always 25 m or 25/cd mrad)

4) and 6) maximum centroid location change, in m .

5) and 7) maximum increase in Drms, in m .

These sensitivities were used to estimate manufacturing and assembly tolerances for the mechanical components such that the expected summation of image errors (added in quadrature) was comparable to the residual aberrations in the optical design. In addition, the sensitivity data also allowed us to set a lens motion tolerance in terms of the final image motion allowable for different gravity orientations.

The camera mechanical system (CMS) for ESI was designed by one of the authors (J.A.S.), (J. Alan Schier Co., La Crescenta, CA 91214) and fabricated by Danco Machine, DPMS Inc (Santa Clara, CA). The CMS is shown in Figure 2. It consists of four cells supported within a single large barrel. The mass of the entire camera assembly, consisting of the CMS and the lens elements is approximately 125 kg. The individual cells with their respective lens elements have masses ranging from 5 to 30 kg. The cells locate radially against locating surfaces (lands) on the barrel inner wall and axially against athermalizing spacers. Each cell consists of an aluminum housing, radial athermalizing spacers and a compressive preload spring. Each of the two oil-coupled cells also contains an oil sealing reservoir system. The lens element spacings within the oil-coupled cells are maintained by 0.10-mm Mylar spacers, placed between the lens elements at their edges. We now describe the four major features of the optomechanical design:

Axial Athermalization:

Radial Athermalization:

Cell #2 Transverse Adjustment:

Optical Coupling Fluid Control:

The camera integration procedure was developed interactively with the aid of a set of simulated lens elements fabricated from aluminum. It was practiced several times with the simulated lens elements until we were comfortable with the amount of manual handling required and the risks involved. With the exception of the translational location of cell #2, the cameraÆs optical alignment is achieved by close-tolerance fabrication of the CMS.

The radial locating surfaces within the cells, i.e., the inner radius of the Delrin II radial athermalizing spacers, as shown in Figure 3, has been final machined to match the as-measured isothermal optic diameters plus a 0.025 mm radial clearance. This was done after the spacers were bolted into the aluminum housings in order to increase the centration accuracy of the hole relative to the outer housing diameter.

The cells were then assembled on a precision rotary table. Both the lens elements and housings were aligned to the rotary table by minimizing their horizontal and vertical run-out. Next the optics were held rigidly in place while the housings were jacked into position. Optics retainers with pre-load springs were then installed into each assembly. The rest of the mechanical components were then installed onto each cell, including the oil handling systems, for doublet and large triplet.

The oil was added to the cells via a gravity feed system. The oil had been previously placed into an evacuated bell jar to remove any dissolved gasses. If bubbles should appear at a later time, it may prove necessary to refill the cells under vacuum.

After the cells were assembled, cell-land locations were measured relative to the lens vertices. Only then were the final lengths of the axial spacers cut. This was done to remove as much axial assembly error stack-up as possible.

Axially, cell #1 locates against the front of the barrel via an internal land. All other cells locate against the back of the barrel through the other cells and associated spacers. A compliant spacer with a pre-load spring provides the necessary force between cell #1 and cells 2, 3 and 4 along with their spacers, which make up a stack whose axial thermal compensation is described in Section 4.

For cells #1, #2, and #3 a hand operated hoist is used to lower the cells into the barrel one at a time. Cell #4 is jacked into place from below. Measurements are made of the "stack height" as the cells are installed to confirm that the cells are seated properly on the spacers.

As mentioned previously, lands within the barrel determine the lateral location of the cells. These barrel lands have been final machined in a single lathe operation such that they are concentric to +/- 0.025 mm. Finally, they were honed individually to match the as-measured isothermal cell diameters. The resulting radial clearance per cell was left at .025 mm, for ease assembly. Radial shims were added afterward to take up these gaps.

The ESI camera was designed to operate under the ambient Keck observatory conditions. However, the integration and testing were carried out at the Instrument Development Laboratory of Lick Observatory in Santa Cruz, CA, which differs substantially in ambient temperature, pressure and humidity. Also, some of the optical tests were performed without oil in the multiplets. These effects were considered and accounted for in all calculations related to the integration and testing. However the details of these corrections are beyond the scope of the present paper.

After Laboratory assembly of the camera, several optical performance tests were carried out. The results of these tests were then compared to the predictions of the lens design and analysis. We now describe two different tests.

Interferometric test:

Point Spread function (PSF) and Back Focal Distance (BFD) measurements:

We have described the optomechanical design, assembly and testing of a 10-lens element, all spherical f/1.07 Epps camera. The camera is part of the ESI spectrograph, which is scheduled for first light on August 29, 1999. The results of our in-house camera-performance testing matched the predictions of the optical design to high precision. Further proof of the cameraÆs performance under actual observatory conditions will be seen at the Keck telescope in Hawaii.

The authors wish to thank Carol Osborne and Mary Poteete for their help with the design drafting and many of the Figures for this paper. We would like to thank George Laopodis for his assistance with the integration of the optics. Also, we would like to thank Joe Miller, Jerry Nelson, Terry Mast, Matt Radovan and Jack Osborne for their many helpful comments.