Memo Review Memo: 422 - The Dual-Load Calibration device revisited Guilloteau, 2002May23 Reviewer: Jeff Mangum Date Received: 2002Aug27 Review: The analysis of the dual-load calibration system in this memo has been thrown into question by David Woody's review. The difficulties with the dual-load calibration system have been well documented by the BIMA team prototyping this system. None of the information regarding this system indicates that its development should be abandoned. Given also that it is the only amplitude calibration system that has been prototyped, development of this system should most certainly be continued within the ALMA project. Specific comments on each section follow... Section 2.1: I don't believe that the dual-load subreflector calibration system was developed to mitigate the adverse affects due to saturation of receivers by ambient loads. The dual-load calibration system allows for the separation of what Larry D'Addario correctly refers to as "sounding" and "calibration" measurements. As the dual-load system is in effect a built-in signal source, it can be used to characterize the instrumental calibration as separate from measurements of the atmospheric parameters (what Larry calls "sounding" parameters). Section 3: The dual-load calibration system operates simultaneously with astronomical observations. Therefore, the fact that sufficient signal-to-noise requires 6-8 seconds of integration time at submillimeter wavelengths should not be considered a drawback of the dual-load calibration device. David Woody has suggested that the chopping frequency analysis in this section is too simplified and leads to an incorrect result. This discrepancy should be considered by the author. Section 4: The statement regarding knowledge of the exact value for the coupling factor (f) is not really correct. In fact, one really only needs to know that the coupling factor is constant to the required precision. Any absolute value differences between the true coupling factor and the assumed coupling factor can be accounted for in the absolute amplitude calibration step (the conversion of Ta* to Tr). Table 2: A number of the "pros" and "cons" listed for the vane and dual-load systems are not really correct: -- Stating that accessibility to the "widget space" above the receiver is easier than the accessibility of the apex is open to interpretation. Access to the apex requires the use of a lift, while access to the widget space requires climbing into the invar cone above the receiver cabin (for the Vertex antenna design, at least). -- There are no facts to base an assessment regarding reliability or maintenance of either of these devices. -- The facts regarding the influence of standing waves for both of these devices is not yet determined.