Nodding the subreflector allows observers to switch between two beams of a high frequency receiver more rapidly than is possible with a standard nod. This has two possible benefits. First, it allows the observer to remove the effects of gain variations or atmospheric temperature or opacity variations on short time scales. This is particularly relevant when the weather conditions are unstable. Second, it has the potential for drastically lowering the observing overhead as compared to the standard nod.
Currently, subreflector nodding tests have involved configuring the subreflector nod in Astrid while running the actual scan via the Scan Coordinator or Antenna Manager in CLEO. This MR is to fully implement subreflector nodding in Astrid so that an observer does not need to interact with CLEO for this observing mode.
During the commissioning of the Ka-band receiver in the winter of 2006-2007, it was determined that the baseline structure seen in the data was due, in part, to gain variations in the signal path of the receiver. The effects of the gain instabilities were exasperated by changes in the atmospheric temperature between two nod positions. Ron Maddalena, Charlie Figura, and D.J. Pisano tested subreflector nodding as a possible solution. In this case, the subreflector nod had a cycle time of 9 seconds. That is 1.5 seconds to place Beam 0 on-source, 3 seconds in this position, 1.5 seconds to switch to Beam 1 on-source, and another 3 seconds in this position. This is compared to standard nodding, where the entire telescope is moved, and has typical cycle times of 2 minutes. One could have shorter cycle times with standard nodding, but the overhead would then be prohibitively high. The resulting baseline shapes were flatter for the short cycle times enabled by subreflector nodding. These baselines remained flat for cycle times as long as 30-60 seconds. For these longer cycle times, the overhead remained only 3 seconds. As a percentage, the overhead dropped from 33% (for 9 second cycles) to 5-10% (for 30-60 second cycles). As such, subreflector nodding has the promise of both improving data quality and observing efficiency.
For these tests, Astrid was used to configure the Spectrometer and the signal path, point the telescope and command the subreflector to tilt between two positions with a given cycle time. We were unable to use a Track command because this would re-initialize the subreflector position, hence no nodding would occur. The script used was project TKA F10214Test_KaSubNod. After using Astrid for setup, then the Scan Coordinator in CLEO was used to initiate the actual scan. All the data from both nod positions were placed in a single scan and the subreflector's position was not recorded with the data (it was in the Antenna fits files). This process was repeated every hour after doing pointing and focus scans in the standard way.
- The user should only have to use Astrid (more specifically, observing scripts) to conduct an observation with subreflector nodding enabled.
- Subreflector Nodding should work for any receiver and any backend. It will initially only be used for dual beam receivers, but additional uses are anticipated.
- The user should always know when subreflector nodding is active and not active, i.e., if a scan is aborted or otherwise ends prematurely, nodding should not carry over to the next scan.
- Nodding cycle times (on+off+move) should be specified as either an integer number of integrations or a time in seconds. In the latter case, a warning should be provided if the observer chooses a time that is not an integer number of integrations. For those backends for which we can not query the integration time, a warning will be provided, but cycle time in seconds will be necessary.
- Subreflector nodding should be usable with different scan types (like track, mapping commands, focus, pointing) and with frequency-switching.
- For this MR, subreflector nodding is intended to switch between the two beams of the dual beam receivers. The system should know what the amount and the direction of the offsets between the beams of different receivers are. The user should not have to specify this information. The priority is for this to work for Ka-band receiver. If easy to implement, then the other dual beam receivers could also be accomodated.
- While the building blocks in 17C507 can be used to specify all types of subreflector motion, the only modes to initially be included in scan types are
SubNod, SubFocus, SubHome, and SubNull. General SubMotion commands will be implemented, but not released for general use.
- A scan type called
SubBeamNod will be created to nod between the two beams of a dual beam receiver using the subreflector. The user only need to specify the two beams to nod between, the cycle time, and the scan length.
- The submotion building blocks will initially be used in the
Track, Slew, and Focus scan types (in addition to SubBeamNod). The remaining scan types should be able to use the subreflector class of objects as well, but are of lower priority.
