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Today is Sunday, January, 11th 

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2015, we're in the SSDIF cleanroom. We're here to 

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do some model testing on the pathfinder structure. My name is Brian 

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Ross, I'm the lead test engineer for JWST.

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And what we're doing today, we've hooked up a model shaker to part of the 

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structure and we're going to drive it with a random signal, and collect response

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data from almost 400 channels of accelerometers that are 

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attached to the structure will be taking data in a couple of different 

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frequency bands. We'll start with a low frequency band goes out to 80 herrtz.

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We're driving it with very low levels 

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for the large set of data we're going to collect it's only a half a pound RMS.

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So it's a very low force input.

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They're trying to correlate their fine line element models do to 

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jitter analysis for the deploy on orbit 

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configuration of the observatory and so they're interested in 

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particular modes of the mirrors and there's a 

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handful of target modes of the mirror themselves and then the entire structure 

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has lots of modes in the bandwidth we're looking at.

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We collect frequency response functions which is a ratio of the 

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acceleration at all the points we've got accelerometers to the input

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force. And we can take that data and 

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curve fit that data and extract what we call mobile parameters which is 

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frequency, damping, and what we call mode shapes. 

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We collect all that 

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data, provide the results to the analysts that create the fine 

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line element models. And they use that to tweak their models. 

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So it's accurately representing the structure.

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