NATIONAL RADIO ASTRONOMY OBSERVATORY
DATE: December 8, 1998
TO: Mark McKinnon
FROM: R.J. Maddalena
SUBJECT: Resources needed for GBT holographic tests on the 140-ft
A goal of the NRAO is to commission the GBT in as fast a way as is practical. One step in commissioning the GBT will be holographic measurements. However, a large fraction of the electronics and software we will be using for holographic measurements has yet to be tested. Many aspects of the software and electronics (for example, the dynamic range of I.F. and backend components and the response of these components to transient and large changes in power levels) cannot be satisfactorily tested in the GBT mockup.
To reduce the time for commissioning the GBT we want to test before commissioning as much of the hardware and software by making holographic maps using an existing NRAO telescope. As you know, the decision is to use the 140-ft for such a test. We already have holographic measurements of the 140-ft to which we can compare measurements made with the new hardware and software. And, the GBT M&C system, as proved by my tests two years ago, already knows how to command the 140-ft through a holographic measurement.
This report summarizes what resources we would need for performing a
holographic measurement of the 140-ft using as much of the GBT hardware and
software as is practical. Except where noted, my estimates are for the extra
effort need for a 140-ft test and do not include the efforts that would be
expended even if we didn't perform the test. To generate this report, I have
canvassed the engineers and programmers who are responsible for the components
we will be testing on the 140-ft. My gut estimate is that the 140-ft tests will
cost NRAO one month of extra labor but should save us about one month in
commissioning the GBT.
I. Hardware required:
According to Steve White, with two weeks notice NRAO can reassemble the receiver we used in 1988-1992 for holographic measurements of the 140-ft and 45-ft telescopes. Due to the differences in telescope receiver packages, we cannot test the GBT holographic receiver on the 140-ft. However, it might be possible to use the L.O. and block converters that will be used for the GBT. Steve is aware of what we would like and, with only a few weeks notice, we can decide what receiver components we can use.
The GBT holography backend was completed more than seven years ago and tested both in the lab and on the 45-ft in a holographic experiment. Thus, we need a minimum of hardware testing in the mockup. However, we will need to test the software interfaces and data collating software for the backend. For the 140-ft experiment, we will most likely decide to keep the backend in the mockup and pipe, via fiber, two I. F.'s from the 140-ft to the lab addition.
The decision as to what L.O. system we will use depends upon the decisions Steve makes regarding receiver components. Most likely we'll either use the existing 140-ft L.O's or will carry to the 140-ft a synthesizer typical of what we'll use on the GBT. In either case, the effort in providing a L.O. should be trivial.
We will attempt to make observations using an I.F. piped from the 140-ft to the mockup using the two fibers that already are in place. Except for the I.F. processing that will be required to get the signals onto the fiber, we'll be using the same hardware setup as we'll be using for the GBT. Some minor work will be needed to prepare equipment to send the 140-ft I.F. onto the fibers. This step will need to be done anyhow as we are planning to perform other system tests with the 140-ft, the spectral processor, and mockup equipment.
Except for mounting the receiver the day the tests start, and some minor
preparing of I.F. and L.O. equipment, the 140-ft is pretty much ready for the
II. Software required:
We will need macros for starting, stopping, and pausing a holographic mapping set; this includes pointing, focus determination (maybe), and starting from a designated point within a map. I tested these routines two years ago but they might need tweaking.
We will need the ability to generate an ASCII table of telescope positions and data so that UniPops can read and process the data. This will allow us to compare the old 140-ft data and the new data using the same data reduction package. By the end of February, it is imperative that Aips++ provides us with a 'table filler' for the holography backend, something that has been on their to-do lists for two years. Note that this effort would be needed regardless of whether we do the 140-ft tests; only the deadline for preparing the software will be different.
We would like to have holographic data reduction routines within Aips++. The
Aips++ software need not be available during the test (since we can feed the
data into UniPops's suite of holographic data reduction software) but we would
like the software to be completed soon afterwards. Again, Aips++ needs to work
on this regardless of whether we do the 140-ft tests and only their deadline has
been pushed forward.
IV. Time line: