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Hydra to Undergo Metamorphosis for WIYN Telescope (1Sep93)
(from KPNO, NOAO Newsletter No. 35, 1 September 1993)

As alluded to in the March 1993 NOAO Newsletter, Hydra will undergo
some modifications during its move over to the WIYN telescope. That
move will be taking place during the spring, summer, and fall of 1994.
This is a summary of the instrumental changes that will be made and a
cursory schedule of the decommissioning from the Mayall 4-m and the
commissioning onto the WIYN.

Hydra is a multi-object fiber positioner and spectrograph first
constructed to allow the simultaneous observation of up to 100 objects
over the 45 arcmin field of view at the Mayall 4-m telescope. Since the
instrument began regular operation during the spring observing semester
of 1992, it has been requested in proposals for a total of more than
360 nights over two years and has been scheduled for 146 nights during
that same time period. An initial driver for the WIYN telescope was to
provide fiber-fed MOS capability in greater supply than would be
possible at the 4-m. NOAO's share of WIYN observing is 40%. MOS
observing will share this time with direct imaging.

Why modify Hydra as it moves over to the WIYN?  First of all, Hydra
will be mounted onto a Nasmyth focus at the WIYN as opposed to the
Cassegrain focus at the 4-m. This means that instead of lying nearly
horizontal during use, the instrument will always be in a vertical
orientation. The current mechanism for accessing the focal plate will
not work in the vertical position. Instead of negator springs and
rails, the new Hydra will utilize a hinge mechanism for such access.

The second major change involves the design of the telescope. To
minimize resolution-throughput losses of a fiber-fed spectrograph, the
fiber input end should be positioned on the telescope focal surface and
pointed toward the telescope exit pupil. The Mayall telescope was
optically designed long before the advent of fiber optic use in
astronomical instruments, and considerations of matching the field
curvature radius to the pupil distance were not of significant concern.
Hence, Hydra's focal plate at the 4-m consists of a "stepped" plate to
approximate the curvature of the focal surface while keeping the fibers
normal to a flat plane for better alignment with the pupil.
Considerable attention was given during the design of the WIYN
telescope to keep it "concentric" where we refer to concentric as
meaning that the radius of field curvature is concentric with the
distance to the telescope exit pupil. Although the final WIYN design
isn't perfectly concentric, it is very close. So close that the desired
position for a fiber should be normal to the curvature of the focal
surface rather than normal to a flat plane.

We tested the ability of the gripper to position a fiber onto such a
curved surface. The tests indicated that our current gripper design was
inadequate and that we would either need to allow a tip-tilt motion for
the gripper, or position the fibers onto a flat plate and then warp
that plate to match the curvature of field. Further investigation led
us to conclude that it was much simpler to warp the focal plate than to
implement gripper tip-tilt articulation. The selected design uses a
vacuum to draw the focal plate against a spherical backstop producing a
curve that very closely matches the theoretical focal surface.

Since we must rebuild the entire focal plate assembly to accommodate
the hinge access and warpable plate, we have the opportunity to make
other enhancements to the instrument which should vastly improve its
performance and reliability. Fiber cable management around the focal
plate is one such item. The current Hydra positioner occasionally
encounters a misplaced button. In most cases, the displaced button is
the result of entangling of that fiber with a neighboring fiber outside
the radius of the pivot blocks. To eliminate this, we are incorporating
long channeled grooves as a replacement to the current pivot scheme. At
WIYN, each fiber will be completely isolated from other fibers beyond
the edge of the focal plate down to where the fibers are clamped and
not allowed to move underneath the focal plate assembly.

Due to flexibility in the pivot redesign process and as a consequence
of the desire of the WIYN consortium to be able to add a small number
of fibers beyond the current 200, we designed the focal plate assembly
and pivots to be capable of accommodating up to 288 fibers. Twelve of
these positions will be occupied by field orientation probes (the
current instrument only has six such probes available), 96 positions
will be filled by the current blue fiber cable, another 96 will hold
the fibers from a newly constructed red fiber cable, and the remaining
84 spots are available for future growth.

As mentioned above, the current blue cable will be transported over for
use on the WIYN. The plate scale is such that these fibers will subtend
nearly 3 arcsec on the sky rather than the 2 arcsec subtended when in
use on the 4-m. It was felt that 3 arcsec blue fibers would accommodate
most observations where atmospheric dispersion might become an issue
since the WIYN wide field corrector will not have any atmospheric
dispersion correction.  Sky limited observations in the blue should not
suffer significantly since there are few sky emission lines to worry
about and since continuum sky subtraction is usually much easier than
line subtraction. In the red, however, atmospheric dispersion is
relatively small while sky subtraction issues are significant
(especially redward of 6000 A).  We felt that a 3 arcsec fiber size for
the red was unacceptable. The new red cable will consist of 200 micron
fibers instead of the 310 micron fibers in the current red and blue
cables. These new red fibers will subtend 2 arcsec on the WIYN
telescope and will also provide slightly better resolving power in the
spectrograph than the blue fibers.

