WIYN Instrumentation:
Bench Spectrograph
Upgrade

These project will commission and document all of the new subsystems, and provide software for scientists to explore and optimize different spectrograph configurations.

A working proto-type of this configuration tool is available. This GUI can be used to optimize setups and calculate exposures for all gratings and fiber cables (calibrated with measured system throughput values where available; see Simulator). Modification of the camera-collimator angle (for conventional gratings) can optimize blaze-wavelength and anamorphic factors.

A more complete description can be found in our Sept 2003 report to the WIYN SAC and Board, which serves as our Concept Design, in the links above, and in the project schedule and status.

Collimator.

As of Apr 2006, the project is currently working toward a preliminary technical design review of the collimator optical design and concept-design review for the opto-mechanical design. An optical design (Feb 2006) and initial tolerancing (Apr 2006) has been completed. Opto-mechanical design and layout is in progress, and in the early stages.

A suite of two initial VPH gratings are part of the Upgrade project.

• Commissioning is complete on one (740 l/mm) VPH grating, and is in regular use. See this link for more details.
• A second (3300 l/mm) VPH grating has been designed, and fabricated by CSL (delivery in Feb 2006). Preliminary testing on-spectrograph and on-sky (April 2006) are under analysis. Future development includes coating for high incidence angles (50-70 degrees), design and fabrication of the grating cell, and final commissioning. Further information about this grating is available below at these links: [1] [2].

CCD and Controller.

A new CCD and controller is planned for the upgraded system. The rear-element of the camera will be replaced assuming the device is significantly flatter than the original CCD used on the spectrograph.

The new CCD system will have smaller pixels (better sampling of small-fiber high anamorphic-demagnification configurations important for the highest-resolution work), lower read-noise (critical for sky-limited work at high dispersion which is read-noise limited with current systems), and faster read-out time. CCDs are currently being culled from test-lots from wafers used in development of OTA devices for ODI. The nominal upgraded controller systems is MONSOON. Fall-back options will be explored if foundry runs yield no suitable devices, and if MONSOON is delayed or cannot yield low-noise environment. So far, results on CCDs look promising.

Optical Design

Description.The new collimator consists of an off-axis parabolic segment with an all-spherical, all-glass, transmissive corrector. The collimator is "faster" than the existing collimator by roughly 20%, i.e. with a focal length decreased from 1021 mm to 800 mm. The primary aim of the shorter focal length is to capture more of the fiber-output beam into the 150mm collimated beam for which the gratings and camera were designed, while preserving the spectral resolution achievable in the highest-resolution settings with the echelle grating where anamorphic factors are large.

The design is heavily constrained by the project-level requirement to keep the existing camera. The initial design consisted of a 3-element corrector with tilted elements (akin to the Wynne triplet, but using tilted, full spherical segments). A preliminary tolerance analysis showed the Bench implementation was likely unbuildable. The current off-axis design has 4 corrector elements and yields improved image performance than the existing on-axis design. Other considerations included:

spectrograph geometry

beam profile

throughput budget

delivered spectral resolution:

geometric demagnification

anamorphic demagnification

optical aberration

pixelization

S/N

The details of these considerations are found in two reports to the WIYN SAC and Board:

• Sep 2003 Concept Design
• Mar 2004 Trade Study: S/N vs line-characterization precision

Spectrograph Geometry: Detailed notes on optical element size and location, as well as obstructions. This is used for determining both the required layout of the upgrade Bench, as well as the throughput budget.

The existing surface-relief grating suite for the WIYN Bench Spectrograph delivers a wide range of coverage in wavelength and resolution, as shown here in the left-hand figure. A completed Bench upgrade may include a set of VPH gratings which replace or augment the current capabilities of the existing gratings in this plane. Some examples of possible VPH grating suites are shown in the right-hand figure.

Two gratings resulting from the initial VPH effort, as contracted to Centre Spatial de Liege (CSL), will be part of the initial Bench Spectrograph upgrade: 740 l/mm and 3550 l/mm gratings. These are shown as red curves in the above, left figure. At this time, testing is underway on the 740 l/mm grating. The development is mature enough to offer the grating in Shared Risk mode for 2005B. We have taken delivery of a test-version of a small high-line-density grating (3300 l/mm). This was made on float glass and is not science grade. The high-density (3550 l/mm) science-grade vph grating is still under manufacture as of April 2005.

Grating Pages:

• v740a:     740   l/mm grating -- science grade
• v3300a:   3300 l/mm grating (CSL/WP3200) -- test grade
• v3550a:   3550 l/mm, large high-density grating (CSL/WP4200) -- science grade
  • Design
  • Optimization:   3550 l/mm, 510.6 nm @ 65^o
  • Expected Performance 

• Opto-mechanical   issues

Notes-

See v740a   page for performance and optomechanical details. The effective d for this grating apparent comes from "RCWA interpolation" (Pierre-Alexandre Blanche, CSL, private communication, 2004.02.09)
See v3300   page for performance and optomechanical details.
Diamant float glass vendor: Saint-Gobain Glass.
Green numbers are measured values as delivered.

• The ESI on Keck, using a MIT-Lincoln Labs 2048 x 4096 device with 15 micron pixels, slow read-out time of 120 sec and RN of 2.7 e-, with QE peaking at or above 80% between 500-650 nm, and above 60% between 400-800 nm. (See this link.)
• The RSS on SALT, using 3 E2V 2048 x 4096 device with 15 µm pixels, and slow read-out time of 42 sec and RN between 2.3 and 2.9 e- (amp dependent), with QE peaking above 75% between 350-650 nm, and above 50% down to 900 nm. Full well is measured at 150-180 k e-.

The specific design goals and requirements have been given to the WIYN SAC for their review. Version 2 is considered the working document.

design requirements - version 1
design requirements - version 2

Early results from foundry run:

As of September 2004, a successful foundry run at Lincoln Labs for OTA CCDs has also produced 10 very promising, standard devices for the Bench. These are 2600x4000 12 micron pixel devices. Three to four of them have a single hot or blocked column. Five of these are now at Mike Lesser's lab for cold testing.

WARM images taken with eight of these are shown below. Note the left side of w10 and w11 are confused, and may be the same half-device - the bad column is exactly in the same place on both. These are all warm, relatively high illumination images - some things get better cold and others get worse.

w01 w03 w04 w05
w07 w09 w10 w11

Results from Lot 2 foundry run:

This URL shows images of each of the 12 CCDs in Lot 2. Each is labeled as wN.gif, where N is the wafer number. Wafers 9-12 are made with the high resistivity material that allows for better red response. Also see George's notes from the report on CCDs for the last telecon (pre 19 Dec 2005).

Surface flatness for one of the devices

This detector (SN 5652) is flat over the great majority of its imaging area to +/- 5 microns.