Last Update: JeffMangum - 12 June 2009
On 12/3/2004, Wilson reported that he had conveyed a "purchase order to Jesus Martin Pintado for further semi-transparent vane lab tests. I hope this is finished by end Feb 05
and we then have a meeting and decided whether 2 loads of semi-transparent vanes are the solution for calibration (that meets spec!)
Tests astronomically are also of urgent interest. A possible plan for comment:
Work to be done for tests of Semi-Tranparent Vane at ATF
There have been two reports on the S/T vane tests at the 30m
Report No. 1
Report No. 2
The first of these identifies the S/T material as a polymer foam with a transmission coefficient of 90-95% at 100 GHz, one of several tested by Lazareff and Carter at IRAM Grenoble. I assume, but do not know, that thisis the material currently under test in Madrid. If a report of the Lazareff and Carter tests is available, it would be useful to have. A report is expected shortly from a lab in Madrid on further tests performed there.
The purpose of this test is to characterize amplitude calibration on an ALMA antenna using an evaluation receiver in conjunction with a semi-transparent vane. Although tests of the semitranparent material are under way in Madrid, they are being done near 60 GHz and in a lab environment. The goals of this test are to:
- Determine the properties of the vane near 100 GHz and 230 GHz
- Observe an astronomical source through the semi-transparent material
Currently, a rotating "bow tie" chopper wheel is employed at the ATF. The experiment proposed would involve placing semitransparent material over one of the clear apertures of the bow tie chopper wheel. Software would need to be modified so that the system could determine when the beam was intercepted by the absorber, when by the semitransparent material, and when it passed through to clear sky.
| Identify material and obtain sample
| Modify ATF bowtie chopper wheel
|| November 2004
| Rework software at ATF to provide for synchronus detection
|| November 2004?
| Perform observations with chopper wheel
|| Winter 2004/5
| Analyze observations
|| Winter 2004/5
| Report on analysis
|| March 2005
- 06 Dec 2004
STV Material Information
The following is from Jesus Martin-Pintado...
We have tested two materials for S/T vanes. The first material was a dense
polystyrene foam (vane #1) with 4 cm thickness. It is usually used for
isolation in buildings. I can send you a piece of this material. I will
also try to find a reference you can use to get it in the USA.
The second material was a dense polyurethane foam (vane #2) with 3 cm
thickness. Santiago Navarro (IRAM Granada) identified the material as
Eccostock SH-8 or SH-10 from Emerson & Cumings. See web page:
The question here is if you want to have the first material with a low
absorption coefficient. The low absorption makes very difficult to
achieve the required precision of 1%. It might be better for the tests at
the ATF to use the second material with a larger absorption coefficient
like 0.2 at 3mm and 0.6 at 1.3 mm. We have also made some test with these
type of material (vane#2) in the second report of the tests of the S/T
vane at the 30-m.
In the last months I have not been very active in the Calibration/ Science
groups because we have not news to report. I hope that we get finally the
contract signed. In any case, I will try to participate in the next
telecom if you plan to discuss the amplitude calibration
- 19 Apr 2005
Do We Need the STV? (Stéphane Guilloteau)
I am not sure I could dial in for this telecon, due to a number of unrelated activites at the same time. So let me mention the couple of points I believe should be discussed.
- The SEMI-TRANSPARENT vane is a USELESS device:
- The sole reason to use semi-transparent vane is to avoid receiver saturation.
- This is worth the effort only if the precision obtained by using such a device is better than the bias introduced by the possible receiver saturation.
- With the current SPECIFICATIONS, the receiver saturation is essentially negligible.
- So we could use "standard" loads.
- The SEMI-TRANSPARENT vane can be damaging:
- Making all the receiver characteristic (standing waves, etc...) sufficiently similar with the vane on or off is challenging.
- Characterizing the transparency is difficult.
- Mastering its polarisation properties perhaps even more.
- By comparison, a "standard" load is much more controlled. Yet it is a difficult item to realize...
- The faster, the better:
- There is a gain in completing a calibration cycle in 1 second as compared to 4 seconds.
- That gain comes from the better compensation of atmosphere AND from the receiver stability limit.
- So, it means the smallest, lightest devices are to be preferred ...
More generally, the ideal calibration device should measure the slope of the Detected power / Incident energy of the receiver under its normal operating conditions, with a sufficient precision and negligible bias (within the specification). With the current (projected) knowledge about receivers and site, and given the numerous memos on that topic, I do believe we are ready to make a choice for the calibration device. In my view, we have two options
- Make a simple device, with just (already sophisticated) ambient and hot loads, and make it as fast as reasonably possible. The ultimate limit will be receiver saturation.
- Go for the full, relatively heavy and bulky, 5 position device which allows to account for receiver saturation, if that saturation behaves as expected.
In view of the revised calibration accuracy, option (1) is largely sufficient. High accuracy relative calibration can be done by referencing on a secondary amplitude calibration when needed, of even by self-calibration on strong sources.
- 10 May 2005
Final Report STV Prototype Test Results
The final report, entitled Amplitude Calibration System, Characterization of Semitransparent Vanes
by Fernando Martin Jimenez, Jesus Martin Pintado, and Manuel Sierra Castaner, describes the test results from the STV prototyping. The final recommends read:
_In view of the results reported in this study, the ALMA calibration system using a STV
as a load will be affected by a number of effects due to the properties of any dielectric slab which will make extremely difficult to reach the specification of 1% accuracy. A calibration system based on absorbers at known temperatures should, in principle, provide a more reliable amplitude calibration systems with the potential to reach an accuracy of 1% if the receiver saturation effects and nonlinearities are smaller than predicted for the ALMA receivers._
- 11 Jul 2007