- There will be limits as to the shortest cycle times usable. The absolute limit is currently a half-cycle time of greater than 4.4 seconds. Longer times may be required depending on the sky position being observed. At present, we do not know the limits as a function of sky position so they can be left blank for starters. These limits may change in the future, so the software group must be able to change these limits.
The initial classes of subreflector motion will be SubNod (to nod between the two beams of a dual beam receiver), Subfocus (to do a focus scan), SubHome (to place the subreflector at its "home" position"), and SubNull (to leave the state of the subreflector unchanged). There are other subreflector objects that may be developed, such as submotion and commands to re-oil the actuators. These are beyond the scope of this MR or 17C507.
The priority should be placed on implementing this class of objects in "Track", "Focus" and "Slew" as well as the creation of SubBeamNod. If time allows, all remaining scan types should allow for subreflector motion. While we can conceive of possible uses for subreflector nodding in the other scan types, it is likely to be used less at present.
There is no requirement that there are an integer number of cycles per scan.
Limits on cycle times as a function of sky position in order to prevent excessive wear on the actuators has been tabled until DennisEgan is able to provide recommendations. For starters, we will set an absolute limit on cycle times at 4.4 seconds per half-cycle.
Any scan procedure providing the submotion parameter as an option passes it as the second argument of
its call to Receiver.GetBeam(beamName, subMotion) rather than the defaulted argument SubHome.
The scan procedure SubBeamNod will behave much akin to a Nod The signature will be:
SubBeamNod(location, scanDuration, beamName=None) where beamName is derived for dual-beam receivers,
but must be specified otherwise.
The primary one is the devlopment of the subreflector building blocks (17C507).
There are a couple of other related MRs that will address other details of observing with subreflector nodding. This MR can be fully tested without them by using procedures written by Ron and D.J., but they will be necessary before fully releasing subreflector nodding to observers.
- The data files will need to include some indication of the subreflector position. (4C507)
- The new calibration routine as outlined in GBT Memo 246 "Recent Astronomical Commissioning Results for the Ka-band (26.0-40 GHz) Receiver" needs to be implemented in GBTIDL for reduction of data taken with subreflector nodding. Vector T(cal) and T(sys) values need to be accommodated as well. (5C507)
- Document the new features. Add to Astrid manual. Send a summary to support scientists and operators.
- Document the policies of how one may or may not use subreflector nodding. For example, elevation limits, maximum time between re-oiling the actuators, maximum scan lengths
- Add description of changes to release notes.
- We must have the limits on maximum time we can nod before re-oiling and any location dependent limits in place before we can deploy this mode for general use.
The submotion parameter for any scan procedure using it will be described as follows:
- submotion is a SubMotion object. It specifies movement of the subreflector during the subscans. The default value is SubHome.
The SubBeamNod scan procedure will be described as follows:
Syntax:
SubBeamNod(location, scanDuration, beamName, nodLength, nodUnit)
Parameter Info:
- location is a Catalog source name or Location object. It specifies the source upon which to do the nod.
- scanDuration is a float. It specifies the length of each subscan in seconds.
- beamName is a string. It specifies the receiver beam pair to use for nodding. beamName can be "MR12" or "MR34".
- nodLength is a number (integer for "integrations", and float or integer for "seconds".
- nodUnit is a string, either "integrations" or "seconds". The default is "seconds".
For two-beam receivers SubBeamNod causes the subreflector to tilt about the x axis between the two feeds at the given periodicity. The primary mirror is centered on the midpoint between the two beams. The beam selections are extracted from the scan's beamName, i.e., MR12 or MR34. The "first" beam ("1" or "3") performs the first integration. The periodicity is specified in seconds (float) per nod (half-cycle). A nod is limited to a minimum of 4.4 seconds. An example:
Slew("3C48", beamName="MR12", submotion=SubNod(4.4826624))
Alternatively, one can specify the nod time in units of the primary backend's integration times (integer) by setting the periodicity units to integrations instead of the default seconds, e.g.,
Track("3C48", None, 60.0, beamName="MR12", submotion=SubNod(nodLength=3, nodUnit="integrations"))
If the backend's actual integration time is obtainable then a warning is issued if the alignment between the integration times and the nod times shift over the duration of the scan by more than 10% of the nod time. A warning is issued in any case if the backend's actual integration time is not obtainable. Attempting to use integrations as the unit when the integration time cannot be obtained from the selected backend will cause a failure.
