The WIYN 3.5M telescope is the facility of choice on Kitt Peak for high-resolution optical imaging. In an effort to further enhance our high-resolution imaging capability over a moderate field, a tip-tilt imager is being constructed for WIYN. The WIYN Tip-Tilt Module (WTTM) is an optical and near-IR reimaging system that utilizes fast tip-tilt compensation and includes real-time focus sensing. The field of view for the WTTM is 4´ 4 arcminutes at a plate scale of 0.12 arcseconds per 15m m pixel. The WTTM will be added to the WIYN Instrument Adapter System as a "second" imaging port, facilitating rapid changing between the WTTM and the MiniMosaic imager by moving a pick-off mirror (in response to changing atmospheric conditions, for example). The WTTM passed its PDR in March 1999 and is proceeding with design, procurement and fabrication. The status of the WTTM project is discussed below.

Over the past 6 months following the PDR the WTTM team has focused on addressing the higher risk items that were indicated by the PDR committee. Included with this report is a copy of the WTTM response to the PDR report.

Prior to the PDR held 19-20 March 1999 the WTTM project lost its mechanical engineer to the higher priority GNIRS effort. The project received valuable assistance from KPNO Engineering to help the project through the PDR. However, during the 6 months following the PDR the WTTM project has been short handed in the mechanical engineering skills. During the 9 months the WTTM project has been without a mechanical engineer NOAO engaged in a recruiting effort which resulted in 5 offers to competent engineers. Unfortunately, these offers were turned down.

Recently, NOAO has extended an offer to Ron Price who is an experienced opto-mechanical design engineer. Ron has accepted NOAO’s offer and will be assigned to the WTTM project team beginning 18 Oct. 1999. Ron spent 7 years with the Gemini project and will bring valued experience to the project.

With our new member the WTTM project team presently consists of:

• Chuck Claver – Project Scientist
• David Vaughnn – Project Engineer
• Earl Pearson – FEA Analysis Engineer
• Ron Price – Opto-Mechanical Engineer as of 18 Oct. 1999
• Phil Daly – Software Engineer
• Rich Gomez – Mechanical Designer
• Rich Reed – Electrical Engineer
• Dave Dryden – Electrical Technician
• Al Camacho – Optician

Other contributing people include:

• Dave Sawyer – WIYN Site Manager
• Charles Corson – WIYN Engineer
• Art Code – Astronomer
• Taft Armandroff – Astronomer

The original estimate for project completion early in 2001 relied heavily on parallel efforts in all skill categories. While, significant progress has been made within the project, the absence of a mechanical engineer has resulted in a schedule slip of approximately 6 months. NOAO management (L. Daggert and R. Reed) is assisting the project team in developing a reliable and accurate project schedule and management plan to assess if schedule recovery is possible with additional resources.

On the basis of our draft schedule (see B sized detail and summary included) and present staffing and resource requirements it is our estimate that at most ~2 months of schedule recover is possible. There is no one or two items where this would occur, rather it would be from many small gains along the critical path of the project.

At this time the IAS is due to under go its modifications in the NOAO instrument shop during the months of February and March 2001. With the return of the IAS to the telescope at the beginning of April the WTTM will be essential complete and enter into its commissioning phase. Following approximately 6 months of on sky commissioning the WTTM will be available for shared risk observing beginning early September 2001.

At the end of April 1999 we completed our contract with Optical Filter Corporation (OFC). This contract for $5K was used to develop in detail the fabrication steps necessary for the diamond turned WTTM optics. David Vaughnn and Rich Gomez iterated with OFC’s engineering staff to produce a complete set of drawing of the fabrication process that were suitable for submitting for bids. NOAO let this package out for competitive bidding in May 1999. We received several "no bids" and two positive responses from OFC at $64.3K and from Contour Metrological Manufacturing (CMM www.cmmoptic.com) at $30.0K. The OFC bid came back significantly higher than expected and the CMM bid was unexpected.

Because of the large differences in the two bids we decided to research the capabilities of CMM. After several lengthy phone calls , David Vaughnn took a trip in June to Troy, Michigan for a site visit to CMM. David returned to Tucson with a sample part that he witnessed being fabricated. Using facilities at the University of Arizona’s Optical Sciences Center we obtained surface roughness measurements showing the part to have an RMS surface of ~50Å, within the quoted spec off the diamond turning machine. A qualitative test with a He-Ne laser showed diffraction grating effects with images in several orders present. The sample mirror was returned to CMM for post polishing using the same methods that were specified for the aspheric M2. After post-polishing at CMM the sample was returned to NOAO to repeat the series of surface tests. We found that the surface quality was improved to ~20Å RMS and all diffraction effects were removed. The original machined figure of the sample remained unchanged. The WTTM team was satisfied that the specifications for surface and figure quality could easily be met by CMM.

