WIYN Instrumentation:
Bench Spectrograph Upgrade

    Project Team

       M. Bershady - Project Scientist
       D. Blanco - Systems & Optical Engineering
       C. Corson - Observatory Engineering
       J. Glaspey - Instrument Scientist
       G. Jacoby - Director, CCD development
       J. Keyes - Mechanical Engineering
       P. Knezek - Project Manager
       M. Liang - Opical Design
       E. McDougall - Systems Design
       D. Mills - Software Engineering
       G. Poczulp - Optical Testing & Integration
       D. Sawyer - CCD Engineer
       D. Willmarth - Instrument Scientist

    Consultants: S. Barden, D. Harmer, C. Harmer,
    B. Schoening, D. Schroeder, D. Vaughnn
    Vendors & Contractors: CSL, SESO, Infinite Optics

     Overview       Collimator       Spectrograph Layout   
     Schedule       VPH Gratings       Throughput Budget   
     Simulator       CCD       Beam Profile   
Internal Team Working Pages
(password protected)
     Links to related sites:

This project is funded by the WIYN Consortium.
Figures and documents on this and related web pages may not be reproduced or published without permission of the Project Team.


The Bench Upgrade project consists of implementing a faster, off-axis, faster collimator, the capability to use VPH transmission gratings over a full range of incidence angles, two VPH gratings, and a new CCD system. Together, these improvements are expected to yield gains in throughput of factors of 2-5 over the existing system while maintaining or improving spectral resolution and sampling for most applications, and at most degrading spectral resolution by 20%.

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.

Schedule & Status

Collimator VPH Capabilities VPH Gratings CCD and controller   Team working page
(password protected)

< 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.

VPH Transmission Grating Capability.

VPH gratings designed for 55 degrees incidence angle or above can be placed in the existing grating turret given the current Bench layout. This is refered to as the "direct" VPH mode. However, low line-density gratings in the optical are used at low incidence angles. Given the size of the Bench and the length of the current camera mount, incidence angles below 55 degrees require use of a fold-flat placed in the first grating turret, with the VPH grating placed in a (new) second turret.

A low-density grating capability has been designed and implemented. A large fold-flat with a good Al coating has a mount for the first grating turret. A mount, turret, and appropriate "bar" (link between first and second turret) have been fabricated and tested at fold angles of 22.5 degrees (full fold of 45 degrees). Some work remains on improving the optomechanics and metrology of these components, as well as some annodization of components. However, the current is now in use and available to the community for general application.

The most significant future development includes

  • Shortening the camera mount to decrease the lower-limit of the "direct" VPH mode below 55 deg. We expect to make the appropriate modifications during Summer Shutdown 2006.
  • Increasing the fold angle up to 45 degrees (full fold of 90 degrees). This will require re-coating the fold-flat. Planning for this is underway.

The combination of these changes will ensure that VPH gratings can be used at all incidence angles on the upgraded Bench Spectrograph.

VPH Transmission Gratings.

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.

Bench Spectrograph Simulator

This link is to a Java-applet calculator for Bench Spectrograph configurations. The applet calculate delivered spectral resolution, vignetting, and total throughput for all of the existing surface-relief gratings and fiber cables. The applet includes a geometric calcuator that traces the ray-bundle through the system to determine realistic system vignetting using a realistic spectrograph beam that includes the effects of fiber FRD. There are a number of diagnostic plotting functions which allow the user to optimize the spectrograph setup in terms of resolution and throughput. It supercedes the old hydra.f. A key free parameter in the new simulator is the camera-collimator angle. Future upgrades (already in beta testing) will include VPH grating geometries and the new collimator.

The Upgraded Bench Spectrograph Collimator

Scope Design Requirements Early Results Delivered System

Optical Design Team working page
(password protected)
  Optomechanics Team working page
(password protected)

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:

    Optomechanical Design

    Spectrograph Layout

    Team working page
    (password protected)

    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.

