GSMT Control Workshop Tucson, September 11-12, 2001 Pointing Control for a giant segmented mirror telescope Patrick Wallace Rutherford Appleton Laboratory United Kingdom

GSMT Control Workshop Tucson, September 11-12, 2001 Presentation Outline l Platforms l Software l GSMT Challenges

GSMT Control Workshop Tucson, September 11-12, 2001 TCS Platforms l Use mass-market hardware and software: PC, running Linux/RTL or even Windows. C++, Java, CORBA. Avoid expensive RTOS, minority-interest middleware and specialized hardware. l Work mostly on the “Unix side”: use strict real-time only when necessary; use computers as intelligent managers, not as mere “programmable hardware”.

GSMT Control Workshop Tucson, September 11-12, 2001 TCS Design Philosophy l The science requirements, plus observing scenarios, merely sample the required functionality. l The TCS must deliver those functions as points in a “functionality envelope”. l The different modes of operation come from parametric control of a single, integrated, system. l As far as possible, all the code runs all the time.

GSMT Control Workshop Tucson, September 11-12, 2001 Specifying the Pointing l Not simply where the optical axis is aimed. l The user tells the TCS three things: »where in the sky to look »where in the focal plane the image is to fall »which way up the image is to be l The TCS predicts the mount and rotator angle demands that will realize the specified image.

GSMT Control Workshop Tucson, September 11-12, 2001 The Pointing Flow l Starts with target position. l Astronomical transformations lead to “observed” [Az,El]. l Allowing for non-perpendicularities, flexures and other pointing effects produces the required mount angles.

GSMT Control Workshop Tucson, September 11-12, 2001 Pointing “Filters” l Science pointing  current pointing: »imposes offsetting speed limit l Current pointing  mount pointing: »apportions motion between mount and M2 l Guider  pointing model: »offloads M2 bias l All filters have adjustable time constants etc. to achieve a variety of effects.

GSMT Control Workshop Tucson, September 11-12, 2001 TCS/Mount Interface l TCS sends timestamped mount coordinates over a LAN at (say) 20 Hz, defining locus. l Mount gets its position/velocity/acceleration demands by interpolation, using the last two or three TCS demands. l Same “locus” strategy for rotator, guide probes, even M2 in principle.

GSMT Control Workshop Tucson, September 11-12, 2001 “Virtual Telescope” celestial transformation pointing model position angle pointing origintarget direction mount coordinates

GSMT Control Workshop Tucson, September 11-12, 2001 Guiding l Each guider is a separate “virtual telescope”. l Given the guide star [ ,  ], the current mount demands define the [x,y] we want the guide star image to occupy. l Differential refraction and atmospheric dispersion are taken care of automatically. l The guider system is more important than the mount in pinning down the WCS.

GSMT Control Workshop Tucson, September 11-12, 2001 World Coordinates l Predicting [x,y]  [ ,  ] is the objective. l Using the current pointing state, the TCS generates the transformation describing the focal plane in [x,y]  [ ,  ] terms. l Packaged support for transformation to instrument coordinates and for writing FITS headers is also required.

GSMT Control Workshop Tucson, September 11-12, 2001 GSMT Challenges l In terms of pointing, not much is new in fact. l Pointing integrity must extend into AO, including adaptive M2. l Probably not possible to locate the rotator axis; the guider probes will define the WCS, so calibration methods need attention. And/or peripheral CCDs? l Encoders not enough. Need accelerometers and structural sensors. l 10 mas PSF means variable refraction across the pupil and atmospheric dispersion need attention.

GSMT Control Workshop Tucson, September 11-12, 2001 The “Servo Engineer” Problem l How do you keep your servo engineer(s) between the design phase and telescope commissioning? l Alternatively, how can the knowledge be mothballed during the construction phase?