The multi-object spectrometer, Hydra, has undergone commissioning at WIYN over the past twelve months. In August 1995, it was made available for "shared-risk" usage while continued improvements and performance evaluation were undertaken during the monthly testing and engineering nights at WIYN.
We are pleased to announce that Hydra is now in the phase of routine operation. All components of the system have been delivered, including the all transmissive camera. A review of the instrument was also part of the overall WIYN operational readiness review conducted in February 1996. The manual and software releases are in the process of being updated to reflect the latest condition of the instrument. Access to the manual can be achieved through anonymous ftp from ftp.noao.edu and through the KPNO home page http://www.noao.edu.
400 line/mm Grating
Initially, four conventional gratings and one echelle grating were planned to be dedicated to the Bench Spectrograph at WIYN. A first-order low-dispersion blue grating was not included among these because one was not available at the time the grating complement was specified. Such a grating has become available from Milton-Roy, and we procured this 400 l/mm grating with a blaze angle of 4.2 degrees. This grating has excellent efficiency in the blue and good scattered light performance. The grating was released for shared-risk use in September and is now used quite frequently. The properties of all the Bench Spectrograph gratings are listed in the Hydra manual and are included in the exportable program which helps users design spectrograph configurations (setup.f).
Bench Spectrograph All-Transmissive Camera
Work on the all-transmissive camera for the Hydra Bench Spectrograph has been underway for some time. This camera, called the Bench Spectrograph (Red) Camera, has the advantages of no central obstruction and the utilization of high-performance broadband anti-reflection coatings. The Simmons camera, the other option, has a significant central obstruction that blocks 40% of the light in the fairly uniform beam produced by fiber scrambling. The glass and coatings of the Bench Spectrograph (Red) Camera are optimized for the wavelength interval 4500-10000 A. Our deployment of the Bench Spectrograph Camera was delayed by the fact that the WIYN CCD T2KC is warped and the Camera has a narrow depth of focus. In order to overcome this difficulty, a new final element was fabricated that optimizes the coincidence of the focal surface of the camera with the measured shape of the CCD. We can happily report that this fix has been quite successful. The images are good ( 2.4 pix FWHM) over 90% of the CCD. There is some degradation in the lowest and highest rows. We list below the grating configurations for which we have tested the new camera. This includes all of the on- blaxe configurations that make sense redwards of 4000 Å. Note that because the camera is fully refractive, the position of the best focal surface is a function of wavelength. In practice, that means that we have a tilt adjustment for the CCD dewar with respect to the final element of the camera (referred to as the dewar azimuth). As part of our testing, we determined the optimum dewar azimuth for all of the tested gratings (listed below). In practice, one sets this dewar azimuth and then performs a focus sequence in dewar-camera spacing (as usual). We have observed numerous flux standards with the new camera and various gratings. Unfortunately, these are not fully reduced. Initial indications are that the Bench Spectrograph Camera is approximately twice as efficient at 7000 A as the Simmons Camera. We plan to incorporate all our information on count rates and efficiency in the Hydra manual. Note that the wavelength coverage, resolutions, etc., with the Bench Spectrograph Camera are currently available via setup.f and Table 1 in the Hydra manual.
The Bench Spectrograph Camera is available for shared-risk use. For now, we recommend only observations that concentrate on the wavelength region 4500-10000 Å. We also recommend only configurations listed in the Table below. Note that the 400 l/mm (blue) grating and the Bench Spectrograph (Red) Camera do not work well in tandem and that unacceptably poor image quality results.
One note of caution regarding changing between the Simmons and Bench Spectrograph cameras. This is a time-consuming job that should not be done during the night. During the design of the camera, we did take numerous steps to enhance the speed and safety of camera changes.
Grating Angle Wavelength Dewar Azimuth (lines/blaze) Order (degrees) Central Blue Red (degrees) 316@7.0 1 21.953 7401 4729 10105 1.310 600@13.9 1 28.900 7735 6306 9166 0.808 860@30.9 1 38.672 9019 8026 9999 0.638 860@30.9 2 41.523 5016 4523 5500 -0.360 1200@28.7 1 43.701 7731 7029 8419 0.293 316@63.4 10 68.900 5633 5479 5768 -0.128 Note: dewar azimuth is not expected to be a function of order for the echelle due to the narrow wavelength interval covered. We have tried only 10th order so far.
Hydra Simulator Caveat
The software which runs a simulator of the Hydra instrument is valuable for inspecting and modifying one's target assignments. Unfortunately, the platform under which the software will operate is restricted to Sun computers running SunOS and OpenWindows. In some cases, the compiled version of the simulator will run on a Sun workstation running Solaris, but we have experienced cases where it does not work.
If you experience difficulty running the simulator at your home institution and your preparation is for the NOAO share of WIYN time, you may contact Jeannette Barnes (jbarnes@noao.edu) or Dave Bell (dbell@noao.edu) to set up a visitor account through which it is possible to run the simulator remotely (though not necessarily efficiently). Please refer to the Hydra manual for further information.
Hydra Positioning
One of the performance evaluations of the past year involved an analysis of Hydra's positioning accuracy, after it was discovered last fall that the fiber positioning could do with some improvement. The smaller plate scale of WIYN in comparison to that of the Mayall 4-m places tighter specifications on the physical positioning of the fibers required to land the desired target onto the fiber. We also determined the need to introduce some distortion terms to eliminate residual "plate scale" terms of the instrument stages. In addition, we found that the new fiber pivots were introducing somewhat stochastic positional errors which would cause some fibers to completely miss their targets. Simple fixtures were recently installed into the pivots to eliminate such errors. As a result of these improvements, it should now be a rare circumstance that leads a fiber to miss its target completely.
On-sky evaluation of Hydra positioning performance showed an improvement in rms positioning from > 0.5", measured last fall, to about 0.3", measured this spring.
Fun Facts
During the "shared-risk" period from August 1995 to March 1996, Hydra was operated on a total of 156 days or nights (of which about 25 were spent on testing and evaluation) and has moved 43500 fibers during that time. The reliability of the instrument is significantly better than when it was in use at the 4-m, with the failure rate of fiber motions being about 1 in 1700 moves, as compared to 1 in 200 moves at the 4-m.
Future Prospects
Attention is now focusing on fabrication of Hydra/CTIO (see related article under the CTIO section in this Newsletter). That instrument will utilize better performing motors and controllers and will involve enhancements in the controlling software. Discussions are underway with WIYN for the possibility of a retrofit project which will upgrade Hydra at WIYN with similar new motors, controllers, and software. We hope that such an upgrade might be possible a few years from now.
Sam Barden, Taft Armandroff