WIYN / Bench Upgrade: VPH 740 l/mm grating
|Overview||Expected Performance||Opto-Mechanics||Performance Verification|
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).
|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).|
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
And finally: YES this will get annodized!
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.
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.
|order||alpha||VPH 740 l/mm||SR 790 l/mm|
|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.
With these two sets of data (Jan and Jun 2005) we will determine answers to the following questions:
Later, we will measure FWHM (sigma) for all good lines in the spectrum and for all fibers.
This result agrees qualitatively with a comparison of system absolute throughput with SparsePak and the blue Hydra cable, as reported in Bershady et al. 2005 (ApJS). The reasons for throughput differences include differences in fiber transmission, and differences in the fiber-exit apeture formed by the cable toes. Based on a comparison of the fiber transmission, SparsePak fibers are considerably redder in the 440-540 region than the red Hydra cables. In the Sep 03 Concept Design Report we estimated differences in the fiber toes should amount to roughly a 20% effect (see Section 7), compared to 47% here. However, the red Hydra cables appear to have the largest FRD (see Figure 18 in the Sep 03 Concept Design Report), which would lead to larger vignetting from the toes than estimated in the Report. We conclude that SparsePak continues to show evidence for significantly higher throughput than existing cables. Further tests should be made (any grating) where observational control is provided (e.g., repeat and bracketing observations) for changes in the dome-lamp intensity and color, and other cables are included.
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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