The following example does a subreflector nod between beams 1 and 2 for 60 seconds, each nod or half-cycle lasts
for three integrations where Rcvr26_40 was selected in the configuration:
SubBeamNod("1011-2610", 60.0, "MR12", nodLength=3, nodUnit="integrations")
Used with these GBT Standard Observing Modes: Position Switched (Dual Beam); Beam Switched
On 10/3/07 Dennis Egan traveled to Nook Industries to discuss our proposed subreflector nodding operation with respect to its impact on their linear actuators which are in place on the subreflector currently. The goal was to find out if the proposed motion would adversely effect the life of the actuators and if so would it still provide an acceptable life to allow nodding to go forward. The proposed nodding operation was outlined and they were shown the worst case profiles provided by D.J. Pisano and Mark Clark.
The main parameters that Nook considered were;
1) that the balls in the ball screw rolled a complete revolution during the operation, and
2) that the worm gear remained lubricated.
In the nodding outlined by D.J. Pisano, requiring 0.1 inches of actuator motion per cycle, the screw rotates 3.2 times per cycle, more than adequate motion to meet both limitations. In 3.2 revolutions of the screw, the balls will rotate many times. The worm screw will move through the grease in the gear box giving it a fresh coating of grease 3 times per cycle, assuming the grease level is high enough to touch the gear somewhere in its rotation.
Another minor concern was voiced by Nook. The motor coupling, a Lovejoy type coupling, was not chosen for this kind of cyclic motion. Its set screws should be checked for loosening and excessive spider wear and tightened or replaced as necessary.
It would be prudent to cycle the actuator on the order of ever hour in order to relubricate the screw and mix the grease in the gear box. Nook also recommended we monitor actuator currents during nodding to determine if the required power is climbing, indicating the necessity of cycling the actuator for relubrication.
Unit tests in ygor/proc containing submotion objects for Track and Slew will be added
as well as a new test suite SubBeamNodTest.
On the simulator we will run SubBeamNod and check through
the antenna simulator via CLEO's xyzSubrSegments table whether the subreflector is responding correctly.
After the scan check the GO FITS file for the proper entries for SubNod.
These will largely be combined with the tests in 17C507.
Using the simulator, we can test the Astrid interface. We can confirm that different scan types work with subreflector nodding and that the subreflector is in the correct position for different receivers and that the limits on cycle times are correct.
On-sky tests would involve nodding on a bright calibrator. This will confirm that the subreflector offset is placing the beams in the correct positions for different receivers. It will also confirm that the nodding cycles and the backend cycles remain in phase. These tests can also confirm that the different scan types yield the expected results when used with subreflector nodding or other subreflector motions. See 17C507 for further tests. We will test that the system behaves according to the requirements when aborted during scans. Cycle time limits will be tested according to 17C507.
Current integration and regression tests should suffice.
APPROVED: I acknowledge that my request is fully contained in this MR, and if the SDD delivers exactly what I specified, I will be happy.
ACCEPTED: I acknowledge that I have validated the completed code according to the acceptance tests, and I am happy with the results.
| Written |  - D.J. Pisano - 23 Aug 2007 |
|---|
| Checked | - MikeMcCarty - 31 Aug 2007 |
|---|
| Approved by Sponsor | - D.J. Pisano - 31 Aug 2007 |
|---|
| Approved by CCC | - RonMaddalena - 5 Sep 2007 |
|---|
| Accepted/Delivered by Sponsor | - D.J. Pisano - 22 Oct 2007 |
|---|
Symbols:
- Use
%X% if MR is not complete (will display 
)
- Use
%Y% if MR is complete (will display )
CCC Discussion Area
|
Revision r1.38 - 22 Oct 2007 - 20:56 GMT - DjPisano
|
Content copyright © 1999-2007 by the contributing authors.
All material on this collaboration platform is the property of the contributing authors.
|
| |
f10214_subrnod.astrid 
subrXtpos.py