NATIONAL RADIO ASTRONOMY OBSERVATORY

MEMORANDUM

DATE: August 27, 1996

TO: Roger Norrod

FROM: Ron Maddalena

SUBJECT: Time line for holographic measurements on the 140-ft with GBT systems

For the most part, the July/August tests of the GBT M&C system on the 140-ft were very encouraging. With a few more tests of the GBT M&C system I think we can attempt with some confidence holographic measurements on the 140-ft in preparation for similar measurements on the GBT.

This report updates the time line of my June 1996 memo on preparing for holography. The tests that I labelled (1) and (2) in that memo were successfully completed in July/August. A full memo describing the results of that test is in preparation. Below I give my views on how to proceed with the tests (3) and (4) in my original memo.

1. Pointing accuracy and repeatability

Pointing accuracy and repeatability was successfully measured in July/August and the results compare very well with the best results obtainable with the existing 140-ft Modcomp system. In brief, the pointing accuracy and repeatability were measured to be around 3" in both hour angle and declination. A broad, smooth 50" shape in the pointing residuals from the GBT M&C tests was identical to what was measured with the existing Modcomp system. Thus, the residuals were not due to the GBT M&C system. In fact, the GBT M&C system was the principal means were by a problem with the 140-ft was detected. I see no reason to repeat these tests.

2. Large-scale pointing accuracy

The large-scale pointing accuracy was also successfully measured in July/August with results that compare favorably with what has been determined before with the existing Modcomp system. The number of data points I could use from the July/August tests was fewer than I usually have on hand when I made similar measurements with the Modcomp system so, if we have some spare time during later GBT tests, we might want to repeat these measurements. But I am confident that the pointing of the GBT M&C system is good enough for an attempt at holography.

3. On-the-fly mapping

I believe we still need to test on-the-fly mapping with the GBT M&C system. The following updates my July memo:

The experiment is designed to test whether the GBT M&C system can generate an on-the-fly map with sufficient accuracy for holographic measurements. This can be accomplished by mapping a typical celestial point-source and mapping a hard-edged source like the moon.

a. Hardware requirements:

• GBT Cassegrain system (probably X-band).
• GBT DCR.
• GBT L.O. (can be replaced with 140-ft L.O. system if necessary).
• GBT I.F. system (can be replaced with 140-ft I.F. system if necessary since this system has not yet been successfully tested on the telescope ).
• 140-ft telescope.
 

b. Software requirements:

• GBT M&C -- mostly the same software from the previous GBT tests with the addition of mapping a 'moving' target like the moon.
• Aips++ -- mostly the same software from previous GBT tests. If possible, two-dimensional mapping or plotting algorithms.
 

c. Personnel:

• Hardware engineer (M. Stennes, W. Grammer, R. Norrod, T. Weadon).
• Software assistance ( R. Fisher, R. Maddalena).
• J. Brandt to develop ability to track the moon.
 

d. Time line:

• Requires some development work.
• Late September or October 1996
• About four days of 140-ft observing (plus overhead).
• Requires one to two weeks of data processing time.
 

4. Holography

To test whether the GBT systems can successfully complete a holographic map of a telescope and to compare the resulting maps with those generated previously of the 140-ft with the Modcomp and UniPops system.

a. Hardware required:

• GBT/140-ft prime-focus holography receiver.
• GBT holography backend.
• GBT L.O. (can be replaced with 140-ft L.O. system if necessary).
• GBT I.F. system (can be replaced with 140-ft I.F. system if necessary since this system has not yet been successfully tested on the telescope ).
• 140-ft telescope.

b. Software required:

• The same as 3.b.
• GBT M&C -- Macro for starting, stopping, and pausing a holographic mapping set; includes pointing, focus determination (maybe), starting from a designated point within a map.
• Aips++ -- Ability to generate an ASCII table of telescope positions and data so that the data can be read and processed by the UniPops suite of holographic data reduction software.
• Aips++ -- Holographic data reduction (need not be completed as long as it is available soon afterwards and as long as we can feed the data into UniPops's suite of holographic data reduction software).
 

c. Personnel:

• The same as 3.c.
• Holography frontend and backend engineers.
 

d. Time line:

• Should only happen after the successful completion of test 3.
• Requires about four to eight weeks for development work.
• Early 1997 (at least one to two months after the completion of test 3).
• About five days of 140-ft observing time.
• About two to four weeks of data processing.