Functional Requirements for the IRTF Adaptive Optics System
Overview
The IRTFAOS will be used with SPEX and NSFCAM for typical observations with an emphasis on support of planetary observations. This strong planetary bent will require a variety of software modes to support the tracking of planets and satellites and the observations of planets while tracking one of its satellites. While not strictly part of the AO system a new offsetguider/ acquisition camera will be installed with the AO system and the requirements for its software are included here.
The list below shows a summary of the Top Level Software requirements. More detail for each of the elements follows the list.
Acquisition of guide star
Beam Switching within the field
Beam switching outside of the field
Dithering from science instrument macros or screen guide buttons
Correction for Instrument rotation (NSFCAM Grism mode)
SPEX or NSFCAM
Static correction
Guide on a point source
Guiding on an extended object
Guiding on a dark feature on a planet
Non-sidereal tracking (drifting steering mirror)
Satellite guiding with differential lookup table (drifting steering mirror)
User Interfaces
Engineering interface
Observing Interface
Performance Display
Calibration Fiber
Pickoff mirror
Steering mirror
Membrane mirror focus
Filter wheel control
Tilt mirror offload
Focus offload
Membrane mirror throw
Loop gain
Offsetguider/acquisition camera operation
Acquisition of guide star
Beam Switching within the field
Beam switching outside of the field
Dithering from science instrument macros or screen guide buttons
Correction for Instrument rotation (NSFCAM Grism mode)
X/y translator
Filter wheel
Focus Stage
Lens slide
APD Power
Control Computer power
Mechanism power
Steering mirror power
APD temperatures
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Details for each of these requirements are given below
Acquisition of guide star - This mode is required for setup. The guide object must be placed in the WFS ~5arcsecond FOV. This mode will give the operator/observer feedback about the signal level and centering of the guide object prior to turning the correction loop on.
Beam Switching within the field This is the most commonly used mode. In this mode the science instrument will be spending 50% of its time in one position and 50% in another. The two positions will be within the 80 arcsecond field of the science instrument so that the science object is visible in both positions.
Beam switches typically are less than 40 arcseconds for NSFCAM or 30 arcseconds for SPEX. The AO loop will be paused while beam switching and the steering mirror moved to the B beam before the AO loop is resumed so that the observations are AO corrected in both beams.
Beam switching outside of the field This mode will allow large beam switches. In this mode the B beam will be outside the FOV of the science instrument and the WFS. AO corrections will only be made in the A beam. The AO loop will remain paused during the telescope movement and the B beam integration. The steering mirror will not move.
Dithering from science instrument macros or screen guide buttons Through the use of a macro command form the science instrument, virtual guide button push, or number entry at the users screen an offset to the lock position can be made. But moving the lock position the image will move in the science field. These offsets must be within the WFS FOV+-40 arcseconds. The offsets will be accomplished by moving the WFS steering mirror. The AO loop will remain on during the moves. The resolution of the moves will be ~1/10th of a pixel or 0.014 arcseconds.
Correction for Instrument rotation (NSFCAM Grism mode) Since NSFCAM will have smaller pixels than SPEX there will be times that the NSFCAM grism mode will be desired. Often to use the grism NSFCAM will have to be rotated. There will be a method to input the rotation angle.
SPEX or NSFCAM The WFS sensor mounts on different sides of these two instruments. A matrix change is required when changing instrument.
In static correction mode a fixed curvature signal is applied to the DM. The data for the fixed signal is stored in a look up table based on telescope position.
Guide on a point source this is the primary mode. There is presently a manual aperture at the membrane mirror so no software changes are required when in this mode. If the remote aperture is added we will need to close it down when observing in this mode.
Guiding on an extended object Presently the only change required in this mode is to open up the aperture either manually or automatically.
Guiding on a dark feature on a planet This mode will not be part of the initial compliment of software if implemented it will require an inversion of the curvature signal. This mode will need more definition before it is implemented.
Non-sidereal tracking (drifting steering mirror) In this mode the science object will typically be a planetary object with non-sidereal rates and the guide object will be a star. Two axis differential track rates will be entered. Once the differential track is turned on, the steering mirror will drift to keep the science object stationary. All of the observing modes must be supported while non-sidereal tracking is used.
Satellite guiding with differential lookup table (drifting steering mirror) Venus, Mars, Jupiter and Saturn are too big to guide on directly. Until a wide field WFS is designed and built we will have to guide on a satellite while imaging these planets. Some of the closer satellites are moving very quickly and therefore have different track rates than the planet and these track rates change significantly during a typical integration. This mode will allow for an observer provided lookup table that gives track rate versus GST. Once started the AO system will drift the steering mirror according to the rates in the table.
