WIYN Instrumentation:
Bench Spectrograph
Upgrade



    Project Team

       M. Bershady - Project Scientist
       D. Harmer - Instrument Scientist
       C. Harmer - Optical Designer
       G. Jacoby - Project Manager

    Consultants: S. Barden, B. Schoening
    Vendors & Contractors: CSL, LLNL

       Contents
       Description       Collimator       Spectrograph Layout   
       Schedule       VPH Gratings       Throughput Budget   
       Simulator       CCD       Beam Profile   
   
    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.


Description

The Bench Upgrade project consists of implementing a new, off-axis, faster collimator, VPH gratings, and a new CCD system to improve throughput by factors of 2-5 while maintaining or improving spectral resolution and sampling.

A GUI is available 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.

Schedule & Status

As of Mar 2004, the project is currently working toward a critical design review of the collimator. Testing is underway on one (740 l/mm) VPH grating, a second (3300 l/mm) VPH test grating has been fabricated, and the parameters for a a final 3300 VPH l/mm grating are being finalized.

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

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:

    Working page

    Optomechanical Design

    Spectrograph Layout

    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.

    Working page

    VPH Gratings

    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
    Grating
    Name
    l/mm
    d
    (um)
    dn n=n2 phi height
    (mm)
    width
    (mm)
    depth
    (mm)
    height
    (mm)
    width
    (mm)
    substrate
    material
    index
    n1=n3
    grating
    man.
    post-polishcoatingmount
    v740a 740 17
    14 effective
    0.03 1.43 0 220
    219.46
    240
    239.55
    24
    24.55
    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
    v3300a
    CSL/WP3200
    3300 12 0.048 1.43 0 120 170 24 100 150 Diamant float glass; 2x12mm thick ? CSL TBD TBD TBD
    v3550a
    CSL/WP4200
    3550 6
    pending
    0.10
    pending
    1.5
    pending
    0 230 500 30 210 480 Zygo
    FS 7980 2F
    1.462
    at 20 C and 1 atmos.
    CSL No Yes; TBD TBD

    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.

    CCD

    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 (4-5 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 (2e- rms goal)

    The specific design goals and requirements are still under draft and review by the project team and the WIYN SAC:

    design requirements - version 1
    design requirements - version 2

    One ramification of this upgrade is that with a flat detector, the last camera element will need to be changed. This may be an opportunity to re-coat the camera elements to improve red and blue response and at the same time decreasing scattering.

    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


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    last updated: 03 Jan 2006 (mab@astro.wisc.edu)