Further discussion with CMM resulted in a refinement in the fabrication process, which increased the cost but substantially reduced the risk in meeting spec with the 3 WTTM mirrors. The change added a surround sub-assembly for each of the three mirrors. Each sub-assembly contains pair of the WTTM mirrors (M1, M2 or M3) and can be removed from the diamond turning machine as a unit with disturbing the location and mounting of the mirrors themselves. The sub-assemblies are axially symmetric and essentially fill in the unused surface of the parent for each of the WTTM’s off-axis mirrors. The mirrors will be delivered to NOAO in their associated sub-assemblies. This allows several risk reduction benefits:

• The spherical mirrors M1 and M3 will be post-polished and tested at NOAO in the sub-assemblies as "normal" on axis optics. Once all quality control assurances have been met the off-axis mirror pair will be removed.
• Each sub-assembly has an unused diameter 90 degrees from the off-axis optic that can be used for contact profilometry with out risk to the desired mirror surface. This will be used for quality control measurements of the aspheric M2 following its post polishing at CMM.

We decided that the benefits of the extra work and costs of the surround sub-assemblies were more than offset by the reduction in the risk of manufacturing. The extra cost amounted to $16.3K, making the total CMM bid come in at $46.3K. The WTTM project was comfortable in awarding the optics contract to CMM in early July.

By the end of July NOAO took delivery from CMM of two sample mirrors for developing in house post-polishing expertise on Nickel surfaces. As of early October the NOAO optics shop is reliably producing £ 15Å on either electro-less or electrolytic Nickel coated aluminum mirrors. This is at least as high a quality surface as is typical produced on glass substrate mirrors. Thus, this risk identified at the PDR with the diamond turning process has been eliminated or reduced to being equivalent to typical glass optics.

We expect delivery by CMM in early November, which will include completed M2 sub-assembly and M1 and M3 sub-assemblies ready for post polishing at NOAO.

In order to further understand the effects caused by diffraction of the diamond turned surfaces we contracted Zemax for $300 to produce a specific curved ruling grating. Using this surface, David Vaughnn modeled each of the diamond turned optics as if they were gratings with curved rulings per the diamond turning process. David found that for wavelengths redward of 5000Å all first order images fell outside the WTTM field of view independent of where the zero order images was in the field. Between 4500Å and 5000Å approximately 10% of light the first order image fell inside the FOV when the source was at the extreme field edge.

The design concept for the WTTM’s error sensor presented at the PDR has been further developed by David Vaughnn. The optical design of the error sensor is complete and has been detailed for fabrication. In the figure below all elements on the right hand side are being made under contract with Liebmann Optical (field lens, astigmatism corrector, and MMA null lens) or purchased from Melles Griot (right-angle prism and negative lens). The left hand assemble is the quadrant Mangin Mirrorlette Array (MMA) and is being fabricated by the NOAO optics lab. David Vaughnn, Rich Gomez and Al Camacho are presently designing fixtures to fabricate and align the assembly. The materials for the MMA have been ordered and delivered to NOAO.

C. Claver has revised his modeling of the error sensor signal performance using the "to be built" design and a more realistic PSF model as recommended by the PDR committee. Following the process outlined in the PDR document a Moffat PSF with b =2.5, which matches observed WIYN PSFs well, was convolved with diffraction images from Zemax ray tracing of the error sensor. The centroid signal for determination of X-Y tilt corrections remained essentially unchanged. The astigmatic focus signal used to keep the WTTM in focus is shown below.

The WTTM team learned that the EGG avalanche photodiode modules that are to be used with the error sense included several optical surfaces that did not have AR coatings. We have estimated that this represents a loss of approximately 10% in overall efficiency. The contract to EGG for 9 APD modules included a special handling item for NOAO to coat these surfaces prior to assembly by EGG. David Vaughnn has completed work with EGG engineering detailing the necessary process. Work on this is presently active and we expect delivery of the APD modules by the end of the calendar year.

In response to the PDR the WTTM project has initiated environmental testing of enhanced silver coatings from three different companies: 1) Melles Griot – raw silver (control) and enhanced silver, 2) Opto Sigma Co. – enhanced silver, and 3) New Focus – protected silver. The test is being conducted at WIYN on the IAS and consists of a set of "exposed" samples as the control and a set of samples purged with dry nitrogen as would be in the WTTM. Charles Corson is making monthly reflectance measurements, which will be evaluated for degradation prior to coating the WTTM optics. We anticipate a base line of nearly 1-year when our decision on coatings is made.

