WIYN / Bench Upgrade: VPH 740 l/mm grating


Overview Expected Performance Opto-Mechanics Performance Verification


One 740 l/mm grating has been made by CSL and delivered. The grating currently has been post-polished at LLNL to high Strehl (0.6), and then coated and mounted at KPNO. Optomechanical fabrication is complete for testing and using grating on the current Bench Spectrograph. Tests of the grating commenced in January 2005, and will continue through Fall 2005.

The grating appears to perform superbly between 600 nm and 1 micron in first order, and around 500 nm in second order. The range of useful angles is about 12 to 22 degrees, with an optimum at 19 degrees (from grating normal, or about 880 nm). The grating area is about 200 by 211.5 mm and the total glass size is 220 by 240 by 24 mm (two, 12 mm plates of glass).

Expected Performance

Calculations are based on rigorous coupled wave analysis (Barden, Bershady); measurements are from CSL. Calibration of efficiency on spectrograph is pending. Note: (1)Barden's calcuations are NOT for the delivered grating; (2) CSL measurements are of transmitivity, i.e., 0th order thoughput; (3) Barden's and Bershady's calculations are for 0th, 1st and 2nd order; (4) Bershady's calcuations have a shift in the superblaze which presumably means adopted parameters for d, n and dn are not correct. This must be resolved.

Predicted efficiency of 770 and 600 l/mm VPH gratings in order 0-2 using RCWA (Barden).
Measured transmitivity of completed 740 l/mm VPH grating (CSL)
RCWA-modeled efficiency of 740 l/mm VPH grating based on quoted parameters of completed grating (Bershady). Dashed line indicates CSL scan region; dotted line is CSL-measured superblaze peak (1st order).

Optomechanical Components for Use on Bench Spectrograph

See v3300 page for illustration of optomechanics for high-density gratings.

Status: Component fabrication is complete.

Design and drawings are courtesy of Bill Schoening. Configuration for low-density grating (small angle). Current grating turret (#1) holds flat for low-density VPH grating configurations. VPH grating is in second grating turret (#2).

Fabrication costs, etc: If you look at the VPH mount in the bottom left panel of 4, there is a stand underneath it. The orange part is a rotating stage that is commercial (from Bill's office). The grey pieces above and below below it adapt to the grating and to the table. Those were fabricated. They are relatively simple pieces, but the costs were (???).

The bottom set of 3 drawings show Grating cell detail for 740 l/mm grating.

"As built" Dimensions:   outer dimension of exterior housing

  • Height = 406.4mm
  • Width = 463.55mm
  • Depth = 111.125 - 155.575mm depending on whether you include the clamping knobs

All dimensions were measured to the nearest 1/8 inch and converted to mm; so the error is +/-3.175mm (an overestimate). These are "as built," as measured measured on the spectrograph by Gene McDougall (28 mar 2006).

Also note:

  • The turret that holds the VPH cell extends beyond the cell anywhere from 2-5 inches depending on where you are measuring.
  • The excessive width of the exterior mount is to accomodate larger-width gratings to be used at larger angles. The negative ramifications of this choice is that larger fold-angles are required to place VPH grating close to fold-flat and not vignette incident beam.

And finally: YES this will get annodized!

Performance Verification

The primary technical goals for performance-verification are as follows, in order of priority:

a. the absolute throughput of Bench Spectrograph with the VPH 740 l/mm grating and large flat;

b. the spectral performance of the VPH 740 l/mm grating; and

c. the relative throughput of the VPH 740 l/mm vs the SR 790 l/mm gratings.

In addition to these measurements, science-demonstration is highly desirable. Data gathering for items (b) and (c) are now complete via on-telescope acquisition of dome-flat and line-lamp exposures. The most important measurement (a) requires on-sky time, which we hope to accomplish during a Sep 2005 T&E run. At that time, we also hope to acquire science-demonstration data. A Shared-risk run in Jun 2005 (Wilcots) was weathered out, but did yield additional performance-verification data. Preliminary analysis is complete for relative throughput measurements (c). Ancilliary measurements concerning the performance of a large flat and spectral performance (b) are underway.

T&E run, Jan 2005. (Obs: D. Harmer, K. Westfall, C. Harmer) Comparison of dome-flats and line-maps for VPH 740 l/mm and SR 790 l/mm gratings. (SR = surface-relief, i.e., conventional, ruled grating. The SR 790 grating is from the RC spectrograph on the Mayall Telescope.) No on-sky measurements. Small flat (circular, 150mm clear diameter) used for VPH configuration. Matching mask used for SR configuration. Tests made in two orders at three angles per order (see table below) with only the red Hydra cable. With these data we were able to directly compare the relative throughput of the VPH vs the SR grating in six configurations in two orders, as shown below, as well as determine the spectral performance (not yet completed).

Preliminary result: comparison of VPH 740 l/mm vs SR 790 l/mm grating throughput. The VPH grating has 2 times (1.7-2.5) the throughput as the SR grating. Note that only the central third of the sampled wavelength range has been used in this analysis because of focus degradation away from the central wavelength. Likewise, this result applies to an average over the central third of the fibers. In June run, more time was available for focus, and significantly higher uniformity was achieved with the VPH grating. To Do: Check the extraction width as function of wavelength. Credit: K. Westfall, D. Harmer, C. Harmer.
orderalpha VPH 740 l/mm SR 790 l/mm
wave (A)
wave (A)
wave (A)
wave (A)
wave (A)
wave (A)
1 15.0 6010 8259 1.08 ~790
1 17.0 6907 9188 1.00 ~790
1 19.0 7860 10044 1.06 ~900
1 21.0 8688 10948 0.99 ~960
2 19.0 3904 5038 0.54 ~460
2 21.0 4332 5435 0.54 ~490
2 23.0 4789 5872 0.53 ~540
Table Note - we estimate spectral FWHM is ~2 pixels for Red Hydra and ~4.5 pixels for SparsePak in Littrow configuration for the VPH 740 l/mm grating. The FWHM will be somewhat smaller for the SR 790 l/mm grating because of additional anamorphic factors (i.e., non-Littrow).

Shared-risk run, Jun 2005. (Obs: D. Harmer, K. Westfall, C. Harmer) 740 l/mm only, but configured with large flat (215mm x 260mm clear aperture). Did not get on sky due to weather, but engineering data were gathered with both SparsePak and Red Hydra cables w/ and w/o the masked used in the Jan'05 T&E run to mimic the small-flat footprint. Setups are summarized in the table below; data included domeflat and calibration lamp spectra.

Jun'05 Shared-Risk SETUPS for VPH 740 l/mm Grating
orderalpha cwl cable
120o~ nmHYDRA-RED
121o~ nmHYDRA-RED
220o~490 nmHYDRA-RED
221o~490 nmHYDRA-RED
223o~540 nmHYDRA-RED
221o~490 nmSPSPK

With these two sets of data (Jan and Jun 2005) we will determine answers to the following questions:

Preliminary results:

T&E run, Sep 2005. -- pending.

The highest priority for performance-verification for this run is on-sky measurement of spectrophotometric standards with SparsePak to determine the absolute throughput of the Bench Spectrograph with the large flat and VPH 740 l/mm grating. Measurement in at least one setup is needed.

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last updated: 28 Mar 2006 (mab@astro.wisc.edu)