    VPH Gratings

    Scope Design Requirements Early Results Delivered System

    The VPH grating development for the Bench was initiated by Barden (see, for example Barden et al. 2000, PASP, 112, 809) as part of a more general NOAO effort in advanced instrumentation. The advantage of VPH gratings relative to conventional surface-relief gratings is their high throughput (up to 90%), large super-blaze (i.e., good efficiency over a broad range of tunable central wavelengths), low scattered light, and transmisivity instead of reflectivity. The latter permits more compact spectrograph designs, particularly for large incidence-angle (i.e., high dispesion) setups, which allows for more optimum pupil placement, and hence less vignetting.

    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:

    Summary of Initial "Upgrade" VPH Grating Parameters
    Grating Substrate
    DCG parameters physical apertureclear aperture
    dn n=n2 phi height
    v740a 740 17
    14 effective
    0.03 1.43 0 220
    200 211.5 Diamant float glass; 2x12mm thick ? CSL Yes; 2D Strehl of 0.7, 0th-order transmission; 2D Strehl of 0.1 for -1 order; LLNLYes; soft MgF2; KPNO completed; KPNO
    3300 12 0.048 1.43 0 120 170 24 100 150 Diamant float glass; 2x12mm thick ? CSL TBD TBD TBD
    3550 6
    0 230 500 30 210 480 Zygo
    FS 7980 2F
    at 20 C and 1 atmos.
    CSL No Yes; TBD TBD


    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.

    CCD Subsystem

    Scope Design Requirements Early Results Delivered System

    CCD Subsystem: Scope

    A new CCD detector will be put in the existing dewar, and upgraded with new control electronics. The current CCD is warped, has large (24 micron) pixels, moderate read-noise (4-10 e- rms delivered), slow read-time (2+ minutes) and no enhanced red or blue response. While the chip is cosmetically very clean, significant improvements can be made with newer technology, including:
    • a flat detector (improved focus over field)
    • smaller pixels (better sampling)
    • higher QE in red and blue
    • lower read-noise (RN; 2e- rms goal)
    • faster-read time at low noise

    These design goals can be compared to as-delievered, on-telescope performance of two state-of-the-art systems using Leach-2 controllers:

    CCD Subsystem: Design Requirements

    version 1 - internal draft
    version 2 - working document reviewed by WIYN SAC
    version 3 - final project document

    One ramification of this upgrade is that with a flat detector, the last camera element will be changed. The opportunity will be taken to re-coat the camera element to improve red and blue response and at the same time decrease scattering.

    CCD Subsystem: Early Results

    Lot 1 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

    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 device

    This detector (SN 5652) is flat over the great majority of its imaging area to +/- 5 microns, and is typical of the devices from Lot 2, and the device (5644) ultimately adopted for the Bench system.

    CCD Subsystem : Delivered Performance

    Device: STA1 (SN 5644), 12 micron 2600x4000 pixels, low-resistivity silicon

    Controller: MONSOON

    Parameter summary     [Detailed information provided in links]

    *Note: These values should be verified at final operating temperature.

    Detector noise and bias structure

    o Overscan vs detector noise
    o Charge-injection and bias structure


    The STA1 device provides a 30% throughput performance gain relative to T2KA due to higher QE alone, flat across wavelength. The improvement relative to the original detector, T2KC, is roughly half this value. The following figure (Bershady et al. 2008, SPIE, in prep) illustrates and compares laboratory and telescope measurements.

    Associated data files:
    o on-telescope measurements
    o lab measurements

    Dark Current

    o As a function of detector temperature.

    Data-taking interface

    Data headers

    The following set of header key-words were populated as part of the requirements document. The only issue outstanding concerns the defintion of a detector-noise parameter as distinct from read-noise (RDNOISE), which is populated with the rms of the overscan (not relevant to data processing or performannce).

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    last updated: 21 Apr 2009 (mab)