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This page is out of date. OOF Holography corrections should automatically be enabled when you enable Zernike coefficients. See Richard Prestage for more details.

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Once testing has been completed an example script will be placed here.

# Preamable : this defines the ZernikeModel procedure
import os
ptcsturtledir = "/home/groups/ptcs/obs/turtle"
DefineScan("ZernikeModel",
           os.path.join( ptcsturtledir ,  "ZernikeModel.py"))


# Perform auto peak/focus near target source.  
# slew to target source
Comment("Slewing to target source...")
Slew("BR1202")
# auto peak/focus
AutoPeakFocus()

# Spectral line observations

# Configure for line
Configure(line)

# Slew to source and balance the IF rack and spectrometer
Comment("Slewing to target source...")
Slew("BR1202")
Balance()

# Turn on appropriate Zernike's. 
# NOTE: This has been called _after_ slewing back to the source!!
ZernikeModel()

# Peform a NOD observation
Nod("BR1202", "1", "2", 90.0)

# Turn off Zernike's 
ZernikeModel(doZernModel=False)

procedure. Since calibrator sources are typically withing ten degrees

procedure. Since calibrator sources are typically within ten degrees

Using the ZernikeModel procedure

Accessing the ZernikeModel procedure

Typical usage of ZernikeModel()

The surface corrections derived from OOF observations have been evaluated at 5 degree elevation intervals and are relatively smooth. Therefore, changes of elevation of less than 5 degrees certainly do not require updating the corrections applied to the active surface, but we would recommend updates for elevation changes of 10 degrees or more.

In a scenario where the observer is switching between widely separated sources, we recommend updates after each slew to the source.

In a scenario that a source is being tracked as it raises/sets, we recommend updates when the source elevation changes by more than five degrees. This implies that duration of the track commands should not be excessively long so as to allow them to be interleaved with the ZernikeModel() commands.

Calling the update command ZernikeModel() should take very little time and no problems should arise from calling this command too often.

Usage example

Once testing has been completed an example script will be placed here.

surface. Documentation on how to do this will be here shortly.

surface, using a new observing procedure ZernikeModel.

The ZernikeModel procedure

The ZernikeModel observing procedure applies surface corrections appropriate for the current telescope elevation. It has two optional parameters:

1. source : the name of a source to slew to before setting the surface corrections. The default is source=None in which case no slewing command is issued.

1. doZernModel : as in the AutoPeakFocuss2 procedure, this specifies the surface model to use. The default is doZernModel="2005WinterV1", which is the currently recommended model. If doZernModel=None is supplied, corrections will be turned off.

Using the ZernikeModel procedure

The ZernikeModel procedure is a custom procedure not yet integrated into the main Astrid tree. Before use, it must be made available by calling the following code close to the top of the scheduling block:

import os
ptcsturtledir = "/home/groups/ptcs/obs/turtle"
DefineScan("ZernikeModel",
           os.path.join( ptcsturtledir ,  "ZernikeModel.py"))

After this the procedure can be called by issuing ZernikeModel() at any point in the scheduling block. It can be placed after every slew which is likely to be of significant distance.

# Import general modules import os import sys

Applying corrections in a more accurate (and intrusive) manner

The AutoPeakFocuss2 procedure applies corrections appropriate to the elevation of the calibrator source selected by the procedure. Since calibrator sources are typically withing ten degrees on the sky of the source being observed the elevation selected in this way is approximately right. The shortcomings of this approach are that the differences in elevation of the source and calibrator may in fact be significant, and that corrections are updated only every time an AutoPeakFocuss2 is executed.

Both of these shortcomings can be resolved by modifying users' observing scripts to explicitly update the correction to the active surface. Documentation on how to do this will be here shortly.

By analysing the OOF measurements we have produced a model of for the

By analysing the OOF measurements we have produced a model for the

displacements of actuators.

actuator displacements.

Out-of-focus (OOF) holography observations have been used to adjust the surface as a function of elevation using Zernike polynomials that can be dialed into the active surface control. Currently, these models are connected to the observing procedure AutoPeakFocus2 . This procedure is very similar to AutoPeakFocus (see ScanTypes), except that three parameters have been added: (1) a parameter "loc" that specifies a location on the sky; (2) a parameter "configure" that will configure the telescope if True or not if False; and (3) a parameter "doZernModel" that defines the Zernike models. The current best model is called "2005WinterV1". The defaults are (1) the current telescope position for loc; (2) to configure the telescope; and (3) not to apply any Zernike model. Listed below is an example scheduling block using AutoPeakFocuss.

Out-Of-Focus (OOF) holography observations have been used to measure total aberrations in the GBT optical system as a function of elevation. Due to the nature of these observations, it is not possible to determine how much of the aberrations arise from deformation of the primary surface as compared to elsewhere in the optical system. The aberrations can, however, be effectively compensated by adjusting the figure of the primary reflector using the active surface.