Additional items underway in the modifications include replacement of
the ILS camera that views the field orientation probes (FOPS). A more
sensitive ICCD camera will be installed. In actuality, we will start
using an ICCD for the FOPS this fall. In addition, the gripper TV
camera is being upgraded to the newer generation of ICCD cameras that
have been installed around the mountain this summer.  Since there are
no offset guide probes on the MOS port at WIYN, we are developing
guiding algorithms that will utilize the FOPS as probes. The increase
of the number of such probes to 12 was done to minimize the impact of
FOPS selection on target acquisition.

The calibration screen will be removed from the positioner unit and
relocated in the instrument mount/spacer assembly. Effort is underway
to determine the best screen material to improve the detected flux of
the calibration lamps. Both Th-Ar and Cu-Ar tubes will be mounted
simultaneously and selected via the computer. We are also investigating
the use of an etalon in front of a quartz lamp (similar to that at the
MMT) to provide relative (not absolute) fiber to fiber wavelength
calibration. Flat fielding will continue to be carried out in a similar
fashion as at the 4-m by viewing a dome spot illuminated by quartz
lamps at the top of the telescope.

Automation is a key goal at the WIYN to allow rapid adjustments for
queue scheduling and for remote observing by the WIYN consortium
universities. As a consequence, much effort will go into upgrading the
software to allow communication and integration between the positioner,
spectrograph, data acquisition, and telescope software and computers.
Enhancements are underway in the controller electronics to allow
computer control of the vacuum system, calibration lamps, and the
monitoring of various important aspects (eg. temperature, TV status,
etc.).

Unfortunately, grating availability for the bench spectrograph will be
more restricted than at the 4-m.  Borrowing R-C gratings will be
difficult given the new scheduling constraints that will exist at the
WIYN. We have however acquired some duplicates of the R-C gratings for
permanent use at the WIYN.  In addition, the 316 line/mm echelle
grating will move over to the WIYN for use with Hydra. See the table
below for a list of the gratings that will be available. Both the
Simmons camera and the new all-refractive camera will also be available
for use at the WIYN. Although both 6 inch collimators will be moved to
WIYN, the large collimator will not be available since it will be
utilized in a future fiber fed echelle at the 4-m.

It appears that Hydra will need to be decommissioned from the 4-m
telescope around June of 1994. We will do our best to keep it
operational until the latest date possible. Once Hydra is
decommissioned, the positioner will undergo the modifications required
to mount the new focal plate assembly. Hardware modifications are
expected to take most of the summer. By September, Hydra should be
ready for installation onto the WIYN telescope and the process of
commissioning initiated.  It is anticipated that full time scientific
availability should resume during the spring of 1995.

                    Hydra/Bench Characteristics at WIYN

    |--------------------------------------------------------------------|
    | Full unvignetted field        |      60 arcmin                     |
    | diameters                     |                                    |
    |-------------------------------|------------------------------------|
    | Fiber diameters               |      3 arcsec (blue fibers)        |
    |                               |      2 arcsec (red fibers)         |
    |-------------------------------|------------------------------------|
    | Fibers available              |      96 blue fibers                |
    |                               |      96 red fibers                 |
    |                               |      12 field orientation probes   |
    |                               |      84 locations for future fibers|
    |-------------------------------|------------------------------------|
    | Minimum fiber separation      |      37 arcsec                     |
    |-------------------------------|------------------------------------|
    | Configuration time            |      20-25 minutes                 |
    |-------------------------------|------------------------------------|
    | Estimated positioning         |      0.22 arcsec (20 um)           |
    | accuracy                      |                                    |
    |--------------------------------------------------------------------|

    |--------------------------------------------------------------------|
    | Grating      |  316 line/mm, blaze angle 7 d (B&L 181)             |
    |              |  600 line/mm, blaze angle 13.9 d (B&L 420)          |
    |              |  860 line/mm, blaze angle 30.9 d (KPC-24)           |
    |              |  1200 line/mm, blaze angle 21.1 d                   |
    |              |  316 line/mm echelle, blaze angle 63.4 d            |
    |--------------|-----------------------------------------------------|
    | Cameras      |  All-refractive bench camera (285 mm focal length)  |
    |              |  Simmons camera (381 mm focal length)               |
    |--------------|-----------------------------------------------------|
    | Collimators  |  6 inch f/6.7 paraboloid                            |
    |              |  6 inch f/6.7 spheroid                              |
    |--------------|-----------------------------------------------------|
    | Detector     |  2048 x 2048 thinned Tektronix CCD (24 um pixels)   |
    |--------------------------------------------------------------------|

Sam Barden, Taft Armandroff

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