The maximum supported differential track rate will be 0.1 arcseconds/second on the sky. This will require a correction signal to the steering mirror at 10 times a second.
User Interfaces
Engineering interface The engineering interface will require a variety of displays that will be used for testing and troubleshooting the system. The core of this interface will be a 36 element graphical display where the user can select from the following information:
Counts from APDs + beam
Counts form APDs beam
Curvature signal
DM voltages
And probably others
There will also be an engineering screen that will allow any settable parameter to be changed. Some changes may require a password.
Observing Interface
The observers interface will be very simple it will provide a display of the total average counts, Tip/tilt values, Focus Values, Membrane mirror through and input, Steering mirror position
Performance display There will be some display or displays to indicate the system performance and current seeing conditions. This is yet to be defined.
Calibration Fiber This is a linear slide with an in and out position. The out and home position will be at one of the limits. The in position will be a programmable number of steps from the home. This in position should be settable through a password accessed engineering page. When the fiber is moved in the source will be turned on and when it is moved out the source will be turned off.
Pickoff mirror - This is a linear slide with an in and out position. The out and home position will be at one of the limits. The in position will be a programmable number of steps from the home. This position should be settable through a password accessed engineering page.
Steering mirror The steering mirror will be controlled through a computer yet to be specified. Required commands will be Zero Mirror Position, Offset Mirror xxx yyy and Read Position xxx yyy
Membrane mirror focus There is a focus stage on the Membrane mirror. This is a linear stage with two limits, one of which will be the home. We will need preset values for each instrument and a way to tweak the focus + or -.
Filter wheel control There is a filter wheel with 6 positions that is controlled through a BCD input. The control computer will send the BCD signal to the wheel. The blank-off position will be identified as the home position. Other positions will be selectable from the operators control screen through a menu.
Tilt mirror offload There is a tip/tilt stage behind the DM. When the tilt angle exceeds a set amount a offset command will be sent to the telescope to off load the tilt and allow the tip/tilt stage to move toward its center position.
Focus offload There are three ways to change the focus with this system. They are 1) the telescope focus, 2) the DM curvature and 3) the membrane mirror focus slide. There is a fourth but I do not believe that we use it, which is moving all three PZTs behind the DM together (I will ignore this one). The membrane mirror focus slide is used to get the instrument focal plane and the WFS focal plane coincident. Change from Spex to NSFCAM or to different plate scales within NSFCAM will probably require different positions of the membrane mirror slide. This will be provided from a lookup table through a menu. Once this is set the AO loop will adjust the DM curvature to achieve best focus. When the DM curvature exceeds a settable limit a command will be sent to the telescope to move the secondary mirror to reduce the DM curvature. Once the AO loop is on small tweaks to the focus at the science instrument are made by moving the membrane mirror slide + or from its lookup table position. Another way to say it is that the secondary and the DM are common mode focus adjustments and the membrane slide is a non-common mode adjustment.
Membrane mirror throw The membrane mirror throw is adjusted to match seeing conditions. We will probably have three preset levels for good, average and poor seeing that will be selectable form the menu.
This parameter may be set in conjunction with the loop gain terms.
Loop gain As the guide star gets fainter the AO loop gain will be adjusted to reduce the gain of the higher order terms. This will probably also be set in three or four steps user/operator selectable.
Tip/tilt Only mode for stars fainter than 15th magnitude or so we will use this mode which only sends corrections to the tip/tilt stage. A static set of voltages will be applied to the DM to make it flat.
Offsetguider/acquisition camera operation
All of the functions of the present offset guider are to be duplicated in the new system. The new system will use one camera on a x/y movable paddle such that it can intercept the science beam and be used as an on axis acquisition camera or moved in a C shaped arc around the science beam. There will be the following positions:
In - in the science beam, this is the on axis position
Out a position that clears the science beam but is close by
Offset - anywhere around the science beam but not vignetting the science beam
These observing modes must be supported when the guider is running
Acquisition of guide star set a guide box size and click on star to acquire
Beam Switching within the field command from the Science Instrument
Beam switching outside of the field - command from the Science Instrument
Dithering from science instrument macros or screen guide buttons
Correction for Instrument rotation (NSFCAM Grism mode)
The system must just pause when data is bad and background subtraction is required.
Screen outputs similar to the present system are required.
Initialize all or one of the mechanisms
X/Y translator - +/- 6 inches step of 0.001 inches with a speed of 1 inch per second
Filter wheel OTS 6 position wheel
Focus Stage Neat stage +- 3 inches
Lens slide three different lenses in a cross slide, one home and two limits
AC power should be controlled remotely via the Ethernet.
The boxes that require remote power control are:
APD Power
Control Computer power
Mechanism power
Steering mirror power