By analysing the OOF measurements we have produced a model of for the aberrations as a function of elevation. The models are currently stored as sets of disk files which contain the corrections to be applied at specific elevations. The corrections themselves are parameterized as coefficients of Zernike's circle polynomials; a section of the telescope M&C system turns these coefficients into displacements of actuators.

Applying corrections

The corrections can be applied in a largely user-transparent way through the use of the observing procedure AutoPeakFocus2 . This procedure is very similar to AutoPeakFocus (see ScanTypes), except that three parameters have been added:

1. Parameter "loc" that specifies a location on the sky around which to search for calibrators.

1. Parameter "configure" that will configure the telescope if it is set to True. The configuration stage will be skipped if configure is False. Note that True and False are Python (and hence, Astrid) keywords and should be entered into observing scripts without quotes.

1. Parameter "doZernModel" that selects the Zernike model to use. Value None applies no correction. Currently the best model is named "2005WinterV1" and is the only model we recommend be used. Note None is also a keyword which should be entered without quotes; model names are strings to be entered with quotes.

All of the new parameters have default values allowing AutoPeakFocuss2 to be used without any further modification in scripts which made use of AutoPeakFocuss. The default values for the parameters are: (1) the current telescope position for loc; (2) to configure the telescope; and (3) not to apply any Zernike model. Attached below is an example scheduling block using AutoPeakFocuss2.

Accessing the AutoPeakFocuss2 procedure

The AutoPeakFocuss2 procedure is not currently a part of the general Astrid release. Hence, to access this procedure, it is necessary to add the following small fragment of code to the beginning of the observing script:

# Add directory to the path
codedir = "/home/groups/ptcs/obs/turtle"
sys.path.append(codedir)

# Define AutoPeakFocus2
DefineScan("AutoPeakFocus2",
           os.path.join( codedir ,  "AutoPeakFocus2.py"))

Daytime performance of the telescope

nighttime under benign conditions. During the day similar observations revealed large distortions that appeared to be astigmatism. Large local pointing corrections (LPCs) were also measured during the day. Preliminary tests indicate that these distortion are not caused by deformations in the primary surface but are caused by sub-reflector collimation. We are developing procedures that allow the user to perform LPCs observations during the day that will use the sub-reflector to correct for these distortions, which appear to be a collimation problem, thereby improving the aperture efficiency and the pointing.

night under benign conditions. Similar observations made during the day reveal relatively large aberrations, mostly in the form of astigmatism. Large local pointing corrections (LPCs, that is, the difference between expected telescope pointing including the pointing model and actual telescope pointing) are also typically measured during the day. Some preliminary tests indicate that the aberrations found during the day are not by and large caused by deformations in the primary surface but are predominantly caused by collimation errors due to mis-positioning of the sub-reflector.

We are developing procedures to allow the user to implement local pointing corrections through adjustment of the sub-reflector position rather than offsetting the entire telescope. In this way we expect to correct the pointing and simultaneously improve collimation leading to improved overall aperture efficiency during the day.

Sample observing script

to AutoPeakFocus, except that three parameters have been added: (1) a parameter "loc" that specifies a location on the sky; (2) a parameter "configure" that will configure the telescope if True or not if False; and (3) a parameter "doZernModel" that defines the Zernike models. The current best model is called "2005WinterV1". The defaults are (1) the current telescope position for loc; (2) to configure the telescope; and (3) not to apply any Zernike model.

to AutoPeakFocus (see ScanTypes), except that three parameters have been added: (1) a parameter "loc" that specifies a location on the sky; (2) a parameter "configure" that will configure the telescope if True or not if False; and (3) a parameter "doZernModel" that defines the Zernike models. The current best model is called "2005WinterV1". The defaults are (1) the current telescope position for loc; (2) to configure the telescope; and (3) not to apply any Zernike model. Listed below is an example scheduling block using AutoPeakFocuss.

• Example Scheduling Block

The OOF holography observations discussed above were made during the nighttime under benign conditions. During the day similar observations revealed large distortions that appeared to be astigmatism. Large local pointing corrections (LPCs) were also measured during the day. Preliminary tests indicate that these distortion are not caused by deformations in the primary surface but are caused by sub-reflector collimation. We are developing procedures that allow the user to perform LPCs observations during the day that will use the sub-reflector to correct for these distortions, which appear to be a collimation problem, thereby improving the aperture efficiency and the pointing.

• Example using AutoPeakFocus2

Out-of-focus (OOF) holography observations have been used to adjust the surface as a function of elevation using Zernike polynomials that can be dialed into the active surface control. Currently, these models are connected to the observing procedure AutoPeakFocus2 . This procedure is very similar to AutoPeakFocus, except that three parameters have been added: (1) a parameter "loc" that specifies a location on the sky; (2) a parameter "configure" that will configure the telescope if True or not if False; and (3) a parameter "doZernModel" that defines the Zernike models. The current best model is called "2005WinterV1". The defaults are (1) the current telescope position for loc; (2) to configure the telescope; and (3) not to apply any Zernike model.

• Example Scheduling Block

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Active Surface - Observing Procedures

-- DanaBalser - 05 Oct 2005