In general the WTTM mechanical design has matured significantly since the PDR, however several areas remain undeveloped. Included with this report are several 3-D drawing showing the state of the mechanical design.

As the mechanical design of the WTTM housing became more developed it was clear that the X-Y stage for the error sensor was a significant area of uncertainty in both physical dimensions and performance. Final FEA modeling of the WTTM housing flexure performance needed definition of the X-Y stage. To this end Rich gomez and Chuck Claver began an extensive vendor search for and X-Y stage that met the specifications for the WTTM. This proved to be a difficult task, but in the end we let a contract to DynaOptic Motion Inc. DynaOptic has designed a 2-axis stage specifically for the WTTM project. The stage has magnetic linear encoders and a closed loop servo for each axis with positioning precision to ± 2m m while carrying a load of 2Kg. We are expecting delivery of the stage the week of 18 Oct. 1999. Once the X-Y stage is delivered we will certify its performance through a series of tests for flexure, repeatability and precision. Once these tests are passed to the team’s satisfaction Phil Daly will begin software development for integration into the WTTM system.

The WTTM optical housing is composed of three "cells", the front which holds the tip-tilt stage and M3, the mid cell which holds M2, and the rear cell which holds M1, the beam splitter, and errors sensor x-y stage. Earl Pearson has compiled a detailed FEA model of the entire WTTM assembly for the purpose of flexure and modal analysis. The FEA modeling shows that the WTTM is well behaved through the entire range of IAS rotation. It should be noted that the WTTM relies on flexure compensation through out the structure where small uncertainties can cause significant imbalance, hence large differential flexure between the CCD imager and error sensor. We have made every attempt to make the FEA model as accurate as possible and not anticipate any significant errors. Should any significant flexure exist between the error sensor and science detector we can use the active servo in the X-Y stage and a look-up table to compensate.

The WTTM mechanically interfaces to the WIYN IAS by a bracket on a 4 point system, three on the front cell and a fourth on the rear. The bracket caries the weight of the WTTM optical house and the imager. This keeps the WTTM optics well isolated from the imager load and allows the tight optical tolerances to be met.

Modal analysis of the WTTM’s FEA model showed that there was significant energy loss between the Tip-Tilt mirror and the supporting structure in the front cell. Initially there was concern that the week link was the "jo-block" used to precisely position the tip-tilt platform, which was noted by the PDR review. However, further analysis showed that the font cell was lacking in stiffness, rather than the "jo-block". Rich Gomez and Earl Pearson iterated on a minor redesign to stiffen the front cell. The redesigned front cell shows little coupling with the tip-tilt stage, where the modeled cut-off frequency with the "jo-block" is above 700hz.

After several iterations the WTTM team has settled on a control system design that is illustrated in the included diagram (see B sized drawing with this report).

The computer for the WTTM, a dual 500Mhz Pentium III Lynux system, was order in October 1999 from A. S. Labs. This is a rack-mounted system that will be located in once of the service racks under the WIYN’s azimuth skirt.

The WTTM control system will utilize two custom made interfaces, one for the error sensor APD pulse counting and the other for generating the analog signal to the x-y tilt stage amplifiers. Rich Reed and Dave Dryden have preliminary designs for these interfaces for a PCI buss. The preliminary design for the APD counter interface consists of 10 programmable channels. Four of these will be used as pulse counters for the erros sensor quad cell, the other can be programmed to provide additional counters or arithmetic functions (e.g. total counts from the 4 quad cell channels). The arithmetic functions will reduce the computation time, hence latency, in the error signal yielding improved performance in the high-speed tip-tilt servo loop. The 10 channels can also be configured as 10 separate counters allowing a potential upgrade path for a more sophisticated low order AO error sensor.

See attached report by Phil Daly.

The NOAO instrument shop has completed modifaction of the 2" filter wheel for the WTTM CCD imager. Dave Dryden is presently assembling and testing the stand-alone motor controller for the filter wheel.

The SITe 2K´ 4K CCD has returned from its time at CTIO for ARCON development. It has under gone some minor header board modifications at the NOAO CCD lab and has passed a suit of tests. Rich Reed and Tom Wolff have made some minor changes to the micro code in order to improve read-noise and dark current performance. In addition, initial position of the CCD in the dear has been done. Final positioning, piston and tip-tilt, will be done during the lab phase of WTTM testing and commissioning.